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Zinc, iron, and copper contents of eggs from hens fed varying levels of zinc
JOURNAL
OF FOOD COMPOSITION
AND ANALYSIS
1,309-3
15 ( 1988)
Zinc, Iron, and Copper Contents of Eggs from Hens Fed Varying Levels of Zinc JAMES L. STAHL,
M. E. COOK, AND J. L. GREGER
Departments of Nutritional Sciences and Poultry Science, University of Wisconsin, Madison, Wisconsin 53706, U.S.A. Received March 22, 1988, and in revised form May 16, 1988 We wanted to determine whether eggs which contained more zinc could be produced by fortifying the diets of hens with zinc. In two studies, hens fed very high levels of zinc ( 1762 or 186 1 r.qg&t/g diet) for 4 to 40 weeks produced eggs that contained 57-90% more zinc than eggs produced by hens fed control diets (28 or 26 pg Zn/g diet). Hens fed high levels of zinc (2 18 or 257 pg Zn/g diet) produced eggs that contained as much as 25% more zinc than eggs produced by hens fed control diets. Ingestion of excess zinc did not have consistent effects on the iron or copper content of eggs or on the zinc content of eggshells. 0 1988 Academic PEW hc.
INTRODUCTION In a recent nationwide survey, women (19-50 years of age) consumed on average only 60% of the Recommended Dietary Allowance (RDA) for zinc (Human Nutrition Information Service, 1985). A number of investigators have reported that elderly Americans generally consume only one-half to two-thirds of the RDA for zinc (Greger, 1984). These data suggest that enrichment of some foods with zinc might increase their market appeal (Schutz et al., 1986). We thought that eggs might be such a food, especially among the elderly. Traditionally the major source of zinc in the diets of Americans is meat, fish, and poultry (Welsh and Marston, 1982). However, elderly (a75 years of age) men in the United States consume only 69% as much meat, fish, and poultry but 153% as many eggs as young (23-34 years of age) men (Science and Education Administration, 1980). Previous claims of nutritionally superior eggs being produced by certain breeds of chickens, i.e., Araucana, have not been proven to be valid (Hickman, 1974; Simmons and Somes, 1985). Hence we wanted to determine whether zinc-enriched eggs could be produced by fortifying the diets of hens with zinc.
MATERIALS
AND
METHODS
Dietary TreatmentsandAnimals In study A, DeKalb single comb white leghorn (SCWL) hens (n = 40/treatment) were fed ad libitum for 12 weeks one of four dietary treatments: a control corn-soybased diet containing 28 pg Zn/g diet and diets supplemented with zinc sulfate to contain 59 (moderate), 2 18 (high), and 1762 (very high) pg Zn/g diet. The basal diet contained 67.15% corn; 2 1 .O%soybean meal; 3.0% alfalfa meal; 1.3% dicalcium phosphate; 6.0% calcium carbonate; 1.0% iodized salt; 0.5% of a vitamin-mineral mix 309
0889- 1575/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of rqmduction in any form reserved.
310
STAHL,
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AND
GREGER
that contained vitamins A, D, and Bi2, riboflavin, and manganese; and 0.5% DLmethionine (Stahl etal., 1986). We had already shown that the control diet contained adequate zinc for normal egg production (Stahl et al., 1986). The average copper and iron contents of the diets were determined to be 5.6 and 267 pg element/g diet, respectively. In study B, SCWL hens (n = 40/treatment) were fed ad libitum for 40 weeks one of four dietary treatments. The dietary treatments were similar to those in study A; diets provided 26 (control), 52 (moderate), 257 (high), and 186 1 (very high) bg Zn/g diet. The average copper and iron contents of the diets were determined to be 6.8 and 240 pg mineral/g diet, respectively. The exact composition of the test diets and the maintenance of the hens have been described previously (Stahl et a/., 1986). At the beginning of study A, hens were 54 weeks of age; they had been fed a corn-soy diet similar to the control diet since 20 weeks of age. At the beginning of study B, hens were 28 weeks of age; they had been fed the control diet since 20 weeks of age.
