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MINERALS IN DAIRY PRODUCTS | Trace Elements, Nutritional Significance
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MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
lactose, phosphopeptides) or inhibitors (e.g. proteins, calcium, phosphate) of macromineral absorption in milk and dairy products remains unclear. However, milk does not appear to contain substances which are strongly inhibitory to mineral absorption, such as phytate or polyphenols in foods from plants. Finally, understanding the nutritional signi®cance of macrominerals in milk and dairy products will also bene®t from the improvement in our knowledge of fundamental aspects of minerals, such as their nutritional roles, requirements and metabolism, and the quantitative relationship between dietary intakes and health. See also: Minerals in Dairy Products: Trace Elements, Nutritional Significance.
Further Reading Allen LH (1983) Calcium bioavailability and absorption: a review. American Journal of Clinical Nutrition 53: 783±808. Fairweather-Tait S and Hurrell RF (1996) Bioavailability of minerals and trace elements. Report of FLAIR Concerted Action no. 10 (Hurrell RF (ed.)). Nutrition Research Reviews 9: 295±324. Fleet JC and Cashman KD (2001) Magnesium. In: Bowman B and Russel R (eds.) Present Knowledge in Nutrition, 8th edn, pp. 292±301. Washington, DC: LSI Press. Flynn A and Cashman K (1997) Nutritional aspects of minerals in bovine and human milks. In: Fox PF (ed.) Advanced Dairy Chemistry, vol. 3, pp. 257±301. London: Chapman & Hall. Flynn A and Cashman K (1999) Calcium forti®cation of foods. In: Hurrell R (ed.) Mineral Forti®cation of Foods, pp. 18±53. Leatherhead: Leatherhead Food Research Association. Holland B, Welch AA, Unwin ID et al. (eds.) (1995) McCance & Widdowson's The Composition of Foods, 5th edn. London: HMSO. Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press. Miller DD (1989) Calcium in the diet: food sources, recommended intakes and nutritional bioavailability. Advances in Food and Nutrition Research 33: 103±156. National Research Council (1989) Recommended Dietary Allowances, 10th edn. Washington, DC: National Research Council. Renner E, Schaafsma G and Scott KJ (1989) Micronutrients in milk. In: Renner E (ed.) Micronutrients in Milk and Milk-Based Food Products, pp. 1±70. London: Elsevier Applied Science. Sadler MJ, Strain JJ and Caballero B (eds.) (1999) Encyclopedia of Human Nutrition, vols. 1±3. San Diego: Academic Press. Scott KJ (1989) Micronutrients in milk products. In: Renner E (ed.) Micronutrients in Milk and Milk-Based
Food Products, pp. 71±123. London: Elsevier Applied Science. Strain JJ and Cashman K (2002) Minerals and trace elements. In: Gibney M (ed.) Introductory Nutrition. London: Nutrition Society. Van Dokkum W (1995) The intake of selected minerals and trace elements in European countries. Nutrition Research Reviews 8: 271±302.
Trace Elements, Nutritional Signi®cance K D Cashman, University College, Cork, Ireland Copyright 2002, Elsevier Science Ltd. All Rights Reserved
Introduction There are 20 minerals that are considered to be nutritionally essential for humans and these are sometimes classi®ed into two groups, i.e. the macroelements (also known as the macrominerals) and trace elements. The macrominerals (sodium, potassium, chloride, calcium, magnesium and phosphorus) are present in the body in amounts greater than about 0.01% by weight and the nutritional aspects of these minerals are dealt with elsewhere (see Minerals in Dairy Products: Macroelements, Nutritional Signi®cance). The trace elements (the remaining 14 essential minerals) occur in the body at much lower levels and are required in the diet in amounts less than about 100 mg dayÿ1. The nutritional aspects of these minerals will be discussed in this article. The 14 trace elements that are considered essential in the human diet are iron, copper, zinc, manganese, selenium, iodine, chromium, cobalt, molybdenum, ¯uoride, arsenic, nickel, silicon and boron. While some of these, e.g. arsenic, nickel, silicon and boron, have not been shown to be essential for humans, they are essential for experimental animals and probably are also essential for humans. A number of other elements occur in milk, e.g. lithium, bromine, aluminium, strontium, silver, lead, tin, vanadium, mercury, cadmium, rubidium and caesium. These are not nutritionally essential and are not discussed here, but many of them are toxic. However, their concentrations in milk are normally well below toxic levels. This article outlines the nutritional roles, recommended intakes and hazards of de®ciency or excess of the 14 trace elements that are considered to
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
be nutritionally essential for humans, all of which occur in milk. In this article, `milk' refers to cows' milk unless otherwise stated. The content, chemical form, bioavailability and nutritional signi®cance of these trace elements in milk and dairy products are considered. In addition, in view of the widespread use of infant formulae based on cows' milk, some nutritional aspects of trace elements in these formulae are discussed. Although the trace elements are treated separately, it is important to realize that interactions of trace elements with each other, with macrominerals, with other constituents of milk, and with other food constituents occur, and that such interactions are assuming an increasing importance in nutrition.
Content and Chemical Form of Trace Elements in Milk and Dairy Products Mineral Content
The trace element content of milk is not constant but is in¯uenced by a number of factors such as stage of lactation, nutritional status of the animal, and environmental and genetic factors. Reported values in the literature for the concentration of many trace elements show a wide variation which is partly due to these factors, but also partly to analytical errors and contamination from milk collection and processing equipment and procedures. Representative values for the average trace element content of milk are presented in Table 1. Iron, zinc, copper Although several investigations have reported lack of a developmental pattern for iron in milk, at least one study has reported that iron Table 1 Mean concentrations of trace elements in cows' milk Trace element
Content (l ÿ1)
Iron (mg) Zinc (mg) Copper (mg) Manganese (mg) Iodine (mg) Fluorine (mg) Selenium (mg) Cobalt (mg) Chromium (mg) Molybdenum (mg) Nickel (mg) Arsenic (mg) Silicon (mg) Boron (mg)
0.5 3.9 0.09 30 100±770 20 10 0.5 2.0 50 26 20±60 3000 500±1000
Adapted from Flynn and Cashman (1997).
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concentration decreased by 35±50% during the ®rst 3 days of lactation and remained relatively constant thereafter. The iron content of milk is resistant to changes in dietary iron intake. Contact with metal containers can increase the concentration of iron in milk. Mean zinc concentration in milk is 3.9 mg lÿ1, but large variations in the zinc content of milk (2.0± 6.0 mg lÿ1) have been reported. There is a large decrease (50%) in the concentration of zinc in cows' colostrum during the ®rst 3 days of lactation, with little change thereafter. Dietary zinc supplementation increases the zinc concentration in milk only slightly. The concentration of copper in milk decreases by up to 50% during the ®rst 3 days of lactation; it can be increased by dietary copper supplementation or by contact with metal containers and processing equipment. Manganese, selenium, iodine The concentration of manganese is higher in colostrum (100±160 mg lÿ1) than in mature milk (20±50 mg lÿ1) and a decrease of over 50% has been reported to occur during the ®rst 3 days of lactation. Oral supplementation of manganese to cows can increase the manganese content of milk provided that large doses are administered over a long period. Mean selenium concentration in cows' milk samples from 15 countries was reported as 10 mg lÿ1 (range 3±40 mg lÿ1). The concentration in milk depends on dietary intake and in areas such as New Zealand and Finland, where the selenium content of soil and plants is low, concentrations as low as 3±5 mg lÿ1 have been reported. It has been shown that the selenium content of milk increased linearly from about 30 to 55 mg lÿ1 when dietary selenium was increased from about 2 to 6 mg dayÿ1. Summarized data from various countries on the iodine content of milk indicate mean values of 100± 770 mg lÿ1 and a wide range of individual values from 20 to >4000 mg lÿ1. The concentration of iodine in milk is in¯uenced by season and is closely related to dietary intake, and feeding winter rations containing mineral supplements results in considerable increases in milk iodine. High concentrations of iodine in milk have been related to the addition of excessive amounts of ethylenediamine dihydriodide (EDDI) to dairy cow rations. The use of iodophors for teat disinfection increases the iodine content in cows' milk. In addition, use of iodophors for the disinfection of containers, milking machines and processing equipment can also cause contamination of milk with iodine. The standardization of mineral feed supplements and supervised and restricted use of iodophor disinfectants have
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MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
been recommended as measures to reduce the iodine content of milk and milk products. Chromium, cobalt, molybdenum, fluoride The mean concentrations of chromium and cobalt in milk are 2 mg lÿ1 (range 0.2±3.6 mg lÿ1) and 0.5 mg lÿ1 (range 0.4±1.1 mg lÿ1), respectively. Dietary supplementation with cobalt increases the concentration of cobalt in milk but does not increase the vitamin B12 content of milk unless the diet is cobalt-de®cient. The concentration of molybdenum in milk has been shown to be dependent on dietary intake, increasing over ®vefold when cows were supplemented with ammonium molybdate. The mean ¯uoride content in milk is 20 mg lÿ1 (range 10±140 mg lÿ1). Arsenic, nickel, silicon, boron Reported concentrations of these elements in milk are presented in Table 1. Little is known about the in¯uence of stage of lactation, maternal nutritional status, environmental and genetic factors on the content of these trace elements in milk. Trace Element Content of Dairy Products
The contents of selected trace elements in other dairy products are shown in Tables 2±5.
