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Milk Salts | Macroelements, Nutritional Significance
Macroelements, Nutritional Significance K D Cashman, University College, Cork Ireland ª 2011 Elsevier Ltd. All rights reserved.
Introduction In this article the term ‘minerals’ refers to those mineral elements that are of nutritional significance and, though the term may be chemically inaccurate, it is a commonly used and widely accepted terminology in the field of nutrition. Mineral elements occur in the body in a number of chemical forms, such as inorganic ions and salts, and constituents of organic molecules such as proteins, fats, carbohydrates, and nucleic acids. They serve a wide variety of essential physiological functions, ranging from structural components of body tissues to essential components of many enzymes and other biologically important molecules. This article, in conjunction with another article within this Encyclopedia (see Milk Salts: Trace Elements, Nutritional Significance), outlines the nutritional roles, recommended intakes, and hazards of deficiency or excess of the 20 minerals that are considered to be nutritionally essential for humans, all of which occur in milk. In this article, ‘milk’ refers to cow’s milk unless otherwise stated. The content, chemical form, bioavailability, and nutritional significance of these minerals in milk and dairy products are discussed. The 20 minerals considered essential in the human diet are sodium, potassium, chloride, calcium, magnesium, phosphorus, iron, copper, zinc, manganese, selenium, iodine, chromium, cobalt, molybdenum, fluoride, arsenic, nickel, silicon, and boron. The essential minerals are sometimes classified into two groups: the macroelements (also known as the macrominerals) and the trace elements. The macrominerals (sodium, potassium, chloride, calcium, magnesium, and phosphorus) are present in the body in amounts larger than 0.01% by weight, whereas the trace elements (the remaining 14 essential minerals) occur in the body at much lower levels and are required in the diet in amounts smaller than 100 mg day 1. The nutritional aspects of the former class of minerals will be discussed in this article, whereas the nutritional aspects of the latter class will be dealt with elsewhere (see Milk Salts: Trace Elements, Nutritional Significance). Although the minerals are treated separately, it is important to realize that interactions of the minerals with each other, 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 Macrominerals in Milk and Dairy Products Mineral Content The mineral content of milk is not constant but is influenced 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 minerals show a wide variation, which is due partly to these factors, but also partly to analytical errors and contamination during milk collection and from processing equipment and procedures. Representative values for the average macromineral content of milk are presented in Table 1.
Sodium, Potassium, Chloride Particularly high levels of sodium occur in cow’s colostrum, but the level decreases within a few days to the value shown in Table 1. The sodium content of milk is not influenced by dietary sodium intake within the normal range. Sodium concentration in milk tends to be higher at the end of lactation when milk yield is low. Removal of milk fat has little effect on milk sodium content. In contrast to most other minerals, potassium concentration in cow’s colostrum is lower than that in mature milk, but increases to normal values within the first 2–3 days of lactation and is independent of potassium intake. The chloride concentration in milk decreases from higher levels in colostrum to lower levels in mature milk but increases sharply toward the end of lactation and is independent of dietary intake. The sodium, potassium, and chloride contents of other dairy products are shown in Tables 2–5. Levels of sodium (and also chloride) in cheese are dependent on the amount of added salt. Levels of potassium are no higher in cheese than in milk. In general, the concentration of macrominerals decreases as the fat concentration increases in milk and dairy products (Table 3).
Calcium, Phosphorus, Magnesium The mean calcium and phosphorus contents of cow’s milk are 1120 and 890 mg l 1, respectively. In cow’s milk, calcium concentration is slightly elevated in colostrum and at the end of lactation but varies little with feeding or
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The calcium, phosphorus, and magnesium contents of other dairy products are shown in Tables 2–5. Levels of calcium and phosphorus are generally highest in hard cheeses (Parmigiano, Gouda, Edam, and Cheddar), that is, up to 10 times that in milk, followed by mold cheeses (Brie, Stilton), that is, 4–5 times that in milk (Table 5). The lowest levels are found in cream and cottage cheese types. Levels of magnesium are ordered similar to calcium but only around 5 times higher in hard cheeses and 2–3 times higher in mold types.
Table 1 Mean concentrations of macrominerals in cow’s milk
Mineral
Content (mg 1 1)
Sodium Potassium Chloride Calcium Phosphorus Magnesium
430 1550 890 1180 930 110
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
Chemical Form of the Macrominerals season. There is little variation in the phosphorus content of milk throughout lactation. The mean concentration of magnesium in milk is 110 mg l 1 and is unaffected by fat removal. Magnesium concentration in colostrum is 2–3 times that in mature milk and decreases to the level in mature milk within the first 1–3 days of lactation, remaining relatively constant thereafter. The concentration of magnesium in milk is unaffected by dietary intake of magnesium.
The chemical form of a mineral is important because it may influence intestinal absorption and utilization (the process of transport, cellular assimilation, and conversion into a biologically active form) and thus bioavailability. Sodium, Potassium, Chloride Sodium, potassium, and chloride are believed to be present in milk almost entirely as free ions. Practically all the sodium, potassium, and chloride in milk is absorbed in the
Table 2 Mean concentrations of macrominerals in concentrated milks Content (mg 100 g 1) Mineral
Pasteurized skimmed
Dried skimmed
Evaporate (whole)
Condensed (whole)
Sodium Potassium Chloride Calcium Phosphorus Magnesium
43 155 89 118 93 11
550 1590 1070 1280 970 130
180 360 250 290 260 29
140 360 230 290 240 29
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
Table 3 Mean concentrations of macrominerals in creams Content (mg 100 g 1) Fresh cream
Soured
Sterilized canned
UHT
Mineral
10% fat
20% fat
35–48% fat
60% fat
20% fat
25% fat
32% fat
Sodium Potassium Chloride Calcium Phosphorus Magnesium
49 120 77 99 89 11
29 104 80 80 79 8
25 86 59 58 59 6
18 55 40 37 40 5
41 110 81 93 81 10
53 110 78 86 73 10
31 107 66 54 57 7
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
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Table 4 Mean concentrations of macrominerals in butter, yogurt, and dairy ice cream Content (mg 100 g 1) Mineral
Butter (whole milk)
Yogurt
Dairy ice cream
Sodium Potassium Chloride Calcium Phosphorus Magnesium
606a 27 994a 18 23 2
80 280 170 200 170 19
60 174 110 100 91 12
a
Unsalted butter contains 9 mg 100 g 1 sodium and 19 mg 100 g 1 chloride. Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
Table 5 Mean concentrations of macrominerals in some cheese varieties Content (mg 100 g 1) Mineral
Brie
Cheddar
Cream
Cottage
Edam
Feta
Gouda
Parmesan
Stilton
Sodium Potassium Chloride Calcium Phosphorus Magnesium
556 91 900 256 232 15
723 75 1040 739 505 29
300 160 480 98 100 10
300 161 490 127 171 13
996 89 1570 795 508 34
1440 95 2350 360 280 20
925 82 1440 773 498 32
756 152 1260 1025 680 41
788 96 1230 326 314 15
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
gastrointestinal tract, although much of what is absorbed is not retained.