SampleCollectionandAnalysis In study A, eggs were collected after hens had ingested the diets for 2, 4, 6, and 8 weeks. In study B, eggs were collected at the initiation of study and every 8 weeks thereafter. Egg production was monitored continuously throughout the studies. In study A, two groups of eggs per dietary treatment (n = 30) were weighed on two occasions. Average weights of eggs were calculated. Five eggs were randomly selected from those produced by hens fed each diet at each time interval. Eggs were stored at 4°C for no more than 2 weeks before being broken. Preliminary studies showed that the mineral concentrations of eggs stored in their shells were constant during a 5-week period. Eggs were rinsed with deionized water and broken into mineral-free plastic containers. Egg contents were thoroughly mixed and then frozen until analysis. Shells and shell membranes were rinsed of adhering material with deionized water, dried, and finely powdered with a mortar and pestle. Egg contents were ashed as described previously (Greger and Snedeker, 1980). Eggshells (0.1 g) were dissolved in 1 ml concentrated nitric acid overnight and then diluted 1O-fold with deionized water prior to analysis. Prepared samples were analyzed using atomic absorption spectrophotometry (Pet&n-Elmer Corp., Norwalk CT; Model 372). Bovine liver standards (SRM 1577a) obtained from the National Bureau of Standards were analyzed with samples. The liver standards (n 3 20) were found to contain 125 -t 2 (SEM) pg Zn/g, 152 + 1 fig Cu/g, and 174 * 2 pugFe/g; the standard was certified to contain 123 pg Zn/g, 158 kg Cu/g, and 194 pg Fe/g. Data were subjected to analysis of variance using the general linear model system of SAS (SAS Institute, Inc., 1985). Repeated measures analysis were used to detect variations in treatment effects over the course of the studies. Protected least-significant differences were determined as appropriate. All statements of significance are based on P < 0.05. RESULTS
AND
DISCUSSION
In study A, hens that ingested the diet with the very high level of zinc produced eggs that contained significantly (P < 0.05) more zinc than eggs produced by hens
311
Zn, Fe, AND Cu CONTENTS OF EGGS
10
! 0
I 2
I 4
I 6
I 8
FIG. 1. StudyA: Concentration of zinc in whole eggsfrom hensfed (-) control diet, (f) moderateZn diet, (*) high Zn diet, and (-O-)very high Zn diet for 8 weeks.SEMbarsincluded. Eggslaid by hensfed the very high Zn diet contained significantly(P < 0.05) more zinc than eggslaid by other hensat all times tested.Eggslaid by hensfed the hi& zincdiet contained significantly(P i 0.05) more zinc than eggslaid by hensfed the control diet for 4,6, and 8 weeks. fed the other diets (Fig. 1). The effect was significant at all time intervals. Statistical evaluations indicated that zinc concentrations of eggs did not vary significantly with time but there tended (P < 0.06) to be an interaction between time and treatments. The zinc concentration of eggs from hens fed the three lower levels of zinc did not differ from each other after 2 weeks of dietary treatment. However, after 4,6, and 8
weeks, eggs from hens fed the high level of zinc contained significantly (P < 0.05) greater concentrations of zinc than eggs produced by hens fed the control diet. Although zinc intake of hens affected the zinc concentration of eggs, zinc intake did not affect the size of eggs in study A. The average weights of 120 eggs from hens fed the control, moderate, high, and very high levels of zinc were 62, 63,62, and 64 g, respectively. Egg production was also unaffected by the dietary treatments (Stahl et al., 1988). Thus hens fed the very high level of zinc for 4 to 8 weeks produced eggs that contained 57-66% more zinc than eggs from hens fed the control diet. Hens fed the high level of zinc produced eggs that contained as much as 25% more zinc than eggs from hens fed the control diet. After 8 weeks of dietary treatment, hens fed even the moderate level of zinc produced eggs that contained 14% more zinc than eggs from hens fed the control diet. Previously Palafox and Ho-A (1980) found that feeding hens 20,000 pg Zn/g diet ( 10 times the level of zinc in our very high zinc diet) for only 5 days reduced the zinc content of eggs and temporarily reduced egg production. We were concerned that similar adverse effects might be observed if hens were fed -2000 pg Zn/g diet for more than 8 weeks, hence we conducted study B. Although study B was much longer than study A and the hens were younger at the initiation of dietary treatments in study B, the results were similar in the two studies. In study B, hens that were fed the very high level of zinc produced eggs that contained more zinc than eggs produced by hens fed the other diets (Fig. 2). Eggs produced by hens fed the very high level of zinc contained 57-90% more zinc than eggs produced by hens fed the control diet.