Chemical Form of the Trace Elements
The chemical form of a trace element is important because it may in¯uence intestinal absorption and utilization (the process of transport, cellular assimilation and conversion into a biologically active form) and thus bioavailability. Iron, zinc, copper In milk, 14% of the iron occurs in milk fat where it is associated with the fat globule membrane. About 24% of the iron is bound to casein, probably to the phosphoserine residues of caseins, while 29% is bound to whey proteins and 32% is associated with a low molecular weight fraction. Most of the zinc in milk is in the skim milk fraction, with only 1±3% in the lipid fraction. Of the zinc in the skim milk fraction, over 95% is associated with the casein micelles, with a small proportion (5%) associated with a low molecular weight compound which has been identi®ed as citrate. Within the casein micelles, one-third of the zinc is loosely bound to casein phosphoserine residues and two-thirds is more tightly bound to colloidal calcium phosphate. The distribution of copper in milk has been reported as: 2% in the fat fraction, 8% bound to whey proteins, 44% to casein and 47% in a low molecular weight fraction.
Table 2 Mean concentrations of selected trace elements in concentrated milks Content (100 gÿ1)
Trace element
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium (mg) Iodine (mg)
Pasteurized skimmed
Dried skimmed
Evaporated (whole)
Condensed (whole)
0.05 Trace 0.4 Trace 1.0 15
0.27 Trace 4.0 Trace 11 150
0.26 0.02 0.9 Trace 3.0 11
0.23 Trace 1.0 Trace 3.0 74
Data from Holland et al. (1995).
Table 3 Mean concentrations of selected trace elements in creams Content (100 gÿ1)
Trace element Fresh cream
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium ( mg)
10% fat
20% fat
35±48% fat
60% fat
0.1 Trace 0.3 Trace Trace
0.1 Trace 0.5 Trace Trace
0.2 Trace 0.2 Trace Trace
0.1 0.09 0.2 Trace Trace
Data from Holland et al. (1995).
Soured 20% fat
Sterilized canned 25% fat
UHT 32% fat
0.4 Trace 0.5 Trace Trace
0.8 Trace 1.1 Trace Trace
1.0 Trace 0.4 Trace Trace
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
Manganese, selenium, iodine In cows' milk, manganese distribution is: 67% bound to caseins, 1% to the milk fat globule membrane, 14% to whey proteins and 18% to a low molecular weight fraction. About 12% of the selenium in milk has been estimated to be incorporated in the enzyme glutathione peroxidase (EC 1.11.1.9). Most (80±90%) of the iodine in milk is in the inorganic form, mainly as iodide and is in the watersoluble fraction, and 5±13% is bound to proteins through either covalent bonds or loose physical associations, with less than 0.1% bound to fat.
Nutritional Significance of Trace Elements in Milk and Dairy Products
Molybdenum, chromium, fluoride All the molybdenum in milk is considered to be associated with xanthine oxidase (EC 1.1.3.22). The chemical form of chromium in milk is unknown, although chromium in foods is generally in the trivalent state. About 46±64% of the ¯uoride in milk occurs as free ¯uoride ions with the remainder bound to proteins.
Iron
Cobalt, arsenic, nickel, silicon, boron Little is known about the chemical form or distribution of these trace elements in milk and dairy products. Table 4 Mean concentrations of selected trace elements in butter, yoghurt and dairy ice cream Content (100 gÿ1)
Trace element
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium (mg) Iodine (mg)
Butter
Yoghurt (whole milk)
Dairy ice cream
0.2 0.03 0.1 Trace Trace 38
0.1 Trace 0.7 Trace 2.0 63
0.1 0.02 0.3 Trace 1.5a Ðb
a
Barclay et al. (1995). Iodine is present in signi®cant amounts but there is no reliable information on the amount. Data from Holland et al. (1995).
b
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A simple and useful evaluation of the nutritional signi®cance of trace elements (as well as macrominerals) in milk can be obtained by comparing the amounts of the different elements provided by 1 litre of milk (Table 1) with the recommended daily intakes for these elements (Table 6). Great progress has been made in our knowledge of the trace element nutrition of infants during the last two decades, particularly in the area of bioavailability.
Iron is an essential trace element that acts as a catalytic centre for a broad spectrum of metabolic functions. Iron, as a component of haem in haemoglobin, myoglobin, cytochromes and other proteins, plays an essential role in the transport, storage and utilization of oxygen. It is also a cofactor for a number of enzymes. De®ciency of iron, resulting in anaemia, af¯icts about 30% of the world's population and occurs in both Western and developing countries. The recently revised recommended daily allowances (RDAs) for iron in the United States are shown in Table 6. Milk and milk products are very poor sources of iron, and milk contributes little to total iron intake. Bioavailability of iron to the infant from human milk has been reported to be in the range 49±70%. Considerably lower absorption ef®ciency of iron from cows' milk by human infants has been reported, usually about 10±34%. To compensate for the relatively low bioavailability of iron in cows' milk, infant formulae are often supplemented with iron. Iron absorption by infants from cows' milk formula containing 12 mg lÿ1 iron as ferrous sulphate is about 4±7%, but because of the much higher concentration of iron in such formulae, the absolute amount of iron absorbed is considerably greater than from human milk.
Table 5 Mean concentrations of selected trace elements in some cheese varieties Content (100 gÿ1)
Trace element
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium (mg) a
Brie
Cheddar
Cream
Cottage
Edam
Feta
Gouda
Parmesan
Stilton
0.8 Trace 2.2 Trace 3.6a
0.3 0.03 2.3 Trace 12
0.1 0.04 0.5 Trace 1.0
0.1 0.04 0.6 Trace 4.0
0.4 0.05 2.2 Trace 6.4a
0.2 0.07 0.9 Trace 5.0a
0.1 Trace 1.8 Trace 8.0a
1.1 0.33 5.3 0.1 11
0.3 0.18 2.5 Trace 11
Barclay et al. (1995). Data from Holland et al. (1995).