Calcium, Phosphorus, Magnesium In cow’s milk, 99% of the calcium is in the skim milk fraction, which explains why the calcium content is not affected by fat removal. Two-thirds of the total calcium occurs in colloidal form associated with the casein micelles, either as a calcium phosphate salt (about half of total milk calcium) or as calcium ions bound to phosphoserine residues (about one-sixth of total calcium); the remaining one-third is soluble. Ionized calcium in the soluble phase accounts for 10% of the total calcium, and most of the remaining soluble calcium occurs as calcium citrate. A small amount of calcium (0.15%) is bound to -lactalbumin. Of the total phosphorus in cow’s milk, 20% occurs as organic phosphate esterified to casein with the remainder occurring as inorganic phosphate. About 44% of the inorganic phosphate is associated with casein micelles as calcium phosphate and 56% is soluble, mainly as free phosphate ions. In cow’s milk, 98–100% of the magnesium is in the skim milk phase, where 65% of the magnesium is in a soluble form (40% as magnesium citrate, 7% as
magnesium phosphate, and 16% as free magnesium ion), and the remainder is colloidal and is associated with the casein micelles (about one-half associated with colloidal calcium phosphate and the other half bound directly to phosphoserine residues in caseins).
Nutritional Significance of Macrominerals in Milk and Dairy Products A simple and useful evaluation of the nutritional significance of macrominerals (as well as trace elements) in milk can be obtained by comparing the amounts of the different elements provided by 1 l of milk (Table 1) with the recommended daily intakes for these elements (Table 6). Another useful impression of the nutritional significance of milk and other dairy products can be obtained by computing, from data on dietary surveys, the actual contribution made by dairy products to the total intake of macrominerals and trace elements. Sodium, Potassium, Chloride Sodium is the principal cation in extracellular fluids and is the primary regulator of extracellular fluid volume. It is
928 Milk Salts | Macroelements, Nutritional Significance Table 6 Recommended dietary intakes of selected macrominerals Minerals
Category
Age (years)
Infants
0.0–0.5 0.5–1.0
Children
Calciuma (mg)
Magnesiumb (mg)
Phosphorusb (mg)
210 270
30a 75a
100a 275a
1–3 4–8 9–13
500 800 1300
80 130 240
460 500 1250
Males
14–18 19–30 31–50 51–70 >70
1300 1000 1000 1200 1200
410 400 420 420 420
1250 700 700 700 700
Females
14–18 19–30 31–50 51–70 >70
1300 1000 1000 1200 1200
360 310 320 320 320
1250 700 700 700 700
Pregnancy
18 19–30 31–50
1300 1000 1000
400 350 360
1250 700 700
Lactation
18 19–30 31–50
1300 1000 1000
360 310 320
1250 700 700
a US adequate intake values; from Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press. b US recommended dietary allowance (RDA); from Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press.
important in the regulation of osmolarity, acid–base balance, and the membrane potential of cells, as well as in active transport across cell membranes. Chloride is the principal extracellular anion and is essential in the maintenance of fluid and electrolyte balance. Potassium is the principal intracellular cation, occurring in the cell at a concentration more than 30 times its concentration in extracellular fluid. Extracellular potassium contributes to the transmission of nerve impulses, to the control of skeletal muscle contraction, and to the maintenance of blood pressure. Under normal circumstances, dietary deficiency of sodium, potassium, or chloride does not occur, but the body can be depleted of sodium and chloride under extreme conditions, for example, heavy perspiration, chronic diarrhea, or renal disease. Depletion of potassium can occur in the body under conditions where there are large alimentary or renal losses. Minimum requirements for sodium, potassium, and chloride (e.g., 500, 2000, and 750 mg day 1, respectively, in adults) have been established. Actual intakes of sodium and chloride are considerably higher than the minimum requirements in many populations and, given the evidence of a relationship of high salt intake to hypertension, it has been recommended that sodium
intake be limited to 2.4 g day 1. Similarly, considering the possible beneficial effect of potassium in hypertension, increasing potassium intake beyond the minimum requirement has been recommended. Cow’s milk contributes little to the dietary intake of sodium (7% in the United Kingdom), but some dairy products such as cheese and butter contain added salt and can be significant sources of sodium in some countries (e.g., 13% of total sodium intake in the United Kingdom). It has been estimated that milk and dairy products provide 20% of total sodium and 24–29% of total potassium in the diet in Ireland and the United Kingdom. The concentrations of sodium, potassium, and chloride in milk are of physiological importance in the feeding of the young infant, and clinical problems may arise if there is an excessive intake of these nutrients. The kidney of the young infant, as compared to that of the adult, has a limited capacity to concentrate solids, and the renal solute load exerts a major effect on water balance. Renal solute load is determined mainly by sodium, potassium, chloride, phosphorus, and protein (which gives rise to urea). Cow’s milk has a much higher potential renal solute load (300 mosmol l 1) than human milk (93 mosmol l 1). The high renal solute
Milk Salts | Macroelements, Nutritional Significance
load resulting from ingestion of cow’s milk may be of relatively little significance in most circumstances because the kidney merely excretes a more concentrated urine. However, it does lead to a smaller margin of safety against dehydration, which can occur in conditions of diarrhea, fever, and low water intake, and for this reason it is recommended that the upper limit of potential renal solute load in formulae for young infants be 220 mosmol l 1. Recommended concentrations of sodium, potassium, and chloride in infant formulae are in the ranges 133–403, 534–1338, and 369–1360 mg l 1, respectively. Cow’s milk-based infant formulae currently in use fall comfortably within these guidelines. Calcium The adult human body contains 1200 g of calcium, which amounts to 1.5–2% of the body weight. Of this, 99% is found in bones and teeth, where it is present as calcium phosphate, providing strength and structure. The remaining 1%, found in extracellular fluids and intracellular structures and cell membranes, is responsible for a number of regulatory functions such as maintenance of normal heart beat, blood coagulation, hormone secretion, integrity of intracellular cement substances and membranes, nerve conduction, muscle contraction, and activation of enzymes. Cow’s milk and milk products, such as cheese and yogurt, are very good sources of dietary calcium. The contribution of dairy products to total calcium intake has been estimated as 75% in The Netherlands, 55% (dietary intake data) to 72% (food disappearance data) in the United States, 60% in the United Kingdom, and 52% in Ireland. In the absence of milk and dairy products from the diet, calcium intakes in excess of 300 mg day 1 are difficult to achieve. This is far below the typical recommended intakes for calcium. For example, the recently established adequate intake (AI) values for calcium in the United States are 1000 mg for adults, 1200 mg for older adults, and 1300 mg for adolescents (Table 6). On this basis, it might be considered that consumption of dairy products is very important in order to achieve an adequate calcium intake. However, there is still considerable disagreement on human calcium requirements, and this is reflected in the wide variation in the recommended adult dietary intakes for calcium (400–1200 mg) that have been set by different authorities. The dietary reference value for calcium is currently under review in the United States, and the new recommendations will be released in Summer 2010. In recent years, considerable attention has been focused on the bioavailability of calcium in milk. Evidence from studies on experimental animals suggests that essentially all of the calcium in human milk, cow’s