312
STAHL,
51 0
COOK, AND GREGER
I 10
I 20
I 30
I 40
week
FIG. 2. Study B: Concentration of zinc in whole eggs from hens fed (-) control diet, (+) moderate Zn diet, (*) high Zn diet, and (G) very high Zn diet for 40 weeks. SEM bars included. Eggs laid by hens fed the very high Zn diet contained significantly (P < 0.05) more zinc than eggs laid by other hens at 8, 16,24, 32, and 40 weeks. Eggs laid by hens fed the high Zn diet contained significantly (P < 0.05) more zinc than eggs laid by hens fed the control diet for 8,16, and 24 weeks.
Hens fed the high level of zinc also produced eggs with significantly increased zinc concentrations as compared to. hens fed the control diet after 8, 16, and 24 weeks of ingesting the diets, but not after 32 or 40 weeks. Generally, the eggs from hens fed the high level of zinc contained 9-20s more zinc than those eggs produced by hens fed the control diet. Statistical evaluations indicated that the concentrations of zinc in eggs varied with time in study B but this response was not linear. The highest zinc concentrations in eggs were observed after hens were fed the diets for 16 weeks. The practical significance of this variation is not known. The dietary treatments had little effect on the concentrations of iron in eggs in either study (Tables 1 and 2). The addition of zinc to the hens’ diets also did not affect copper concentrations in eggs at any time interval in study A. In study B, significant differences in egg copper concentrations were observed after the hens consumed the diets for 8, 16,24, and 32 weeks (Table 2). At first (Weeks 8 and 16), copper concentrations were low in eggs produced by hens fed very high levels of zinc. After 24 and 32 weeks, the effects of dietary zinc on egg concentrations of copper were not consistent. Eggshell zinc was monitored in study A because birds and mammals fed high levels of zinc accumulate zinc in bone (Hambidge et al., 1986). Medullary bone, found in the long bones of sexually mature hens, acts as a labile reserve of calcium for eggshell formation (Candlish, 197 1). We thought that as hens mobilized calcium for eggshell formation, zinc would also be mobilized and deposited in eggshells. Moreover, we thought that this additional eggshell zinc might migrate into the egg during storage. In study A, the addition of zinc to the diets of hens resulted in inconsistent changes in eggshell zinc concentrations (Table 3). Except at 2 weeks, eggshells produced by hens fed the very high level of zinc did not contain more zinc than shells produced by hens fed the control diet, even though hens fed the high and very high levels of zinc accumulated zinc in their tibias (Stahl et al., 1988). There are at least two possible reasons. Although efforts were made to minimize contamination of shells and shells
Zn, Fe, AND Cu CONTENTS
313
OF EGGS
TABLE 1 CONCENTRATION
OF IRON AND COPPER IN WHOLE EGGS FROM HENS FED FOUR LEVELS OF ZINC FOR 8 WEEKS
Dietary treatments Weeks of dietary exposure
Control
Moderate Zn
High Zn
Very high Zn
Pooled standard error’
18.5 19.46 17.4 15.9
1.6 2.1 2.4 3.1
0.89 0.72 0.76 0.89
0.23 0.20 0.18 0.18
bg Fe/g wet wt 2 4 6 8
16.4 16.1Cvd 15.2 16.9
16.3 17.46*’ 17.3 15.4
17.1 14.3d 14.3 17.0
wg Cu/g wet wt 2 4 6 8
0.89 0.94 0.65 0.98
0.87 0.98 0.75 0.74
1.04 1.00 0.85 0.98
bd Means within a row that do not share a common superscript letter are significantly (P < 0.05) different. TABLE 2 CONCENTRATION
OF IRON AND COPPER IN WHOLE EGGS FROM HENS FED FOUR LEVELS OF ZINC FOR 40 WEEKS
Dietary treatments Weeks of dietary exposure
Control
Moderate Zn
High Zn
Very high Zn
Pooled standard error’
pg Fe/g wet wt 0 8 16 24 32 40
16.1 16.8 16.0 14.3 17.2 18.7
18.1 16.3 14.3 15.6 14.6 19.5
15.3 16.6 16.1 15.9 15.9 17.9
15.0 16.7 14.0 17.8 16.1 18.8
1.8 1.4 1.6 2.5 1.3 1.2
1.02 0.67’ 0.596 0.73’ 0.84’ 0.84
0.15 0.09 0.11 0.07 0.06 0.09
pg Cu/g wet wt 0 8 16 24 32 40
1.05 0.82’ 0.89’ 0.576 0.93’ 0.86
0.92 o.86c 0.85’ 0.63b,C 0.77b 0.89
0.93 0.72b~c 0.78’ 0.76’ 1.08d 0.74
’ n = 4 at time 0; n = 5 at other time intervals. b-d Means within rows followed by different superscript letter are significantly (P < 0.05) different.