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Table 6 Recommended dietary intakes of selected trace elements Category
Infants Children Males
Females
Pregnant Lactating a b
Age (years)
0.0±0.5 0.5±1.0 1±3 4±6 7±10 11±14 15±18 19±24 25±50 50 11±14 15±18 19±24 25±50 50 18 19±50
Trace elements Iron (mg)a
Zinc (mg)a
Iodine (g)a
Selenium (g)a
Copper (mg)a
Manganese (mg)b
Fluoride (mg)b
Chromium (g)b
Molybdenum (g)a
0.27b 11 7 10 8±10 8 11 8 8 8 8 15 18 18 8 27 10 9
2b 3 3 5 5±8 8 11 11 11 11 8 9 8 8 8 11±13 14 12
110b 130b 90 90 90±120 120 150 150 150 150 120 150 150 150 150 220 290 290
15b 20b 20 30 30±40 40±55 55 55 55 55 55 55 55 55 55 60 70 70
200b 220b 340 440 440±700 700 890 900 900 900 700 890 900 900 900 1000 1300 1300
0.003 0.6 1.2 1.5 1.5±1.9 1.9 2.2 2.3 2.3 2.3 1.6 1.6 1.8 1.8 1.8 2.0 2.6 2.6
0.01 0.5 0.7 1.0 1.0±2.0 2.0±3.0 3.0 4.0 4.0 4.0 2.0±3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
0.2 5.5 11 15 15±25 25 35 35 35 30 21 24 25 25 20 29±30 44 45
2b 3b 17 22 22±34 34 43 45 45 45 34 43 45 45 45 50 50 50
US Recommended Daily Allowance; from Institute of Medicine (2000, 2001). US Adequate Intake values, from Institute of Medicine (1997, 2000, 2001).
The reason for the exceptionally high bioavailability of iron in human milk is not understood. It has been suggested that it may be related to the high concentration of lactoferrin (a glycoprotein in milk which can bind to ferric ions per molecule of protein) in human milk but evidence for a role for lactoferrin in iron absorption is con¯icting. Others have suggested that the apparent higher bioavailability of iron in human milk compared to cows' milk could be due to a number of possible factors, including a lower concentration of proteins, calcium and phosphorus (potential inhibitors of absorption) and higher concentrations of lactose and ascorbate (enhancers of iron absorption) in human milk. However, studies on suckling rats, which have a very high capacity for iron absorption, have shown that a very high and similar proportion (90%) of iron is absorbed from human milk, cows' milk and an iron-supplemented cows' milk-based formula, suggesting that there are no unabsorbable iron complexes in these milks. It is likely that the lower concentration of iron in human milk may be a contributory factor to the higher absorption ef®ciency of iron from human milk in infants, since iron absorption is subject to homeostatic control and small amounts are absorbed more ef®ciently than large amounts. Iron de®ciency is one of the most common nutritional de®ciences in infancy and childhood due to a rapid growth and marginal supply of iron in the diet. It has been suggested that the iron stores of
a breastfed infant 4±6 months of age may become compromised if they are not replenished from dietary sources, and dietary iron supplementation has been recommended at not later than 4 months of age for full-term infants and not later than 2 months of age for premature infants. Iron-forti®ed cows' milk-based formulae are effective in preventing iron de®ciency, which may be partly attributable to the fact that ascorbate is also added to formulae at levels which markedly enhance the absorption of added iron. Zinc
Zinc is essential for growth and development, sexual maturation and wound healing, and it may also be involved in the normal functioning of the immune system and other physiological processes. It is a component of the hormone insulin and aids in the action of a number of hormones involved in reproduction, as well as being required for the synthesis of DNA, RNA and proteins and as a cofactor for many enzymes involved in most major metabolic processes. Zinc de®ciency in humans was ®rst reported in the Middle East in the early 1960s, giving rise to dwar®sm, impaired sexual development and anaemia. Mild de®ciencies of zinc, although dif®cult to detect, have been shown to occur in Western countries, particularly in infants and young children, giving rise to low hair zinc levels, suboptimal growth, poor appetite and impaired taste acuity.
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
The recently revised RDAs for zinc in the United States are shown in Table 6. Dairy products such as milk, cheese and yoghurt are moderately good sources of zinc, and it has been estimated that milk and dairy products contribute between 19% and 31% of the total zinc intake in Western countries. A number of lines of evidence suggest that the bioavailability of zinc in human milk is greater than in cows' milk. Human milk (but not cows' milk) has a therapeutic value in the treatment of acrodermatitis enteropathica, a hereditary zinc malabsorption syndrome. The plasma zinc concentration of breastfed infants has been reported to be signi®cantly higher than that in infants fed a cows' milk-based formula containing 1.8 mg lÿ1 zinc. Studies on human adults using extrinsic labelling with 65Zn with whole-body counting showed that zinc absorption from human milk (41 9%) was signi®cantly greater than from cows' milk (28 15%) or cows' milk-based infant formula (31 7%). Short-term studies on human adults, pigs and suckling rats show that zinc is absorbed more rapidly from human milk than from cows' milk or cows' milk-based infant formulae. There are two main theories that have been proposed to explain the higher bioavailability of zinc from human milk compared with cows' milk: 1. Binding of a signi®cant fraction of the zinc in human milk (but not in cows' milk) to a low molecular weight zinc-binding ligand (e.g. citrate) may enhance zinc absorption. 2. Binding of a large fraction of zinc in cows' milk to casein (present at about 10 times its concentration in human milk), which may result in the entrapment of zinc in casein curds formed in the stomach and thus render a signi®cant proportion of the zinc unavailable for absorption. However, studies on suckling rats, which have a high capacity for zinc absorption, showed that a very high and similar proportion of zinc (85±95%) was absorbed from human milk, cows' milk and cows' milk-based infant formulae, suggesting that there are no unabsorbable forms of zinc in either human or cows' milk. It is likely that the lower concentration of zinc in human milk may be a contributory factor to the higher absorption ef®ciency of zinc from human milk by infants, since zinc absorption is subject to homeostatic control and small amounts are absorbed more ef®ciently than large amounts. Copper
Copper is an essential element for a wide range of animal species. It is required for iron utilization and is a cofactor for enzymes involved in the metabolism
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of glucose and synthesis of haemoglobin, connective tissue and phospholipids. Dietary de®ciency of copper is uncommon except in conditions of severe malnutrition. The recently established RDAs for copper in the United States are shown in Table 6. Milk and milk products are considered poor sources of copper, and cows' milk contributes little to the total dietary intake of copper. Copper de®ciency is rare in breastfed infants, but has been reported in formula-fed infants in some countries. Feeding infant formula containing 0.03 or 0.4 mg lÿ1 copper has been shown to yield normal parameters of copper status similar to those of breastfed infants. Knowledge of copper absorption from milks is very limited. In a study on suckling rats, copper absorption was 83% from human milk, 76% from cows' milk and 86±87% from cows' milk-based formulae, suggesting that there are no unabsorbable forms of copper in these milks. Manganese
Manganese is an essential element for every animal species studied. It is a speci®c cofactor for glycosyl transferases which are involved in mucopolysaccharide synthesis and a nonspeci®c cofactor for a wide variety of other enzymes. There are two known manganese metalloenzymes: pyruvate carboxylase (EC 6.4.1.1) and superoxide dismutase (EC 1.15.1.1). Manganese is widely distributed in foods, and dietary de®ciency is not known to occur in humans. The recently established Adequate Intake values for manganese in the United States are shown in Table 6. Cows' milk is a poor source of manganese and contributes little (1±3% in Western countries) to the total dietary intake of this mineral. Very little is known about the bioavailability of manganese in milks. One study reported that manganese absorption in healthy adults from human milk (8.2 2.9%) was signi®cantly higher than from cows' milk (2.4 1.7%), while absorption from cows' milk-based infant formulae was 1.7±5.9%. However, the absolute amount of manganese absorbed from cows' milk and formulae was greater than from human milk due to the higher manganese concentrations in these milks. Studies with suckling rats showed that absorption of manganese was not signi®cantly different from human milk (81%) than from cows' milk (89%) or cows' milk-based formulae (80%), suggesting that there are no unabsorbable forms of manganese in these milks.