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milk, and cow’s milk-based infant formulae is potentially available for absorption in the gastrointestinal tract. In addition, it has often been suggested that the bioavailability of calcium is higher in human milk than in cow’s milk or cow’s milk-based infant formulae. This arises from the findings that calcium is absorbed more efficiently by human infants from human milk than from cow’s milk or a cow’s milk-based formula containing 738 mg Ca l 1. However, the absolute amount of calcium absorbed is higher from cow’s milk and formulae, and calcium absorption from cow’s milk formulae with lower calcium contents (363–458 mg l 1) is similar to that from human milk. Similarly, the efficiency of calcium absorption from human milk by weanling Rhesus monkeys has been reported as being significantly higher (72%) than from cow’s milk-based infant formulae (45–53%), but the absolute absorption from human milk and formulae were similar due to the higher calcium concentration in the formulae. Moreover, bone mineralization in infants fed with cow’s milk formula has been reported to be as good as, if not slightly better than, in infants fed with human milk. Mean calcium absorption from cow’s milk by healthy human adults has been variably reported in the range 21–45%. Calcium absorption has been reported as being similar from milk and from CaCO3 by pregnant women at 36–47%, but calcium absorption from cow’s milk by postmenopausal women has been variably reported in the range 5–41%. Calcium absorption from cow’s milk by -galactosidase-deficient subjects has been reported as being higher (36.2% of dose) as compared to a -galactosidase-sufficient group (25.7%), and it has been suggested that this reflects lower habitual calcium intake owing to reduced milk consumption by the -galactosidase-deficient subjects. It has been reported that calcium from various dairy products (including whole milk, chocolate milk, yogurt, Cheddar cheese, processed cheese) is absorbed equally well. Thus, though it appears that all of the calcium in human milk, cow’s milk, and cow’s milk-based infant formulae is potentially available for absorption, the absolute amount of calcium absorbed by animals or humans is determined by physiological factors such as the efficiency of calcium absorption mechanisms in the gastrointestinal tract, which may be influenced by calcium needs, vitamin D status, and age, as well as by the calcium concentration in milk. In addition, some components of milk (lactose, phosphopeptides) may enhance calcium absorption. There is strong evidence that lactose promotes intestinal absorption (particularly in the ileum) and body retention of calcium in rats. This effect of lactose is independent of vitamin D, but the mechanism by which it occurs remains unresolved. It has been suggested that undigested lactose reaching the ileum
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interacts with the brush border membrane, increasing its permeability to calcium. Lactose also appears to increase calcium absorption in human infants. For example, calcium absorption has been reported to be significantly higher from a soy-based infant formula containing lactose (48%) than from a similar formula in which the carbohydrate source was a mixture of starch hydrolysate and sucrose (33%). However, studies on the effect of lactose on calcium absorption by human adults have produced conflicting results. One study found no significant difference in calcium absorption from milk or lactose-hydrolyzed milk by either -galactosidasedeficient or -galactosidase-sufficient subjects, whereas another study reported that calcium absorption was similar from milk (21.4%) and lactose-free milk (lactose replaced by glucose) (26.8%) by healthy adult subjects, but lactose increased calcium absorption by -galactosidase-deficient subjects. Overall, it is likely that lactose, at the level normally present in milk, does not have a significant effect on calcium absorption in healthy adults consuming normal diets. It is possible that lactose affects the non-saturable paracellular pathway for calcium absorption in the gut and its effect is most likely to occur in vitamin D deficiency, or when elevated levels of calcium are fed. It has been suggested that phosphopeptides formed during the digestion of bovine caseins may be involved in promoting calcium absorption. Such phosphopeptides have the capacity to chelate calcium and to prevent the precipitation of calcium phosphate salts and may help to maintain a high concentration of soluble calcium in the intestinal lumen. There is evidence that phosphopeptides are present in the lumen of the small intestine of rats and pigs after a casein-containing meal; purified phosphopeptides have been shown to enhance the absorption of calcium in rats and chicks. The observation that calcium absorption in rats is enhanced by high-casein meals is consistent with this. However, the nutritional significance of these phosphopeptides in humans consuming milk remains unclear. Age-related osteoporosis, a common bone disease and a major cause of disability in Western countries, is characterized by reduced bone density resulting in increased bone fragility and susceptibility to fracture. The condition is particularly common in elderly women, especially in Caucasians. It has been estimated that osteoporosis afflicts 10 million Americans (half of the women over 45 years of age and 90% of the women over 75 years of age). Osteoporosis is a multifactorial disorder, but there is increasing evidence that inadequate calcium intake, particularly during early life, is a contributory factor. Adequate calcium intake is required in early life in order to develop maximum bone mass at maturity (during the third decade of life), and there is evidence that the