314
STAHL,
COOK, AND GREGER TABLE 3
CONCENTRATION OF ZINC IN EGGSHELIS FROM HENS FED FOUR LEVELS OF ZINC FOR 8 WEEKS Dietary treatments Weeks of dietary exposure
Control
Moderate Zn
High Zn
Very nigh Zn
Pooled standard error’
2 4 6 8
6.9”’ 5.8 6.4’ 6.3
7.4&C 5.9 5.8 ‘9’ 6.5
6.2d 7.6 5.1’ 7.2
8.0b 7.0 6.2’ 7.0
0.7 1.6 0.6 0.7
“n=5. hd Means within a row that do not share a common superscript letter are significantly (P < 0.05) different.
were washed with deionized water, it is possible that contamination occurred before eggs were collected for analysis. Another possibility is that hens consumed enough calcium carbonate as limestone that medullary bone was not mobilized. Thus we concluded that eggshells could not be a source of zinc to eggs during storage even if hens were fed very high levels of zinc. Accordingly, eggshells were not analyzed in study B. CONCLUSIONS The zinc content of eggs can be increased by increasing dramatically the level of zinc in the diets of the laying hens. The practical significance of this effect in terms of human nutrition is debatable. The average American consumes 0.69 eggs per day (Economic Research Service, 1987); the average weight of an egg is 50 g (Posati and On; 1976). Thus consumption of eggs produced by hens fed the very high level of zinc rather than the control diet, would increase zinc intake of the average American by only 0.3 mg zinc daily. But among typical elderly (275 years of age) men, who consume 80 g of egg daily (Science and Education Administration, 1980), this substitution would result in the consumption of an additional 0.7 mg zinc daily. This would be almost a 10% increase in total daily intake of zinc. ACKNOWLEDGMENTS The investigators acknowledge the support of the College of Agricultural of Wisconsin-Madison, Projects 2623 and 302.
and Life Sciences, University
REFERENCES CANDLISH, J. K. ( I97 1). The formation of mineral and organic matrix of fowl cortical and medullary Bone during shell calcification. Brit. Poult. Sci. 12, 119- 127. Economic Research Service (1987). Livestock and Poultry Situation and Outlook. LPS-26, U.S. Dept. of Agriculture, Washington, DC.