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Selenium
Selenium is an essential component of the enzyme glutathione peroxidase which occurs in many human tissues where, together with vitamin E and the enzymes catalase (EC 1.11.1.6) and superoxide dismutase, it functions as an antioxidant, protecting cells against oxidative damage. In areas of China where the concentration of selenium in the soil is low, selenium de®ciency causes Keshan disease, a cardiomyopathy that affects primarily young children and women of childbearing years. Low selenium status has also been reported in New Zealand and Finland, countries where the concentration of selenium in the soil is also low. The recently revised RDAs for selenium in the United States are shown in Table 6. The contribution of dairy products to daily dietary intake of selenium has been estimated as: 5 mg (8% of the total intake) in the United Kingdom; 13 mg (10%) in the United States; 13 mg (21±26%) in Finland and 11 mg (39%) in New Zealand. Selenium intake of full-term infants fed on nonforti®ed, milk-based formula have been shown to be near or below the RDA, whereas infants in the United States fed breast milk have a selenium intake meeting or exceeding the US RDA. Moreover, some studies have reported that the selenium status of milk-based formula-fed infants was lower than breastfed infants. The ®nding that unsupplemented infant formula has a lower selenium concentration than breast milk has meant that selenium in now being added to some infants formulae. Iodine
Iodine is an essential component of the thyroid hormones thyroxine and triiodothyronine, which are important in controlling the rate of basal metabolism and in reproduction. Dietary de®ciency of iodine causes enlargement of the thyroid gland and goitre, while a large excess of iodine in the diet reduces the uptake of iodine by this gland and also produces signs of thyroid de®ciency (thyrotoxicosis). Worldwide, 1.6 billion people (30% of the world's population) are at risk of iodine de®ciency. Of these, some 655 million have goitre. While the highest prevalence of iodine de®ciency is in less-developed regions of the world, it persists in parts of the industrialized world, too, in countries such as Germany and Luxembourg. Iodine is the only trace element for which there has been any suggestion of excessive amounts in cows' milk. Excessive addition of EDDI and use of iodophors for disinfection purposes have certainly contributed to the increased iodine content of milk in past years. However, there is evidence of a decline
in the concentration of iodine in milk in the United States in recent years, although the situation in other countries is less clear. The recently revised RDAs for iodine in the United States are shown in Table 6. The contribution of milk and dairy products to the daily dietary intake of iodine has been estimated as: 37% of the total intake in the United Kingdom and only 6±7% in Germany. Molybdenum
Molybdenum is an essential component of several enzymes, including xanthine oxidase, aldehyde oxidase (EC 1.2.3.1) and sulphite oxidase (EC 1.8.3.1), where it occurs in the prosthetic group, molybopterin. It is not known whether the human requirement is for molybdenum per se or for molybopterin (or a precursor). Although molybdenum de®ciency has been reported in a patient on long-term total parenteral nutrition therapy, dietary de®ciency of molybdenum has not been observed in humans. The recently established RDAs for molybdenum in the United States are shown in Table 6. Milk may contribute substantially to the intake of molybdenum (36% of total molybdenum intake). Chromium, Cobalt, Fluoride, Arsenic, Nickel, Silicon, Boron
Chromium is regarded as an essential nutrient for humans, and the earliest detectable effect of de®ciency is an impairment of glucose tolerance. The only known function of cobalt in humans is its presence as an essential component of vitamin B12. Fluoride accumulates in the hard tissues of the body (bones and teeth), and although it is not strictly an essential element, it is regarded as bene®cial for humans because of its protective role against dental caries. The recently established Adequate Intake values for ¯uoride and chromium in the United States are shown in Table 6. As there is no evidence that the intake of cobalt is ever limiting in the human diet, no RDA is necessary. There is substantial evidence to establish the essentiality of arsenic, nickel, silicon and boron in animals and it is likely that these trace elements are also essential for humans. However, the nutritional functions of these elements are still unclear and there are no reliable data on which to base estimates of human requirements. Milk and dairy products do not contribute signi®cantly to the intake of ¯uoride, arsenic, silicon and boron. Milk may make a signi®cant contribution to the intake of chromium (21%) and nickel (11%).
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Conclusions
Further Reading
There is much less information on the nutritional aspects of some trace elements than others, and a considerable amount of current research is being carried out to clarify the roles of such minerals in nutrition. Reliable information on the content, and the principal factors affecting it, of at least some of the trace elements in milk is now available. However, for other trace elements, there is still a wide variation in reported values in the literature which is due, at least in part, to analytical dif®culties. The contribution of cows' milk and milk products to the diet in Western countries is signi®cant for zinc and iodine. While the chemical forms of the some of the trace elements (iron, zinc, copper, manganese) in milks are fairly well de®ned, the forms of many of the trace elements are unknown. Studies are required to characterize the chemical forms of a number of trace elements in milks. Iodine is believed to be almost totally absorbed from milks and infant formulae. The bioavailability of some other trace elements (e.g. iron, zinc) in human milk appears to be very high (generally higher than in cows' milk), but the reason for this remains unclear. There is little information on the bioavailability of copper, manganese, selenium, ¯uoride or other trace elements in milks, formulae and dairy products. Factors affecting the bioavailability of minerals are still poorly understood and the role of possible enhancers (e.g. lactose, ascorbate, citrate, phosphopeptides, lactoferrin) or inhibitors (e.g. proteins, calcium, phosphate) of minerals from milks remains unclear. However, milks do not appear to contain substances which are strongly inhibitory to mineral absorption, such as phytate or polyphenols in foods from plants. Finally, understanding the nutritional signi®cance of trace elements in milk and dairy products will also bene®t from the improvement in our knowledge of fundamental aspects of minerals, such as their nutritional roles, requirements and metabolism, and the quantitative relationship between dietary intakes and health.
Barclay MNI, MacPherson A and Dixon J (1995) Selenium content of a range of UK foods. Journal of Food Composition and Analysis 8: 307±318. Fairweather-Tait S and Hurrell RF (1996) Bioavailability of minerals and trace elements. Report of FLAIR Concerted Action No. 10 (Hurrell RF (ed.)). Nutrition Research Reviews 9: 295±324. Flynn A and Cashman K (1997) Nutritional aspects of minerals in bovine and human milks. In: Fox PF (ed.) Advanced Dairy Chemistry, vol. 3, pp. 257±301. London: Chapman & Hall. Holland B, Welch AA, Unwin ID et al. (eds.) (1995) McCance & Widdowson's The Composition of Foods, 5th edn. London: HMSO. Hurrell R (ed.) (1999) Mineral Forti®cation of Foods. Leatherhead: Leatherhead Food Research Association. Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press. Institute of Medicine (2000) Dietary Reference Intakes: Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press. Institute of Medicine (2001) Dietary Reference Intakes: Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press. Mertz W (ed.) (1986) Trace Elements in Human and Animal Nutrition, 5th edn. San Diego: Academic Press. National Research Council (1989) Recommended Dietary Allowances, 10th edn. Washington, DC: National Research Council. Renner E, Schaafsma G and Scott KJ (1989) Micronutrients in milk. In: Renner E (ed.) Micronutrients in Milk and Milk-Based Food Products, pp. 1±70. London: Elsevier Applied Science. Sadler MJ, Strain JJ and Caballero B (eds.) (1999) Encyclopedia of Human Nutrition, vols. 1±3. San Diego: Academic Press. SandstroÈm B and Walter P (eds.) (1998) Role of Trace Elements for Health Promotion and Disease Prevention. Basel, Switzerland: Karger. Scott KJ (1989) Micronutrients in milk products. In: Renner E (ed.) Micronutrients in Milk and Milk-Based Food Products, pp. 71±123. London: Elsevier Applied Science. Strain JJ and Cashman K (2002) Minerals and trace elements. In: Gibney M (ed.) Introductory Nutrition. London: Nutrition Society. Van Dokkum W (1995) The intake of selected minerals and trace elements in European countries. Nutrition Research Reviews 8: 271±302.
See also: Minerals in Dairy Products: Macroelements, Nutritional Significance.
Minerals in Feeds
see Feed Supplements: Macrominerals; Microminerals.