amount of bone mass present at maturity is an important factor influencing fracture susceptibility in the elderly. Peak bone mass at maturity appears to be related to the intake of calcium during the years of bone mineralization. Inadequate dietary intake of calcium during the critical growth and building periods may result in failure to reach peak bone mass, causing osteopenia, osteoporosis, decreased skeletal integrity, and increased risk of fracture in later life. Many believe that the most promising nutritional approach to reduce the risk of osteoporosis in later life is to ensure a calcium intake that allows the development of each individual’s genetically programmed peak bone mass during the formative years, that is, throughout childhood to the age of 25 years. In this regard, the prevalence of lower-than-recommended calcium intake among adolescent and young adult females in many countries, for example, in the United States and Europe, is of particular concern. In recent years, evidence has been presented for a protective role of dietary calcium against hypertension, hypercholesterolemia, diabetes mellitus, and colon and rectal cancer. Dietary calcium has also been proposed to help regulate and maintain body weight. However, consensus on the role of calcium in these conditions is lacking and further research is required. Phosphorus Phosphorus is an essential nutrient for humans and serves a number of important biological functions. Phosphorus occurs as organic and inorganic phosphates in all body tissues and fluids; is an essential component of many biological molecules, including lipids, proteins, carbohydrates, and nucleic acids; and plays a central role in metabolism. As calcium phosphate, it is a major structural component of bones and teeth. Because almost all foods contain phosphorus, dietary phosphorus deficiency does not usually occur. The recently revised recommended dietary allowances (RDAs) for phosphorus are shown in Table 6. Cow’s milk and milk products, such as cheese and yogurt, are good dietary sources of phosphorus, and the contribution of milk and dairy products to total phosphorus intake has been reported as 30–45% in Western countries. In the first few weeks of life, the infant’s ability to regulate plasma calcium concentration is not fully developed, and hypocalcemia may result in neonatal tetany, which used to occur more frequently in artificially fed than in breast-fed infants. Excessive phosphorus intake contributes to this condition, and feeding unmodified cow’s milk, which has a high content of phosphorus, increases serum phosphorus and lowers serum (ionized) calcium in the newborn infant. For this reason, it is recommended that the calcium-to-phosphorus ratio in artificial
Milk Salts | Macroelements, Nutritional Significance
infant formulae be higher than that in cow’s milk (1.2:1) and more similar to that in human milk (2.2:1). In experimental animals, a high intake of phosphorus or a low calcium-to-phosphorus ratio in the diet could lead to bone loss. However, it is generally agreed that wide variations in phosphorus intake or in the calciumto-phosphorus ratio in the diet do not adversely affect bone mass in adult humans. Magnesium Magnesium has an essential role in a wide variety of physiological processes, including protein and nucleic acid metabolism, neuromuscular transmission and muscle contraction, bone growth and metabolism, and regulation of blood pressure, and it acts as a cofactor for many enzymes. There is little information from human studies on the bioavailability of magnesium from milk. Metabolicbalance studies in infants showed that 16–43% of magnesium is absorbed from cow’s milk-based infant formulae and that lactose enhances the absorption of magnesium. Cow’s milk and milk products contribute 16–21% of the total magnesium intake in Western countries. Dietary deficiency of magnesium is uncommon except in conditions of severe malnutrition and certain disease states. There is evidence that many young adults, especially women, in various European countries and in the United States fail to achieve the recommended intakes of magnesium and this raises the issue of possible adverse effects of lower-than-recommended intakes. Scientists have attempted to demonstrate that suboptimal intake of magnesium (i.e., below the RDA, but not frank deficiency) is a contributor to the development of chronic maladies such as cardiovascular disease, hypertension, disorders of skeletal growth and osteoporosis, and diabetes mellitus. However, the results of studies in this area are ambiguous. The fact that the RDA for magnesium has been raised recently for most groups (Table 6) reflects nutrition scientists’ belief that there is a negative consequence to suboptimal magnesium intake. Additional research is needed to fully justify this concern.
Conclusion Even though the nutritional roles, requirements, and metabolism, and the quantitative relationship between dietary intakes and health for a number of macrominerals have been more clearly defined in recent years, there are still considerable deficiencies in our understanding of these issues, for example, the significance of calcium and magnesium in the etiology and treatment of osteoporosis and hypertension.
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Reliable information on the content, and the principal factors affecting it, of most of the macrominerals in cow’s milk is now available. The contribution of cow’s milk and milk products to the diet in Western countries is significant for sodium, potassium, chloride, calcium, and phosphorus. Sodium, potassium, and chloride are believed to be almost totally absorbed from milk and infant formulae. Though the bioavailability of calcium in milk has received considerable attention in recent years, there is little information on the bioavailability of magnesium in milk and infant formulae. In addition, there is a paucity of data regarding the bioavailability of macrominerals from other dairy products. Factors affecting the bioavailability of minerals are still poorly understood, and the role of possible enhancers (e.g., 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 that are strongly inhibitory to mineral absorption, such as phytate or polyphenols contained in foods from plants. Finally, understanding the nutritional significance of macrominerals in milk and dairy products will also benefit 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: Milk Salts: Trace Elements, Nutritional Significance.
Further Reading Allen LH (1983) Calcium bioavailability and absorption: A review. The American Journal of Clinical Nutrition 53: 783–808. Fairweather-Tait S and Hurrell RF (1996) Bioavailability of minerals and trace elements. Nutrition Research Reviews 9: 295–324. Fleet JC and Cashman KD (2001) Magnesium. In: Bowman B and Russel R (ed.) 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 fortification of foods. In: Hurrell R (ed.) Mineral Fortification of Foods, pp. 18–53. Leatherhead: Leatherhead Food Research Association. Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry. 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.
932 Milk Salts | Macroelements, Nutritional Significance Sadler MJ, Strain JJ, and Caballero B (eds.) (1999) Encyclopedia of Human Nutrition, Vols. 1–3. San Diego, CA: 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 KD (2009) Minerals and trace elements. In: Gibney M, Lanham-New SA, Cassidy A, Vorster HH, et al. (eds.) Introduction to Human Nutrition, 2nd edn. London: Nutrition Society. Van Dokkum W (1995) The intake of selected minerals and trace elements in European countries. Nutrition Research Reviews 8: 271–302.