Zn, Fe, AND Cu CONTENTS
OF EGGS
315
GREGER, J. L. (1984). Zinc and copper requirements of the elderly. In Trace Substances in Environmental Health-XVZIZ(D. D. Hemphill, Ed.), pp. 463-475. University of Missouri, Columbia. GREGER, J. L., AND SNEDEKER, S. M. (1980). Effect of dietary protein and phosphorus levels on the utilization of zinc, copper and manganese by adult males. J. Nutr. 110,2243-2253. HAMBIDGE, K. M., CASEY, C. E., AND Knnns, N. F. (1986). Zinc. In Trace Elements in Human and Animal Nutrition (W. Mertz, Ed.), pp. 1-137. Academic Press, Orlando, FL. HICKMAN, M. (1974). The amazing Araucana! Org. Gard. Farming 21,58-6 1. Human Nutrition Information Service (1985). Nationwide Food Consumption Survey: Continuing Survey ofFood Intake by Individuals. NFCS:CSFII: Report No. 85-1, pp. 25. U.S. Dept. of Agriculture, Washington, DC. PALAFOX, A. L., AND HO-A, E. (1980). Effect of zinc toxicity in laying white leghorn pullets and hens. Poult. Sci. 59,2024-2028. POSATI, L. P., AND ORR, M. L. (1976). Composition of Foods: Dairy and Egg Products. Agric. Handbook 8-1, pp. 01-123. U.S. Dept. of Agriculture, Washington, DC. SAS Institute, Inc. (1985). SAS User’s Guide: Statistics. 5 ed., pp. 416-504. SAS Institute, Inc., Cary, NC. SCHUTZ, H. G., JUDGE, D. S., AND GENTRY, J. (1986). The importance of nutrition, brand, cost and sensory attributes to food purchase and consumption. Food Technol. 40( 1 l), 79-82. Science and Education Administration (1980). Food and Nutrient Intakes of Individuals in 1 Day in the United States, Spring 1977. Preliminary Report No. 2, pp. 45-50. U.S. Dept. of Agriculture, Washington, DC. SIMMONS, R. W., AND SOMES, R. G. (1985). Chemical characteristics of Araucana chicken eggs. Pouh. Sci. 64,1264-1268. STAHL, J. L., COOK, M. E., AND SUNDE, M. (1986). Zinc supplementation: its effect on egg production, feed conversion, fertility and hatchability. Pot&. Sci. 65,2 104-2109. STAHL, J. L., COOK, M. E., AND GREGER, J. L. (1988). Long-term zinc supplementation: Effect on hens. Unpublished data. WELSH, S. O., AND MARSTON, R. M. (1982). Zinc levels of the U.S. food supply: 1909-1980. Food Technol. 36(l), 70-76.
OF FOOD COMPOSITION
AND ANALYSIS
1,309-3
15 ( 1988)
Zinc, Iron, and Copper Contents of Eggs from Hens Fed Varying Levels of Zinc JAMES L. STAHL,
M. E. COOK, AND J. L. GREGER
Departments of Nutritional Sciences and Poultry Science, University of Wisconsin, Madison, Wisconsin 53706, U.S.A. Received March 22, 1988, and in revised form May 16, 1988 We wanted to determine whether eggs which contained more zinc could be produced by fortifying the diets of hens with zinc. In two studies, hens fed very high levels of zinc ( 1762 or 186 1 r.qg&t/g diet) for 4 to 40 weeks produced eggs that contained 57-90% more zinc than eggs produced by hens fed control diets (28 or 26 pg Zn/g diet). Hens fed high levels of zinc (2 18 or 257 pg Zn/g diet) produced eggs that contained as much as 25% more zinc than eggs produced by hens fed control diets. Ingestion of excess zinc did not have consistent effects on the iron or copper content of eggs or on the zinc content of eggshells. 0 1988 Academic PEW hc.
INTRODUCTION In a recent nationwide survey, women (19-50 years of age) consumed on average only 60% of the Recommended Dietary Allowance (RDA) for zinc (Human Nutrition Information Service, 1985). A number of investigators have reported that elderly Americans generally consume only one-half to two-thirds of the RDA for zinc (Greger, 1984). These data suggest that enrichment of some foods with zinc might increase their market appeal (Schutz et al., 1986). We thought that eggs might be such a food, especially among the elderly. Traditionally the major source of zinc in the diets of Americans is meat, fish, and poultry (Welsh and Marston, 1982). However, elderly (a75 years of age) men in the United States consume only 69% as much meat, fish, and poultry but 153% as many eggs as young (23-34 years of age) men (Science and Education Administration, 1980). Previous claims of nutritionally superior eggs being produced by certain breeds of chickens, i.e., Araucana, have not been proven to be valid (Hickman, 1974; Simmons and Somes, 1985). Hence we wanted to determine whether zinc-enriched eggs could be produced by fortifying the diets of hens with zinc.
MATERIALS
AND
METHODS
Dietary TreatmentsandAnimals In study A, DeKalb single comb white leghorn (SCWL) hens (n = 40/treatment) were fed ad libitum for 12 weeks one of four dietary treatments: a control corn-soybased diet containing 28 pg Zn/g diet and diets supplemented with zinc sulfate to contain 59 (moderate), 2 18 (high), and 1762 (very high) pg Zn/g diet. The basal diet contained 67.15% corn; 2 1 .O%soybean meal; 3.0% alfalfa meal; 1.3% dicalcium phosphate; 6.0% calcium carbonate; 1.0% iodized salt; 0.5% of a vitamin-mineral mix 309
0889- 1575/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of rqmduction in any form reserved.