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
lactose, phosphopeptides) or inhibitors (e.g. proteins, calcium, phosphate) of macromineral absorption in milk and dairy products remains unclear. However, milk does not appear to contain substances which are strongly inhibitory to mineral absorption, such as phytate or polyphenols in foods from plants. Finally, understanding the nutritional signi®cance of macrominerals in milk and dairy products will also bene®t from the improvement in our knowledge of fundamental aspects of minerals, such as their nutritional roles, requirements and metabolism, and the quantitative relationship between dietary intakes and health. See also: Minerals in Dairy Products: Trace Elements, Nutritional Significance.
Further Reading Allen LH (1983) Calcium bioavailability and absorption: a review. American Journal of Clinical Nutrition 53: 783±808. Fairweather-Tait S and Hurrell RF (1996) Bioavailability of minerals and trace elements. Report of FLAIR Concerted Action no. 10 (Hurrell RF (ed.)). Nutrition Research Reviews 9: 295±324. Fleet JC and Cashman KD (2001) Magnesium. In: Bowman B and Russel R (eds.) Present Knowledge in Nutrition, 8th edn, pp. 292±301. Washington, DC: LSI Press. Flynn A and Cashman K (1997) Nutritional aspects of minerals in bovine and human milks. In: Fox PF (ed.) Advanced Dairy Chemistry, vol. 3, pp. 257±301. London: Chapman & Hall. Flynn A and Cashman K (1999) Calcium forti®cation of foods. In: Hurrell R (ed.) Mineral Forti®cation of Foods, pp. 18±53. Leatherhead: Leatherhead Food Research Association. Holland B, Welch AA, Unwin ID et al. (eds.) (1995) McCance & Widdowson's The Composition of Foods, 5th edn. London: HMSO. Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press. Miller DD (1989) Calcium in the diet: food sources, recommended intakes and nutritional bioavailability. Advances in Food and Nutrition Research 33: 103±156. National Research Council (1989) Recommended Dietary Allowances, 10th edn. Washington, DC: National Research Council. Renner E, Schaafsma G and Scott KJ (1989) Micronutrients in milk. In: Renner E (ed.) Micronutrients in Milk and Milk-Based Food Products, pp. 1±70. London: Elsevier Applied Science. Sadler MJ, Strain JJ and Caballero B (eds.) (1999) Encyclopedia of Human Nutrition, vols. 1±3. San Diego: Academic Press. Scott KJ (1989) Micronutrients in milk products. In: Renner E (ed.) Micronutrients in Milk and Milk-Based
Food Products, pp. 71±123. London: Elsevier Applied Science. Strain JJ and Cashman K (2002) Minerals and trace elements. In: Gibney M (ed.) Introductory Nutrition. London: Nutrition Society. Van Dokkum W (1995) The intake of selected minerals and trace elements in European countries. Nutrition Research Reviews 8: 271±302.
Trace Elements, Nutritional Signi®cance K D Cashman, University College, Cork, Ireland Copyright 2002, Elsevier Science Ltd. All Rights Reserved
Introduction There are 20 minerals that are considered to be nutritionally essential for humans and these are sometimes classi®ed into two groups, i.e. the macroelements (also known as the macrominerals) and trace elements. The macrominerals (sodium, potassium, chloride, calcium, magnesium and phosphorus) are present in the body in amounts greater than about 0.01% by weight and the nutritional aspects of these minerals are dealt with elsewhere (see Minerals in Dairy Products: Macroelements, Nutritional Signi®cance). The trace elements (the remaining 14 essential minerals) occur in the body at much lower levels and are required in the diet in amounts less than about 100 mg dayÿ1. The nutritional aspects of these minerals will be discussed in this article. The 14 trace elements that are considered essential in the human diet are iron, copper, zinc, manganese, selenium, iodine, chromium, cobalt, molybdenum, ¯uoride, arsenic, nickel, silicon and boron. While some of these, e.g. arsenic, nickel, silicon and boron, have not been shown to be essential for humans, they are essential for experimental animals and probably are also essential for humans. A number of other elements occur in milk, e.g. lithium, bromine, aluminium, strontium, silver, lead, tin, vanadium, mercury, cadmium, rubidium and caesium. These are not nutritionally essential and are not discussed here, but many of them are toxic. However, their concentrations in milk are normally well below toxic levels. This article outlines the nutritional roles, recommended intakes and hazards of de®ciency or excess of the 14 trace elements that are considered to
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
be nutritionally essential for humans, all of which occur in milk. In this article, `milk' refers to cows' milk unless otherwise stated. The content, chemical form, bioavailability and nutritional signi®cance of these trace elements in milk and dairy products are considered. In addition, in view of the widespread use of infant formulae based on cows' milk, some nutritional aspects of trace elements in these formulae are discussed. Although the trace elements are treated separately, it is important to realize that interactions of trace elements with each other, with macrominerals, with other constituents of milk, and with other food constituents occur, and that such interactions are assuming an increasing importance in nutrition.
Content and Chemical Form of Trace Elements in Milk and Dairy Products Mineral Content
The trace element content of milk is not constant but is in¯uenced by a number of factors such as stage of lactation, nutritional status of the animal, and environmental and genetic factors. Reported values in the literature for the concentration of many trace elements show a wide variation which is partly due to these factors, but also partly to analytical errors and contamination from milk collection and processing equipment and procedures. Representative values for the average trace element content of milk are presented in Table 1. Iron, zinc, copper Although several investigations have reported lack of a developmental pattern for iron in milk, at least one study has reported that iron Table 1 Mean concentrations of trace elements in cows' milk Trace element
Content (l ÿ1)
Iron (mg) Zinc (mg) Copper (mg) Manganese (mg) Iodine (mg) Fluorine (mg) Selenium (mg) Cobalt (mg) Chromium (mg) Molybdenum (mg) Nickel (mg) Arsenic (mg) Silicon (mg) Boron (mg)
0.5 3.9 0.09 30 100±770 20 10 0.5 2.0 50 26 20±60 3000 500±1000
Adapted from Flynn and Cashman (1997).
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concentration decreased by 35±50% during the ®rst 3 days of lactation and remained relatively constant thereafter. The iron content of milk is resistant to changes in dietary iron intake. Contact with metal containers can increase the concentration of iron in milk. Mean zinc concentration in milk is 3.9 mg lÿ1, but large variations in the zinc content of milk (2.0± 6.0 mg lÿ1) have been reported. There is a large decrease (50%) in the concentration of zinc in cows' colostrum during the ®rst 3 days of lactation, with little change thereafter. Dietary zinc supplementation increases the zinc concentration in milk only slightly. The concentration of copper in milk decreases by up to 50% during the ®rst 3 days of lactation; it can be increased by dietary copper supplementation or by contact with metal containers and processing equipment. Manganese, selenium, iodine The concentration of manganese is higher in colostrum (100±160 mg lÿ1) than in mature milk (20±50 mg lÿ1) and a decrease of over 50% has been reported to occur during the ®rst 3 days of lactation. Oral supplementation of manganese to cows can increase the manganese content of milk provided that large doses are administered over a long period. Mean selenium concentration in cows' milk samples from 15 countries was reported as 10 mg lÿ1 (range 3±40 mg lÿ1). The concentration in milk depends on dietary intake and in areas such as New Zealand and Finland, where the selenium content of soil and plants is low, concentrations as low as 3±5 mg lÿ1 have been reported. It has been shown that the selenium content of milk increased linearly from about 30 to 55 mg lÿ1 when dietary selenium was increased from about 2 to 6 mg dayÿ1. Summarized data from various countries on the iodine content of milk indicate mean values of 100± 770 mg lÿ1 and a wide range of individual values from 20 to >4000 mg lÿ1. The concentration of iodine in milk is in¯uenced by season and is closely related to dietary intake, and feeding winter rations containing mineral supplements results in considerable increases in milk iodine. High concentrations of iodine in milk have been related to the addition of excessive amounts of ethylenediamine dihydriodide (EDDI) to dairy cow rations. The use of iodophors for teat disinfection increases the iodine content in cows' milk. In addition, use of iodophors for the disinfection of containers, milking machines and processing equipment can also cause contamination of milk with iodine. The standardization of mineral feed supplements and supervised and restricted use of iodophor disinfectants have
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MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
been recommended as measures to reduce the iodine content of milk and milk products. Chromium, cobalt, molybdenum, fluoride The mean concentrations of chromium and cobalt in milk are 2 mg lÿ1 (range 0.2±3.6 mg lÿ1) and 0.5 mg lÿ1 (range 0.4±1.1 mg lÿ1), respectively. Dietary supplementation with cobalt increases the concentration of cobalt in milk but does not increase the vitamin B12 content of milk unless the diet is cobalt-de®cient. The concentration of molybdenum in milk has been shown to be dependent on dietary intake, increasing over ®vefold when cows were supplemented with ammonium molybdate. The mean ¯uoride content in milk is 20 mg lÿ1 (range 10±140 mg lÿ1). Arsenic, nickel, silicon, boron Reported concentrations of these elements in milk are presented in Table 1. Little is known about the in¯uence of stage of lactation, maternal nutritional status, environmental and genetic factors on the content of these trace elements in milk. Trace Element Content of Dairy Products
The contents of selected trace elements in other dairy products are shown in Tables 2±5.