Introduction In this article the term ‘minerals’ refers to those mineral elements that are of nutritional significance and, though the term may be chemically inaccurate, it is a commonly used and widely accepted terminology in the field of nutrition. Mineral elements occur in the body in a number of chemical forms, such as inorganic ions and salts, and constituents of organic molecules such as proteins, fats, carbohydrates, and nucleic acids. They serve a wide variety of essential physiological functions, ranging from structural components of body tissues to essential components of many enzymes and other biologically important molecules. This article, in conjunction with another article within this Encyclopedia (see Milk Salts: Trace Elements, Nutritional Significance), outlines the nutritional roles, recommended intakes, and hazards of deficiency or excess of the 20 minerals that are considered to be nutritionally essential for humans, all of which occur in milk. In this article, ‘milk’ refers to cow’s milk unless otherwise stated. The content, chemical form, bioavailability, and nutritional significance of these minerals in milk and dairy products are discussed. The 20 minerals considered essential in the human diet are sodium, potassium, chloride, calcium, magnesium, phosphorus, iron, copper, zinc, manganese, selenium, iodine, chromium, cobalt, molybdenum, fluoride, arsenic, nickel, silicon, and boron. The essential minerals are sometimes classified into two groups: the macroelements (also known as the macrominerals) and the trace elements. The macrominerals (sodium, potassium, chloride, calcium, magnesium, and phosphorus) are present in the body in amounts larger than 0.01% by weight, whereas the trace elements (the remaining 14 essential minerals) occur in the body at much lower levels and are required in the diet in amounts smaller than 100 mg day 1. The nutritional aspects of the former class of minerals will be discussed in this article, whereas the nutritional aspects of the latter class will be dealt with elsewhere (see Milk Salts: Trace Elements, Nutritional Significance). Although the minerals are treated separately, it is important to realize that interactions of the minerals with each other, 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 Macrominerals in Milk and Dairy Products Mineral Content The mineral content of milk is not constant but is influenced 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 minerals show a wide variation, which is due partly to these factors, but also partly to analytical errors and contamination during milk collection and from processing equipment and procedures. Representative values for the average macromineral content of milk are presented in Table 1.
Sodium, Potassium, Chloride Particularly high levels of sodium occur in cow’s colostrum, but the level decreases within a few days to the value shown in Table 1. The sodium content of milk is not influenced by dietary sodium intake within the normal range. Sodium concentration in milk tends to be higher at the end of lactation when milk yield is low. Removal of milk fat has little effect on milk sodium content. In contrast to most other minerals, potassium concentration in cow’s colostrum is lower than that in mature milk, but increases to normal values within the first 2–3 days of lactation and is independent of potassium intake. The chloride concentration in milk decreases from higher levels in colostrum to lower levels in mature milk but increases sharply toward the end of lactation and is independent of dietary intake. The sodium, potassium, and chloride contents of other dairy products are shown in Tables 2–5. Levels of sodium (and also chloride) in cheese are dependent on the amount of added salt. Levels of potassium are no higher in cheese than in milk. In general, the concentration of macrominerals decreases as the fat concentration increases in milk and dairy products (Table 3).
Calcium, Phosphorus, Magnesium The mean calcium and phosphorus contents of cow’s milk are 1120 and 890 mg l 1, respectively. In cow’s milk, calcium concentration is slightly elevated in colostrum and at the end of lactation but varies little with feeding or
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The calcium, phosphorus, and magnesium contents of other dairy products are shown in Tables 2–5. Levels of calcium and phosphorus are generally highest in hard cheeses (Parmigiano, Gouda, Edam, and Cheddar), that is, up to 10 times that in milk, followed by mold cheeses (Brie, Stilton), that is, 4–5 times that in milk (Table 5). The lowest levels are found in cream and cottage cheese types. Levels of magnesium are ordered similar to calcium but only around 5 times higher in hard cheeses and 2–3 times higher in mold types.
Table 1 Mean concentrations of macrominerals in cow’s milk
Mineral
Content (mg 1 1)
Sodium Potassium Chloride Calcium Phosphorus Magnesium
430 1550 890 1180 930 110
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
Chemical Form of the Macrominerals season. There is little variation in the phosphorus content of milk throughout lactation. The mean concentration of magnesium in milk is 110 mg l 1 and is unaffected by fat removal. Magnesium concentration in colostrum is 2–3 times that in mature milk and decreases to the level in mature milk within the first 1–3 days of lactation, remaining relatively constant thereafter. The concentration of magnesium in milk is unaffected by dietary intake of magnesium.
The chemical form of a mineral is important because it may influence intestinal absorption and utilization (the process of transport, cellular assimilation, and conversion into a biologically active form) and thus bioavailability. Sodium, Potassium, Chloride Sodium, potassium, and chloride are believed to be present in milk almost entirely as free ions. Practically all the sodium, potassium, and chloride in milk is absorbed in the
Table 2 Mean concentrations of macrominerals in concentrated milks Content (mg 100 g 1) Mineral
Pasteurized skimmed
Dried skimmed
Evaporate (whole)
Condensed (whole)
Sodium Potassium Chloride Calcium Phosphorus Magnesium
43 155 89 118 93 11
550 1590 1070 1280 970 130
180 360 250 290 260 29
140 360 230 290 240 29
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
Table 3 Mean concentrations of macrominerals in creams Content (mg 100 g 1) Fresh cream
Soured
Sterilized canned
UHT
Mineral
10% fat
20% fat
35–48% fat
60% fat
20% fat
25% fat
32% fat
Sodium Potassium Chloride Calcium Phosphorus Magnesium
49 120 77 99 89 11
29 104 80 80 79 8
25 86 59 58 59 6
18 55 40 37 40 5
41 110 81 93 81 10
53 110 78 86 73 10
31 107 66 54 57 7
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
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Table 4 Mean concentrations of macrominerals in butter, yogurt, and dairy ice cream Content (mg 100 g 1) Mineral
Butter (whole milk)
Yogurt
Dairy ice cream
Sodium Potassium Chloride Calcium Phosphorus Magnesium
606a 27 994a 18 23 2
80 280 170 200 170 19
60 174 110 100 91 12
a
Unsalted butter contains 9 mg 100 g 1 sodium and 19 mg 100 g 1 chloride. Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
Table 5 Mean concentrations of macrominerals in some cheese varieties Content (mg 100 g 1) Mineral
Brie
Cheddar
Cream
Cottage
Edam
Feta
Gouda
Parmesan
Stilton
Sodium Potassium Chloride Calcium Phosphorus Magnesium
556 91 900 256 232 15
723 75 1040 739 505 29
300 160 480 98 100 10
300 161 490 127 171 13
996 89 1570 795 508 34
1440 95 2350 360 280 20
925 82 1440 773 498 32
756 152 1260 1025 680 41
788 96 1230 326 314 15
Data from Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry.
gastrointestinal tract, although much of what is absorbed is not retained.