310
STAHL,
COOK,
AND
GREGER
that contained vitamins A, D, and Bi2, riboflavin, and manganese; and 0.5% DLmethionine (Stahl etal., 1986). We had already shown that the control diet contained adequate zinc for normal egg production (Stahl et al., 1986). The average copper and iron contents of the diets were determined to be 5.6 and 267 pg element/g diet, respectively. In study B, SCWL hens (n = 40/treatment) were fed ad libitum for 40 weeks one of four dietary treatments. The dietary treatments were similar to those in study A; diets provided 26 (control), 52 (moderate), 257 (high), and 186 1 (very high) bg Zn/g diet. The average copper and iron contents of the diets were determined to be 6.8 and 240 pg mineral/g diet, respectively. The exact composition of the test diets and the maintenance of the hens have been described previously (Stahl et a/., 1986). At the beginning of study A, hens were 54 weeks of age; they had been fed a corn-soy diet similar to the control diet since 20 weeks of age. At the beginning of study B, hens were 28 weeks of age; they had been fed the control diet since 20 weeks of age.
SampleCollectionandAnalysis In study A, eggs were collected after hens had ingested the diets for 2, 4, 6, and 8 weeks. In study B, eggs were collected at the initiation of study and every 8 weeks thereafter. Egg production was monitored continuously throughout the studies. In study A, two groups of eggs per dietary treatment (n = 30) were weighed on two occasions. Average weights of eggs were calculated. Five eggs were randomly selected from those produced by hens fed each diet at each time interval. Eggs were stored at 4°C for no more than 2 weeks before being broken. Preliminary studies showed that the mineral concentrations of eggs stored in their shells were constant during a 5-week period. Eggs were rinsed with deionized water and broken into mineral-free plastic containers. Egg contents were thoroughly mixed and then frozen until analysis. Shells and shell membranes were rinsed of adhering material with deionized water, dried, and finely powdered with a mortar and pestle. Egg contents were ashed as described previously (Greger and Snedeker, 1980). Eggshells (0.1 g) were dissolved in 1 ml concentrated nitric acid overnight and then diluted 1O-fold with deionized water prior to analysis. Prepared samples were analyzed using atomic absorption spectrophotometry (Pet&n-Elmer Corp., Norwalk CT; Model 372). Bovine liver standards (SRM 1577a) obtained from the National Bureau of Standards were analyzed with samples. The liver standards (n 3 20) were found to contain 125 -t 2 (SEM) pg Zn/g, 152 + 1 fig Cu/g, and 174 * 2 pugFe/g; the standard was certified to contain 123 pg Zn/g, 158 kg Cu/g, and 194 pg Fe/g. Data were subjected to analysis of variance using the general linear model system of SAS (SAS Institute, Inc., 1985). Repeated measures analysis were used to detect variations in treatment effects over the course of the studies. Protected least-significant differences were determined as appropriate. All statements of significance are based on P < 0.05. RESULTS
AND
DISCUSSION
In study A, hens that ingested the diet with the very high level of zinc produced eggs that contained significantly (P < 0.05) more zinc than eggs produced by hens
311
Zn, Fe, AND Cu CONTENTS OF EGGS
10
! 0
I 2
I 4
I 6
I 8
FIG. 1. StudyA: Concentration of zinc in whole eggsfrom hensfed (-) control diet, (f) moderateZn diet, (*) high Zn diet, and (-O-)very high Zn diet for 8 weeks.SEMbarsincluded. Eggslaid by hensfed the very high Zn diet contained significantly(P < 0.05) more zinc than eggslaid by other hensat all times tested.Eggslaid by hensfed the hi& zincdiet contained significantly(P i 0.05) more zinc than eggslaid by hensfed the control diet for 4,6, and 8 weeks. fed the other diets (Fig. 1). The effect was significant at all time intervals. Statistical evaluations indicated that zinc concentrations of eggs did not vary significantly with time but there tended (P < 0.06) to be an interaction between time and treatments. The zinc concentration of eggs from hens fed the three lower levels of zinc did not differ from each other after 2 weeks of dietary treatment. However, after 4,6, and 8