Chemical Form of the Trace Elements
The chemical form of a trace element is important because it may in¯uence intestinal absorption and utilization (the process of transport, cellular assimilation and conversion into a biologically active form) and thus bioavailability. Iron, zinc, copper In milk, 14% of the iron occurs in milk fat where it is associated with the fat globule membrane. About 24% of the iron is bound to casein, probably to the phosphoserine residues of caseins, while 29% is bound to whey proteins and 32% is associated with a low molecular weight fraction. Most of the zinc in milk is in the skim milk fraction, with only 1±3% in the lipid fraction. Of the zinc in the skim milk fraction, over 95% is associated with the casein micelles, with a small proportion (5%) associated with a low molecular weight compound which has been identi®ed as citrate. Within the casein micelles, one-third of the zinc is loosely bound to casein phosphoserine residues and two-thirds is more tightly bound to colloidal calcium phosphate. The distribution of copper in milk has been reported as: 2% in the fat fraction, 8% bound to whey proteins, 44% to casein and 47% in a low molecular weight fraction.
Table 2 Mean concentrations of selected trace elements in concentrated milks Content (100 gÿ1)
Trace element
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium (mg) Iodine (mg)
Pasteurized skimmed
Dried skimmed
Evaporated (whole)
Condensed (whole)
0.05 Trace 0.4 Trace 1.0 15
0.27 Trace 4.0 Trace 11 150
0.26 0.02 0.9 Trace 3.0 11
0.23 Trace 1.0 Trace 3.0 74
Data from Holland et al. (1995).
Table 3 Mean concentrations of selected trace elements in creams Content (100 gÿ1)
Trace element Fresh cream
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium ( mg)
10% fat
20% fat
35±48% fat
60% fat
0.1 Trace 0.3 Trace Trace
0.1 Trace 0.5 Trace Trace
0.2 Trace 0.2 Trace Trace
0.1 0.09 0.2 Trace Trace
Data from Holland et al. (1995).
Soured 20% fat
Sterilized canned 25% fat
UHT 32% fat
0.4 Trace 0.5 Trace Trace
0.8 Trace 1.1 Trace Trace
1.0 Trace 0.4 Trace Trace
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
Manganese, selenium, iodine In cows' milk, manganese distribution is: 67% bound to caseins, 1% to the milk fat globule membrane, 14% to whey proteins and 18% to a low molecular weight fraction. About 12% of the selenium in milk has been estimated to be incorporated in the enzyme glutathione peroxidase (EC 1.11.1.9). Most (80±90%) of the iodine in milk is in the inorganic form, mainly as iodide and is in the watersoluble fraction, and 5±13% is bound to proteins through either covalent bonds or loose physical associations, with less than 0.1% bound to fat.
Nutritional Significance of Trace Elements in Milk and Dairy Products
Molybdenum, chromium, fluoride All the molybdenum in milk is considered to be associated with xanthine oxidase (EC 1.1.3.22). The chemical form of chromium in milk is unknown, although chromium in foods is generally in the trivalent state. About 46±64% of the ¯uoride in milk occurs as free ¯uoride ions with the remainder bound to proteins.
Iron
Cobalt, arsenic, nickel, silicon, boron Little is known about the chemical form or distribution of these trace elements in milk and dairy products. Table 4 Mean concentrations of selected trace elements in butter, yoghurt and dairy ice cream Content (100 gÿ1)
Trace element
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium (mg) Iodine (mg)
Butter
Yoghurt (whole milk)
Dairy ice cream
0.2 0.03 0.1 Trace Trace 38
0.1 Trace 0.7 Trace 2.0 63
0.1 0.02 0.3 Trace 1.5a Ðb
a
Barclay et al. (1995). Iodine is present in signi®cant amounts but there is no reliable information on the amount. Data from Holland et al. (1995).
b
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A simple and useful evaluation of the nutritional signi®cance of trace elements (as well as macrominerals) in milk can be obtained by comparing the amounts of the different elements provided by 1 litre of milk (Table 1) with the recommended daily intakes for these elements (Table 6). Great progress has been made in our knowledge of the trace element nutrition of infants during the last two decades, particularly in the area of bioavailability.
Iron is an essential trace element that acts as a catalytic centre for a broad spectrum of metabolic functions. Iron, as a component of haem in haemoglobin, myoglobin, cytochromes and other proteins, plays an essential role in the transport, storage and utilization of oxygen. It is also a cofactor for a number of enzymes. De®ciency of iron, resulting in anaemia, af¯icts about 30% of the world's population and occurs in both Western and developing countries. The recently revised recommended daily allowances (RDAs) for iron in the United States are shown in Table 6. Milk and milk products are very poor sources of iron, and milk contributes little to total iron intake. Bioavailability of iron to the infant from human milk has been reported to be in the range 49±70%. Considerably lower absorption ef®ciency of iron from cows' milk by human infants has been reported, usually about 10±34%. To compensate for the relatively low bioavailability of iron in cows' milk, infant formulae are often supplemented with iron. Iron absorption by infants from cows' milk formula containing 12 mg lÿ1 iron as ferrous sulphate is about 4±7%, but because of the much higher concentration of iron in such formulae, the absolute amount of iron absorbed is considerably greater than from human milk.
Table 5 Mean concentrations of selected trace elements in some cheese varieties Content (100 gÿ1)
Trace element
Iron (mg) Copper (mg) Zinc (mg) Manganese (mg) Selenium (mg) a
Brie
Cheddar
Cream
Cottage
Edam
Feta
Gouda
Parmesan
Stilton
0.8 Trace 2.2 Trace 3.6a
0.3 0.03 2.3 Trace 12
0.1 0.04 0.5 Trace 1.0
0.1 0.04 0.6 Trace 4.0
0.4 0.05 2.2 Trace 6.4a
0.2 0.07 0.9 Trace 5.0a
0.1 Trace 1.8 Trace 8.0a
1.1 0.33 5.3 0.1 11
0.3 0.18 2.5 Trace 11
Barclay et al. (1995). Data from Holland et al. (1995).