Calcium, Phosphorus, Magnesium In cow’s milk, 99% of the calcium is in the skim milk fraction, which explains why the calcium content is not affected by fat removal. Two-thirds of the total calcium occurs in colloidal form associated with the casein micelles, either as a calcium phosphate salt (about half of total milk calcium) or as calcium ions bound to phosphoserine residues (about one-sixth of total calcium); the remaining one-third is soluble. Ionized calcium in the soluble phase accounts for 10% of the total calcium, and most of the remaining soluble calcium occurs as calcium citrate. A small amount of calcium (0.15%) is bound to -lactalbumin. Of the total phosphorus in cow’s milk, 20% occurs as organic phosphate esterified to casein with the remainder occurring as inorganic phosphate. About 44% of the inorganic phosphate is associated with casein micelles as calcium phosphate and 56% is soluble, mainly as free phosphate ions. In cow’s milk, 98–100% of the magnesium is in the skim milk phase, where 65% of the magnesium is in a soluble form (40% as magnesium citrate, 7% as
magnesium phosphate, and 16% as free magnesium ion), and the remainder is colloidal and is associated with the casein micelles (about one-half associated with colloidal calcium phosphate and the other half bound directly to phosphoserine residues in caseins).
Nutritional Significance of Macrominerals in Milk and Dairy Products A simple and useful evaluation of the nutritional significance of macrominerals (as well as trace elements) in milk can be obtained by comparing the amounts of the different elements provided by 1 l of milk (Table 1) with the recommended daily intakes for these elements (Table 6). Another useful impression of the nutritional significance of milk and other dairy products can be obtained by computing, from data on dietary surveys, the actual contribution made by dairy products to the total intake of macrominerals and trace elements. Sodium, Potassium, Chloride Sodium is the principal cation in extracellular fluids and is the primary regulator of extracellular fluid volume. It is
928 Milk Salts | Macroelements, Nutritional Significance Table 6 Recommended dietary intakes of selected macrominerals Minerals
Category
Age (years)
Infants
0.0–0.5 0.5–1.0
Children
Calciuma (mg)
Magnesiumb (mg)
Phosphorusb (mg)
210 270
30a 75a
100a 275a
1–3 4–8 9–13
500 800 1300
80 130 240
460 500 1250
Males
14–18 19–30 31–50 51–70 >70
1300 1000 1000 1200 1200
410 400 420 420 420
1250 700 700 700 700
Females
14–18 19–30 31–50 51–70 >70
1300 1000 1000 1200 1200
360 310 320 320 320
1250 700 700 700 700
Pregnancy
18 19–30 31–50
1300 1000 1000
400 350 360
1250 700 700
Lactation
18 19–30 31–50
1300 1000 1000
360 310 320
1250 700 700
a US adequate intake values; from Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press. b US recommended dietary allowance (RDA); from Institute of Medicine (1997) Dietary Reference Intakes: Calcium, Magnesium, Phosphorus, Vitamin D, and Fluoride. Washington, DC: National Academy Press.
important in the regulation of osmolarity, acid–base balance, and the membrane potential of cells, as well as in active transport across cell membranes. Chloride is the principal extracellular anion and is essential in the maintenance of fluid and electrolyte balance. Potassium is the principal intracellular cation, occurring in the cell at a concentration more than 30 times its concentration in extracellular fluid. Extracellular potassium contributes to the transmission of nerve impulses, to the control of skeletal muscle contraction, and to the maintenance of blood pressure. Under normal circumstances, dietary deficiency of sodium, potassium, or chloride does not occur, but the body can be depleted of sodium and chloride under extreme conditions, for example, heavy perspiration, chronic diarrhea, or renal disease. Depletion of potassium can occur in the body under conditions where there are large alimentary or renal losses. Minimum requirements for sodium, potassium, and chloride (e.g., 500, 2000, and 750 mg day 1, respectively, in adults) have been established. Actual intakes of sodium and chloride are considerably higher than the minimum requirements in many populations and, given the evidence of a relationship of high salt intake to hypertension, it has been recommended that sodium
intake be limited to 2.4 g day 1. Similarly, considering the possible beneficial effect of potassium in hypertension, increasing potassium intake beyond the minimum requirement has been recommended. Cow’s milk contributes little to the dietary intake of sodium (7% in the United Kingdom), but some dairy products such as cheese and butter contain added salt and can be significant sources of sodium in some countries (e.g., 13% of total sodium intake in the United Kingdom). It has been estimated that milk and dairy products provide 20% of total sodium and 24–29% of total potassium in the diet in Ireland and the United Kingdom. The concentrations of sodium, potassium, and chloride in milk are of physiological importance in the feeding of the young infant, and clinical problems may arise if there is an excessive intake of these nutrients. The kidney of the young infant, as compared to that of the adult, has a limited capacity to concentrate solids, and the renal solute load exerts a major effect on water balance. Renal solute load is determined mainly by sodium, potassium, chloride, phosphorus, and protein (which gives rise to urea). Cow’s milk has a much higher potential renal solute load (300 mosmol l 1) than human milk (93 mosmol l 1). The high renal solute
Milk Salts | Macroelements, Nutritional Significance
load resulting from ingestion of cow’s milk may be of relatively little significance in most circumstances because the kidney merely excretes a more concentrated urine. However, it does lead to a smaller margin of safety against dehydration, which can occur in conditions of diarrhea, fever, and low water intake, and for this reason it is recommended that the upper limit of potential renal solute load in formulae for young infants be 220 mosmol l 1. Recommended concentrations of sodium, potassium, and chloride in infant formulae are in the ranges 133–403, 534–1338, and 369–1360 mg l 1, respectively. Cow’s milk-based infant formulae currently in use fall comfortably within these guidelines. Calcium The adult human body contains 1200 g of calcium, which amounts to 1.5–2% of the body weight. Of this, 99% is found in bones and teeth, where it is present as calcium phosphate, providing strength and structure. The remaining 1%, found in extracellular fluids and intracellular structures and cell membranes, is responsible for a number of regulatory functions such as maintenance of normal heart beat, blood coagulation, hormone secretion, integrity of intracellular cement substances and membranes, nerve conduction, muscle contraction, and activation of enzymes. Cow’s milk and milk products, such as cheese and yogurt, are very good sources of dietary calcium. The contribution of dairy products to total calcium intake has been estimated as 75% in The Netherlands, 55% (dietary intake data) to 72% (food disappearance data) in the United States, 60% in the United Kingdom, and 52% in Ireland. In the absence of milk and dairy products from the diet, calcium intakes in excess of 300 mg day 1 are difficult to achieve. This is far below the typical recommended intakes for calcium. For example, the recently established adequate intake (AI) values for calcium in the United States are 1000 mg for adults, 1200 mg for older adults, and 1300 mg for adolescents (Table 6). On this basis, it might be considered that consumption of dairy products is very important in order to achieve an adequate calcium intake. However, there is still considerable disagreement on human calcium requirements, and this is reflected in the wide variation in the recommended adult dietary intakes for calcium (400–1200 mg) that have been set by different authorities. The dietary reference value for calcium is currently under review in the United States, and the new recommendations will be released in Summer 2010. In recent years, considerable attention has been focused on the bioavailability of calcium in milk. Evidence from studies on experimental animals suggests that essentially all of the calcium in human milk, cow’s