weeks, eggs from hens fed the high level of zinc contained significantly (P < 0.05) greater concentrations of zinc than eggs produced by hens fed the control diet. Although zinc intake of hens affected the zinc concentration of eggs, zinc intake did not affect the size of eggs in study A. The average weights of 120 eggs from hens fed the control, moderate, high, and very high levels of zinc were 62, 63,62, and 64 g, respectively. Egg production was also unaffected by the dietary treatments (Stahl et al., 1988). Thus hens fed the very high level of zinc for 4 to 8 weeks produced eggs that contained 57-66% more zinc than eggs from hens fed the control diet. Hens fed the high level of zinc produced eggs that contained as much as 25% more zinc than eggs from hens fed the control diet. After 8 weeks of dietary treatment, hens fed even the moderate level of zinc produced eggs that contained 14% more zinc than eggs from hens fed the control diet. Previously Palafox and Ho-A (1980) found that feeding hens 20,000 pg Zn/g diet ( 10 times the level of zinc in our very high zinc diet) for only 5 days reduced the zinc content of eggs and temporarily reduced egg production. We were concerned that similar adverse effects might be observed if hens were fed -2000 pg Zn/g diet for more than 8 weeks, hence we conducted study B. Although study B was much longer than study A and the hens were younger at the initiation of dietary treatments in study B, the results were similar in the two studies. In study B, hens that were fed the very high level of zinc produced eggs that contained more zinc than eggs produced by hens fed the other diets (Fig. 2). Eggs produced by hens fed the very high level of zinc contained 57-90% more zinc than eggs produced by hens fed the control diet.
312
STAHL,
51 0
COOK, AND GREGER
I 10
I 20
I 30
I 40
week
FIG. 2. Study B: Concentration of zinc in whole eggs from hens fed (-) control diet, (+) moderate Zn diet, (*) high Zn diet, and (G) very high Zn diet for 40 weeks. SEM bars included. Eggs laid by hens fed the very high Zn diet contained significantly (P < 0.05) more zinc than eggs laid by other hens at 8, 16,24, 32, and 40 weeks. Eggs laid by hens fed the high Zn diet contained significantly (P < 0.05) more zinc than eggs laid by hens fed the control diet for 8,16, and 24 weeks.
Hens fed the high level of zinc also produced eggs with significantly increased zinc concentrations as compared to. hens fed the control diet after 8, 16, and 24 weeks of ingesting the diets, but not after 32 or 40 weeks. Generally, the eggs from hens fed the high level of zinc contained 9-20s more zinc than those eggs produced by hens fed the control diet. Statistical evaluations indicated that the concentrations of zinc in eggs varied with time in study B but this response was not linear. The highest zinc concentrations in eggs were observed after hens were fed the diets for 16 weeks. The practical significance of this variation is not known. The dietary treatments had little effect on the concentrations of iron in eggs in either study (Tables 1 and 2). The addition of zinc to the hens’ diets also did not affect copper concentrations in eggs at any time interval in study A. In study B, significant differences in egg copper concentrations were observed after the hens consumed the diets for 8, 16,24, and 32 weeks (Table 2). At first (Weeks 8 and 16), copper concentrations were low in eggs produced by hens fed very high levels of zinc. After 24 and 32 weeks, the effects of dietary zinc on egg concentrations of copper were not consistent. Eggshell zinc was monitored in study A because birds and mammals fed high levels of zinc accumulate zinc in bone (Hambidge et al., 1986). Medullary bone, found in the long bones of sexually mature hens, acts as a labile reserve of calcium for eggshell formation (Candlish, 197 1). We thought