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MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
Table 6 Recommended dietary intakes of selected trace elements Category
Infants Children Males
Females
Pregnant Lactating a b
Age (years)
0.0±0.5 0.5±1.0 1±3 4±6 7±10 11±14 15±18 19±24 25±50 50 11±14 15±18 19±24 25±50 50 18 19±50
Trace elements Iron (mg)a
Zinc (mg)a
Iodine (g)a
Selenium (g)a
Copper (mg)a
Manganese (mg)b
Fluoride (mg)b
Chromium (g)b
Molybdenum (g)a
0.27b 11 7 10 8±10 8 11 8 8 8 8 15 18 18 8 27 10 9
2b 3 3 5 5±8 8 11 11 11 11 8 9 8 8 8 11±13 14 12
110b 130b 90 90 90±120 120 150 150 150 150 120 150 150 150 150 220 290 290
15b 20b 20 30 30±40 40±55 55 55 55 55 55 55 55 55 55 60 70 70
200b 220b 340 440 440±700 700 890 900 900 900 700 890 900 900 900 1000 1300 1300
0.003 0.6 1.2 1.5 1.5±1.9 1.9 2.2 2.3 2.3 2.3 1.6 1.6 1.8 1.8 1.8 2.0 2.6 2.6
0.01 0.5 0.7 1.0 1.0±2.0 2.0±3.0 3.0 4.0 4.0 4.0 2.0±3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
0.2 5.5 11 15 15±25 25 35 35 35 30 21 24 25 25 20 29±30 44 45
2b 3b 17 22 22±34 34 43 45 45 45 34 43 45 45 45 50 50 50
US Recommended Daily Allowance; from Institute of Medicine (2000, 2001). US Adequate Intake values, from Institute of Medicine (1997, 2000, 2001).
The reason for the exceptionally high bioavailability of iron in human milk is not understood. It has been suggested that it may be related to the high concentration of lactoferrin (a glycoprotein in milk which can bind to ferric ions per molecule of protein) in human milk but evidence for a role for lactoferrin in iron absorption is con¯icting. Others have suggested that the apparent higher bioavailability of iron in human milk compared to cows' milk could be due to a number of possible factors, including a lower concentration of proteins, calcium and phosphorus (potential inhibitors of absorption) and higher concentrations of lactose and ascorbate (enhancers of iron absorption) in human milk. However, studies on suckling rats, which have a very high capacity for iron absorption, have shown that a very high and similar proportion (90%) of iron is absorbed from human milk, cows' milk and an iron-supplemented cows' milk-based formula, suggesting that there are no unabsorbable iron complexes in these milks. It is likely that the lower concentration of iron in human milk may be a contributory factor to the higher absorption ef®ciency of iron from human milk in infants, since iron absorption is subject to homeostatic control and small amounts are absorbed more ef®ciently than large amounts. Iron de®ciency is one of the most common nutritional de®ciences in infancy and childhood due to a rapid growth and marginal supply of iron in the diet. It has been suggested that the iron stores of
a breastfed infant 4±6 months of age may become compromised if they are not replenished from dietary sources, and dietary iron supplementation has been recommended at not later than 4 months of age for full-term infants and not later than 2 months of age for premature infants. Iron-forti®ed cows' milk-based formulae are effective in preventing iron de®ciency, which may be partly attributable to the fact that ascorbate is also added to formulae at levels which markedly enhance the absorption of added iron. Zinc
Zinc is essential for growth and development, sexual maturation and wound healing, and it may also be involved in the normal functioning of the immune system and other physiological processes. It is a component of the hormone insulin and aids in the action of a number of hormones involved in reproduction, as well as being required for the synthesis of DNA, RNA and proteins and as a cofactor for many enzymes involved in most major metabolic processes. Zinc de®ciency in humans was ®rst reported in the Middle East in the early 1960s, giving rise to dwar®sm, impaired sexual development and anaemia. Mild de®ciencies of zinc, although dif®cult to detect, have been shown to occur in Western countries, particularly in infants and young children, giving rise to low hair zinc levels, suboptimal growth, poor appetite and impaired taste acuity.
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
The recently revised RDAs for zinc in the United States are shown in Table 6. Dairy products such as milk, cheese and yoghurt are moderately good sources of zinc, and it has been estimated that milk and dairy products contribute between 19% and 31% of the total zinc intake in Western countries. A number of lines of evidence suggest that the bioavailability of zinc in human milk is greater than in cows' milk. Human milk (but not cows' milk) has a therapeutic value in the treatment of acrodermatitis enteropathica, a hereditary zinc malabsorption syndrome. The plasma zinc concentration of breastfed infants has been reported to be signi®cantly higher than that in infants fed a cows' milk-based formula containing 1.8 mg lÿ1 zinc. Studies on human adults using extrinsic labelling with 65Zn with whole-body counting showed that zinc absorption from human milk (41 9%) was signi®cantly greater than from cows' milk (28 15%) or cows' milk-based infant formula (31 7%). Short-term studies on human adults, pigs and suckling rats show that zinc is absorbed more rapidly from human milk than from cows' milk or cows' milk-based infant formulae. There are two main theories that have been proposed to explain the higher bioavailability of zinc from human milk compared with cows' milk: 1. Binding of a signi®cant fraction of the zinc in human milk (but not in cows' milk) to a low molecular weight zinc-binding ligand (e.g. citrate) may enhance zinc absorption. 2. Binding of a large fraction of zinc in cows' milk to casein (present at about 10 times its concentration in human milk), which may result in the entrapment of zinc in casein curds formed in the stomach and thus render a signi®cant proportion of the zinc unavailable for absorption. However, studies on suckling rats, which have a high capacity for zinc absorption, showed that a very high and similar proportion of zinc (85±95%) was absorbed from human milk, cows' milk and cows' milk-based infant formulae, suggesting that there are no unabsorbable forms of zinc in either human or cows' milk. It is likely that the lower concentration of zinc in human milk may be a contributory factor to the higher absorption ef®ciency of zinc from human milk by infants, since zinc absorption is subject to homeostatic control and small amounts are absorbed more ef®ciently than large amounts. Copper
Copper is an essential element for a wide range of animal species. It is required for iron utilization and is a cofactor for enzymes involved in the metabolism
2063
of glucose and synthesis of haemoglobin, connective tissue and phospholipids. Dietary de®ciency of copper is uncommon except in conditions of severe malnutrition. The recently established RDAs for copper in the United States are shown in Table 6. Milk and milk products are considered poor sources of copper, and cows' milk contributes little to the total dietary intake of copper. Copper de®ciency is rare in breastfed infants, but has been reported in formula-fed infants in some countries. Feeding infant formula containing 0.03 or 0.4 mg lÿ1 copper has been shown to yield normal parameters of copper status similar to those of breastfed infants. Knowledge of copper absorption from milks is very limited. In a study on suckling rats, copper absorption was 83% from human milk, 76% from cows' milk and 86±87% from cows' milk-based formulae, suggesting that there are no unabsorbable forms of copper in these milks. Manganese
Manganese is an essential element for every animal species studied. It is a speci®c cofactor for glycosyl transferases which are involved in mucopolysaccharide synthesis and a nonspeci®c cofactor for a wide variety of other enzymes. There are two known manganese metalloenzymes: pyruvate carboxylase (EC 6.4.1.1) and superoxide dismutase (EC 1.15.1.1). Manganese is widely distributed in foods, and dietary de®ciency is not known to occur in humans. The recently established Adequate Intake values for manganese in the United States are shown in Table 6. Cows' milk is a poor source of manganese and contributes little (1±3% in Western countries) to the total dietary intake of this mineral. Very little is known about the bioavailability of manganese in milks. One study reported that manganese absorption in healthy adults from human milk (8.2 2.9%) was signi®cantly higher than from cows' milk (2.4 1.7%), while absorption from cows' milk-based infant formulae was 1.7±5.9%. However, the absolute amount of manganese absorbed from cows' milk and formulae was greater than from human milk due to the higher manganese concentrations in these milks. Studies with suckling rats showed that absorption of manganese was not signi®cantly different from human milk (81%) than from cows' milk (89%) or cows' milk-based formulae (80%), suggesting that there are no unabsorbable forms of manganese in these milks.