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milk, and cow’s milk-based infant formulae is potentially available for absorption in the gastrointestinal tract. In addition, it has often been suggested that the bioavailability of calcium is higher in human milk than in cow’s milk or cow’s milk-based infant formulae. This arises from the findings that calcium is absorbed more efficiently by human infants from human milk than from cow’s milk or a cow’s milk-based formula containing 738 mg Ca l 1. However, the absolute amount of calcium absorbed is higher from cow’s milk and formulae, and calcium absorption from cow’s milk formulae with lower calcium contents (363–458 mg l 1) is similar to that from human milk. Similarly, the efficiency of calcium absorption from human milk by weanling Rhesus monkeys has been reported as being significantly higher (72%) than from cow’s milk-based infant formulae (45–53%), but the absolute absorption from human milk and formulae were similar due to the higher calcium concentration in the formulae. Moreover, bone mineralization in infants fed with cow’s milk formula has been reported to be as good as, if not slightly better than, in infants fed with human milk. Mean calcium absorption from cow’s milk by healthy human adults has been variably reported in the range 21–45%. Calcium absorption has been reported as being similar from milk and from CaCO3 by pregnant women at 36–47%, but calcium absorption from cow’s milk by postmenopausal women has been variably reported in the range 5–41%. Calcium absorption from cow’s milk by -galactosidase-deficient subjects has been reported as being higher (36.2% of dose) as compared to a -galactosidase-sufficient group (25.7%), and it has been suggested that this reflects lower habitual calcium intake owing to reduced milk consumption by the -galactosidase-deficient subjects. It has been reported that calcium from various dairy products (including whole milk, chocolate milk, yogurt, Cheddar cheese, processed cheese) is absorbed equally well. Thus, though it appears that all of the calcium in human milk, cow’s milk, and cow’s milk-based infant formulae is potentially available for absorption, the absolute amount of calcium absorbed by animals or humans is determined by physiological factors such as the efficiency of calcium absorption mechanisms in the gastrointestinal tract, which may be influenced by calcium needs, vitamin D status, and age, as well as by the calcium concentration in milk. In addition, some components of milk (lactose, phosphopeptides) may enhance calcium absorption. There is strong evidence that lactose promotes intestinal absorption (particularly in the ileum) and body retention of calcium in rats. This effect of lactose is independent of vitamin D, but the mechanism by which it occurs remains unresolved. It has been suggested that undigested lactose reaching the ileum
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interacts with the brush border membrane, increasing its permeability to calcium. Lactose also appears to increase calcium absorption in human infants. For example, calcium absorption has been reported to be significantly higher from a soy-based infant formula containing lactose (48%) than from a similar formula in which the carbohydrate source was a mixture of starch hydrolysate and sucrose (33%). However, studies on the effect of lactose on calcium absorption by human adults have produced conflicting results. One study found no significant difference in calcium absorption from milk or lactose-hydrolyzed milk by either -galactosidasedeficient or -galactosidase-sufficient subjects, whereas another study reported that calcium absorption was similar from milk (21.4%) and lactose-free milk (lactose replaced by glucose) (26.8%) by healthy adult subjects, but lactose increased calcium absorption by -galactosidase-deficient subjects. Overall, it is likely that lactose, at the level normally present in milk, does not have a significant effect on calcium absorption in healthy adults consuming normal diets. It is possible that lactose affects the non-saturable paracellular pathway for calcium absorption in the gut and its effect is most likely to occur in vitamin D deficiency, or when elevated levels of calcium are fed. It has been suggested that phosphopeptides formed during the digestion of bovine caseins may be involved in promoting calcium absorption. Such phosphopeptides have the capacity to chelate calcium and to prevent the precipitation of calcium phosphate salts and may help to maintain a high concentration of soluble calcium in the intestinal lumen. There is evidence that phosphopeptides are present in the lumen of the small intestine of rats and pigs after a casein-containing meal; purified phosphopeptides have been shown to enhance the absorption of calcium in rats and chicks. The observation that calcium absorption in rats is enhanced by high-casein meals is consistent with this. However, the nutritional significance of these phosphopeptides in humans consuming milk remains unclear. Age-related osteoporosis, a common bone disease and a major cause of disability in Western countries, is characterized by reduced bone density resulting in increased bone fragility and susceptibility to fracture. The condition is particularly common in elderly women, especially in Caucasians. It has been estimated that osteoporosis afflicts 10 million Americans (half of the women over 45 years of age and 90% of the women over 75 years of age). Osteoporosis is a multifactorial disorder, but there is increasing evidence that inadequate calcium intake, particularly during early life, is a contributory factor. Adequate calcium intake is required in early life in order to develop maximum bone mass at maturity (during the third decade of life), and there is evidence that the