that as hens mobilized calcium for eggshell formation, zinc would also be mobilized and deposited in eggshells. Moreover, we thought that this additional eggshell zinc might migrate into the egg during storage. In study A, the addition of zinc to the diets of hens resulted in inconsistent changes in eggshell zinc concentrations (Table 3). Except at 2 weeks, eggshells produced by hens fed the very high level of zinc did not contain more zinc than shells produced by hens fed the control diet, even though hens fed the high and very high levels of zinc accumulated zinc in their tibias (Stahl et al., 1988). There are at least two possible reasons. Although efforts were made to minimize contamination of shells and shells
Zn, Fe, AND Cu CONTENTS
313
OF EGGS
TABLE 1 CONCENTRATION
OF IRON AND COPPER IN WHOLE EGGS FROM HENS FED FOUR LEVELS OF ZINC FOR 8 WEEKS
Dietary treatments Weeks of dietary exposure
Control
Moderate Zn
High Zn
Very high Zn
Pooled standard error’
18.5 19.46 17.4 15.9
1.6 2.1 2.4 3.1
0.89 0.72 0.76 0.89
0.23 0.20 0.18 0.18
bg Fe/g wet wt 2 4 6 8
16.4 16.1Cvd 15.2 16.9
16.3 17.46*’ 17.3 15.4
17.1 14.3d 14.3 17.0
wg Cu/g wet wt 2 4 6 8
0.89 0.94 0.65 0.98
0.87 0.98 0.75 0.74
1.04 1.00 0.85 0.98
bd Means within a row that do not share a common superscript letter are significantly (P < 0.05) different. TABLE 2 CONCENTRATION
OF IRON AND COPPER IN WHOLE EGGS FROM HENS FED FOUR LEVELS OF ZINC FOR 40 WEEKS
Dietary treatments Weeks of dietary exposure
Control
Moderate Zn
High Zn
Very high Zn
Pooled standard error’
pg Fe/g wet wt 0 8 16 24 32 40
16.1 16.8 16.0 14.3 17.2 18.7
18.1 16.3 14.3 15.6 14.6 19.5
15.3 16.6 16.1 15.9 15.9 17.9
15.0 16.7 14.0 17.8 16.1 18.8
1.8 1.4 1.6 2.5 1.3 1.2
1.02 0.67’ 0.596 0.73’ 0.84’ 0.84
0.15 0.09 0.11 0.07 0.06 0.09
pg Cu/g wet wt 0 8 16 24 32 40
1.05 0.82’ 0.89’ 0.576 0.93’ 0.86
0.92 o.86c 0.85’ 0.63b,C 0.77b 0.89
0.93 0.72b~c 0.78’ 0.76’ 1.08d 0.74
’ n = 4 at time 0; n = 5 at other time intervals. b-d Means within rows followed by different superscript letter are significantly (P < 0.05) different.
314
STAHL,
COOK, AND GREGER TABLE 3
CONCENTRATION OF ZINC IN EGGSHELIS FROM HENS FED FOUR LEVELS OF ZINC FOR 8 WEEKS Dietary treatments Weeks of dietary exposure
Control
Moderate Zn
High Zn
Very nigh Zn
Pooled standard error’
2 4 6 8
6.9”’ 5.8 6.4’ 6.3
7.4&C 5.9 5.8 ‘9’ 6.5
6.2d 7.6 5.1’ 7.2
8.0b 7.0 6.2’ 7.0
0.7 1.6 0.6 0.7
“n=5. hd Means within a row that do not share a common superscript letter are significantly (P < 0.05) different.
were washed with deionized water, it is possible that contamination occurred before eggs were collected for analysis. Another possibility is that hens consumed enough calcium carbonate as limestone that medullary bone was not mobilized. Thus we concluded that eggshells could not be a source of zinc to eggs during storage even if hens were fed very high levels of zinc. Accordingly, eggshells were not analyzed in study B. CONCLUSIONS The zinc content of eggs can be increased by increasing dramatically the level of zinc in the diets of the laying hens. The practical significance of this effect in terms of human nutrition is debatable. The average American consumes 0.69 eggs per day (Economic Research Service, 1987); the average weight of an egg is 50 g (Posati and On; 1976). Thus consumption of eggs produced by hens fed the very high level of zinc rather than the control diet, would increase zinc intake of the average American by only 0.3 mg zinc daily. But among typical elderly (275 years of age) men, who consume 80 g of egg daily (Science and Education Administration, 1980), this substitution would result in the consumption of an additional 0.7 mg zinc daily. This would be almost a 10% increase in total daily intake of zinc. ACKNOWLEDGMENTS The investigators acknowledge the support of the College of Agricultural of Wisconsin-Madison, Projects 2623 and 302.
and Life Sciences, University
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Zn, Fe, AND Cu CONTENTS
OF EGGS
315
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