2064
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
Selenium
Selenium is an essential component of the enzyme glutathione peroxidase which occurs in many human tissues where, together with vitamin E and the enzymes catalase (EC 1.11.1.6) and superoxide dismutase, it functions as an antioxidant, protecting cells against oxidative damage. In areas of China where the concentration of selenium in the soil is low, selenium de®ciency causes Keshan disease, a cardiomyopathy that affects primarily young children and women of childbearing years. Low selenium status has also been reported in New Zealand and Finland, countries where the concentration of selenium in the soil is also low. The recently revised RDAs for selenium in the United States are shown in Table 6. The contribution of dairy products to daily dietary intake of selenium has been estimated as: 5 mg (8% of the total intake) in the United Kingdom; 13 mg (10%) in the United States; 13 mg (21±26%) in Finland and 11 mg (39%) in New Zealand. Selenium intake of full-term infants fed on nonforti®ed, milk-based formula have been shown to be near or below the RDA, whereas infants in the United States fed breast milk have a selenium intake meeting or exceeding the US RDA. Moreover, some studies have reported that the selenium status of milk-based formula-fed infants was lower than breastfed infants. The ®nding that unsupplemented infant formula has a lower selenium concentration than breast milk has meant that selenium in now being added to some infants formulae. Iodine
Iodine is an essential component of the thyroid hormones thyroxine and triiodothyronine, which are important in controlling the rate of basal metabolism and in reproduction. Dietary de®ciency of iodine causes enlargement of the thyroid gland and goitre, while a large excess of iodine in the diet reduces the uptake of iodine by this gland and also produces signs of thyroid de®ciency (thyrotoxicosis). Worldwide, 1.6 billion people (30% of the world's population) are at risk of iodine de®ciency. Of these, some 655 million have goitre. While the highest prevalence of iodine de®ciency is in less-developed regions of the world, it persists in parts of the industrialized world, too, in countries such as Germany and Luxembourg. Iodine is the only trace element for which there has been any suggestion of excessive amounts in cows' milk. Excessive addition of EDDI and use of iodophors for disinfection purposes have certainly contributed to the increased iodine content of milk in past years. However, there is evidence of a decline
in the concentration of iodine in milk in the United States in recent years, although the situation in other countries is less clear. The recently revised RDAs for iodine in the United States are shown in Table 6. The contribution of milk and dairy products to the daily dietary intake of iodine has been estimated as: 37% of the total intake in the United Kingdom and only 6±7% in Germany. Molybdenum
Molybdenum is an essential component of several enzymes, including xanthine oxidase, aldehyde oxidase (EC 1.2.3.1) and sulphite oxidase (EC 1.8.3.1), where it occurs in the prosthetic group, molybopterin. It is not known whether the human requirement is for molybdenum per se or for molybopterin (or a precursor). Although molybdenum de®ciency has been reported in a patient on long-term total parenteral nutrition therapy, dietary de®ciency of molybdenum has not been observed in humans. The recently established RDAs for molybdenum in the United States are shown in Table 6. Milk may contribute substantially to the intake of molybdenum (36% of total molybdenum intake). Chromium, Cobalt, Fluoride, Arsenic, Nickel, Silicon, Boron
Chromium is regarded as an essential nutrient for humans, and the earliest detectable effect of de®ciency is an impairment of glucose tolerance. The only known function of cobalt in humans is its presence as an essential component of vitamin B12. Fluoride accumulates in the hard tissues of the body (bones and teeth), and although it is not strictly an essential element, it is regarded as bene®cial for humans because of its protective role against dental caries. The recently established Adequate Intake values for ¯uoride and chromium in the United States are shown in Table 6. As there is no evidence that the intake of cobalt is ever limiting in the human diet, no RDA is necessary. There is substantial evidence to establish the essentiality of arsenic, nickel, silicon and boron in animals and it is likely that these trace elements are also essential for humans. However, the nutritional functions of these elements are still unclear and there are no reliable data on which to base estimates of human requirements. Milk and dairy products do not contribute signi®cantly to the intake of ¯uoride, arsenic, silicon and boron. Milk may make a signi®cant contribution to the intake of chromium (21%) and nickel (11%).
MINERALS IN DAIRY PRODUCTS/Trace Elements, Nutritional Signi®cance
2065
Conclusions
Further Reading
There is much less information on the nutritional aspects of some trace elements than others, and a considerable amount of current research is being carried out to clarify the roles of such minerals in nutrition. Reliable information on the content, and the principal factors affecting it, of at least some of the trace elements in milk is now available. However, for other trace elements, there is still a wide variation in reported values in the literature which is due, at least in part, to analytical dif®culties. The contribution of cows' milk and milk products to the diet in Western countries is signi®cant for zinc and iodine. While the chemical forms of the some of the trace elements (iron, zinc, copper, manganese) in milks are fairly well de®ned, the forms of many of the trace elements are unknown. Studies are required to characterize the chemical forms of a number of trace elements in milks. Iodine is believed to be almost totally absorbed from milks and infant formulae. The bioavailability of some other trace elements (e.g. iron, zinc) in human milk appears to be very high (generally higher than in cows' milk), but the reason for this remains unclear. There is little information on the bioavailability of copper, manganese, selenium, ¯uoride or other trace elements in milks, formulae and dairy products. Factors affecting the bioavailability of minerals are still poorly understood and the role of possible enhancers (e.g. lactose, ascorbate, citrate, phosphopeptides, lactoferrin) or inhibitors (e.g. proteins, calcium, phosphate) of minerals from milks remains unclear. However, milks do not appear to contain substances which are strongly inhibitory to mineral absorption, such as phytate or polyphenols in foods from plants. Finally, understanding the nutritional signi®cance of trace elements in milk and dairy products will also bene®t from the improvement in our knowledge of fundamental aspects of minerals, such as their nutritional roles, requirements and metabolism, and the quantitative relationship between dietary intakes and health.
Barclay MNI, MacPherson A and Dixon J (1995) Selenium content of a range of UK foods. Journal of Food Composition and Analysis 8: 307±318. Fairweather-Tait S and Hurrell RF (1996) Bioavailability of minerals and trace elements. Report of FLAIR Concerted Action No. 10 (Hurrell RF (ed.)). Nutrition Research Reviews 9: 295±324. Flynn A and Cashman K (1997) Nutritional aspects of minerals in bovine and human milks. In: Fox PF (ed.) Advanced Dairy Chemistry, vol. 3, pp. 257±301. London: Chapman & Hall. Holland B, Welch AA, Unwin ID et al. (eds.) (1995) McCance & Widdowson's The Composition of Foods, 5th edn. London: HMSO. Hurrell R (ed.) (1999) Mineral Forti®cation of Foods. Leatherhead: Leatherhead Food Research Association. Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press. Institute of Medicine (2000) Dietary Reference Intakes: Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press. Institute of Medicine (2001) Dietary Reference Intakes: Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press. Mertz W (ed.) (1986) Trace Elements in Human and Animal Nutrition, 5th edn. San Diego: Academic Press. National Research Council (1989) Recommended Dietary Allowances, 10th edn. Washington, DC: National Research Council. Renner E, Schaafsma G and Scott KJ (1989) Micronutrients in milk. In: Renner E (ed.) Micronutrients in Milk and Milk-Based Food Products, pp. 1±70. London: Elsevier Applied Science. Sadler MJ, Strain JJ and Caballero B (eds.) (1999) Encyclopedia of Human Nutrition, vols. 1±3. San Diego: Academic Press. SandstroÈm B and Walter P (eds.) (1998) Role of Trace Elements for Health Promotion and Disease Prevention. Basel, Switzerland: Karger. Scott KJ (1989) Micronutrients in milk products. In: Renner E (ed.) Micronutrients in Milk and Milk-Based Food Products, pp. 71±123. London: Elsevier Applied Science. Strain JJ and Cashman K (2002) Minerals and trace elements. In: Gibney M (ed.) Introductory Nutrition. London: Nutrition Society. Van Dokkum W (1995) The intake of selected minerals and trace elements in European countries. Nutrition Research Reviews 8: 271±302.
See also: Minerals in Dairy Products: Macroelements, Nutritional Significance.
Minerals in Feeds
see Feed Supplements: Macrominerals; Microminerals.