amount of bone mass present at maturity is an important factor influencing fracture susceptibility in the elderly. Peak bone mass at maturity appears to be related to the intake of calcium during the years of bone mineralization. Inadequate dietary intake of calcium during the critical growth and building periods may result in failure to reach peak bone mass, causing osteopenia, osteoporosis, decreased skeletal integrity, and increased risk of fracture in later life. Many believe that the most promising nutritional approach to reduce the risk of osteoporosis in later life is to ensure a calcium intake that allows the development of each individual’s genetically programmed peak bone mass during the formative years, that is, throughout childhood to the age of 25 years. In this regard, the prevalence of lower-than-recommended calcium intake among adolescent and young adult females in many countries, for example, in the United States and Europe, is of particular concern. In recent years, evidence has been presented for a protective role of dietary calcium against hypertension, hypercholesterolemia, diabetes mellitus, and colon and rectal cancer. Dietary calcium has also been proposed to help regulate and maintain body weight. However, consensus on the role of calcium in these conditions is lacking and further research is required. Phosphorus Phosphorus is an essential nutrient for humans and serves a number of important biological functions. Phosphorus occurs as organic and inorganic phosphates in all body tissues and fluids; is an essential component of many biological molecules, including lipids, proteins, carbohydrates, and nucleic acids; and plays a central role in metabolism. As calcium phosphate, it is a major structural component of bones and teeth. Because almost all foods contain phosphorus, dietary phosphorus deficiency does not usually occur. The recently revised recommended dietary allowances (RDAs) for phosphorus are shown in Table 6. Cow’s milk and milk products, such as cheese and yogurt, are good dietary sources of phosphorus, and the contribution of milk and dairy products to total phosphorus intake has been reported as 30–45% in Western countries. In the first few weeks of life, the infant’s ability to regulate plasma calcium concentration is not fully developed, and hypocalcemia may result in neonatal tetany, which used to occur more frequently in artificially fed than in breast-fed infants. Excessive phosphorus intake contributes to this condition, and feeding unmodified cow’s milk, which has a high content of phosphorus, increases serum phosphorus and lowers serum (ionized) calcium in the newborn infant. For this reason, it is recommended that the calcium-to-phosphorus ratio in artificial
Milk Salts | Macroelements, Nutritional Significance
infant formulae be higher than that in cow’s milk (1.2:1) and more similar to that in human milk (2.2:1). In experimental animals, a high intake of phosphorus or a low calcium-to-phosphorus ratio in the diet could lead to bone loss. However, it is generally agreed that wide variations in phosphorus intake or in the calciumto-phosphorus ratio in the diet do not adversely affect bone mass in adult humans. Magnesium Magnesium has an essential role in a wide variety of physiological processes, including protein and nucleic acid metabolism, neuromuscular transmission and muscle contraction, bone growth and metabolism, and regulation of blood pressure, and it acts as a cofactor for many enzymes. There is little information from human studies on the bioavailability of magnesium from milk. Metabolicbalance studies in infants showed that 16–43% of magnesium is absorbed from cow’s milk-based infant formulae and that lactose enhances the absorption of magnesium. Cow’s milk and milk products contribute 16–21% of the total magnesium intake in Western countries. Dietary deficiency of magnesium is uncommon except in conditions of severe malnutrition and certain disease states. There is evidence that many young adults, especially women, in various European countries and in the United States fail to achieve the recommended intakes of magnesium and this raises the issue of possible adverse effects of lower-than-recommended intakes. Scientists have attempted to demonstrate that suboptimal intake of magnesium (i.e., below the RDA, but not frank deficiency) is a contributor to the development of chronic maladies such as cardiovascular disease, hypertension, disorders of skeletal growth and osteoporosis, and diabetes mellitus. However, the results of studies in this area are ambiguous. The fact that the RDA for magnesium has been raised recently for most groups (Table 6) reflects nutrition scientists’ belief that there is a negative consequence to suboptimal magnesium intake. Additional research is needed to fully justify this concern.
Conclusion Even though the nutritional roles, requirements, and metabolism, and the quantitative relationship between dietary intakes and health for a number of macrominerals have been more clearly defined in recent years, there are still considerable deficiencies in our understanding of these issues, for example, the significance of calcium and magnesium in the etiology and treatment of osteoporosis and hypertension.
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Reliable information on the content, and the principal factors affecting it, of most of the macrominerals in cow’s milk is now available. The contribution of cow’s milk and milk products to the diet in Western countries is significant for sodium, potassium, chloride, calcium, and phosphorus. Sodium, potassium, and chloride are believed to be almost totally absorbed from milk and infant formulae. Though the bioavailability of calcium in milk has received considerable attention in recent years, there is little information on the bioavailability of magnesium in milk and infant formulae. In addition, there is a paucity of data regarding the bioavailability of macrominerals from other dairy products. Factors affecting the bioavailability of minerals are still poorly understood, and the role of possible enhancers (e.g., 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 that are strongly inhibitory to mineral absorption, such as phytate or polyphenols contained in foods from plants. Finally, understanding the nutritional significance of macrominerals in milk and dairy products will also benefit 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: Milk Salts: Trace Elements, Nutritional Significance.
Further Reading Allen LH (1983) Calcium bioavailability and absorption: A review. The American Journal of Clinical Nutrition 53: 783–808. Fairweather-Tait S and Hurrell RF (1996) Bioavailability of minerals and trace elements. Nutrition Research Reviews 9: 295–324. Fleet JC and Cashman KD (2001) Magnesium. In: Bowman B and Russel R (ed.) 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 fortification of foods. In: Hurrell R (ed.) Mineral Fortification of Foods, pp. 18–53. Leatherhead: Leatherhead Food Research Association. Food Standards Agency (2002) McCance & Widdowson’s. The Composition of Foods, 6th edn. Cambridge: Royal Society of Chemistry. 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.
932 Milk Salts | Macroelements, Nutritional Significance Sadler MJ, Strain JJ, and Caballero B (eds.) (1999) Encyclopedia of Human Nutrition, Vols. 1–3. San Diego, CA: 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 KD (2009) Minerals and trace elements. In: Gibney M, Lanham-New SA, Cassidy A, Vorster HH, et al. (eds.) Introduction to Human Nutrition, 2nd edn. London: Nutrition Society. Van Dokkum W (1995) The intake of selected minerals and trace elements in European countries. Nutrition Research Reviews 8: 271–302.