Nutritional Bioavailability of Calcium - ACS Publications - American

14 Influence of Zinc on Calcium Bioavailability ADEN C. MAGEE

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Department of Food, Nutrition, and Food Service Management, School of Home Economics, University of North Carolina, Greensboro, NC 27412 High levels of dietary zinc were associated with marked decreases in bone calcium deposition and in the apparent retention of calcium in male weanling albino rats. Marked increases in fecal calcium levels were also observed in the zinc-fed rats. Excessive dietary zinc was associated with a shifting of phosphorus excretion from the urine to the feces. This resulted in an increase in fecal phosphorus and provided an environmental condition which would increase the possibility of the formation of insoluble calcium phosphate salts and a subsequent decrease in calcium bioavailability. The adverse effect of high dietary zinc on calcium status in young rats could be alleviated and/or reversed with calcium supplements. Most of the research on the influence of zinc on calcium b i o a v a i l a b i l i t y has been i n connection with zinc t o x i c i t y or the effects of high levels of dietary zinc on various animal systems. Such studies and/or i n v e s t i g a t i o n s have been conducted on a v a r i e t y of animal species and humans, but those studies which have revealed a possible effect of zinc on calcium b i o a v a i l a b i l i t y have generally involved the lamb, pig, and rat. This paper w i l l be primarily a discussion of the effects of high l e v e l s of dietary zinc on calcium status i n the rat. The e f f e c t of z i n c on phosphorus s t a t u s , however, has been included because there i s the p o s s i b i l i t y that the effect of zinc on calcium b i o a v a i l a b i l i t y may be dependent upon the phosphorus status of the system. The f i r s t e x p e r i m e n t a l i n d i c a t i o n of an adverse e f f e c t of excess dietary zinc on animals was revealed by Sutton and Nelson (1) in 1937. These researchers observed decreased growth, anemia, and reproductive f a i l u r e s i n rats fed diets containing 0.5% to 1% zinc. F o l l o w i n g t h i s i n i t i a l r e p o r t , s e v e r a l s t u d i e s were conducted by v a r i o u s r e s e a r c h e r s , but most of the s t u d i e s p e r t a i n i n g to z i n c t o x i c i t y between the l a t e 1930's and the e a r l y 1950's p r i m a r i l y focused on the effects of zinc t o x i c i t y on growth and on copper and i r o n metabolism. During 1951 and 1952 a s e r i e s of r e p o r t s by

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Kies; Nutritional Bioavailability of Calcium ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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NUTRITIONAL BIOAVAILABILITY O F C A L C I U M

Sadasivan (2-4) r e v e a l e d that 0.5% and 1% l e v e l s of d i e t a r y z i n c increased the f e c a l and urinary excretion of phosphorus and caused an impairment i n the development and the mineralization of the bones of young r a t s . Sadasivan suggested that the impairment i n bone development could be a t t r i b u t e d to an a n t a g o n i s t i c e f f e c t of z i n c upon normal bone d e p o s i t i o n of c a l c i u m and phosphorus. In 1958 Whiting and Bezeau (5) r e p o r t e d that d i e t a r y z i n c i n c r e a s e d the r e t e n t i o n of c a l c i u m i n p i g s , but had no i n f l u e n c e on phosphorus excretion. Thompson, Hansard, and B e l l (6), however, reported the following year that high levels of dietary zinc were associated with decreases i n the retentions of both calcium and phosphorus i n lambs. My interest i n zinc was stimulated i n the late 1950's, and for s e v e r a l years I was one of s e v e r a l r e s e a r c h e r s l o o k i n g at v a r i o u s aspects of zinc toxicosis i n connection with a variety of b i o l o g i c a l parameters. Beginning i n 1961 one area of c o n c e n t r a t i o n was the e f f e c t s of high levels of zinc on calcium and phosphorus status i n young r a t s , and some of the r e s u l t s of s t u d i e s (7 ,8) conducted i n our laboratory form the major portion of this paper. A l l of the s t u d i e s were conducted w i t h weanling, male a l b i n o r a t s of the Sprague-Dawley s t r a i n (Holtzman company). The b a s a l diet used for these studies consisted of casein, starch, vegetable o i l , vitamin and mineral mixtures, and cellulose. The Wesson Modif i c a t i o n of the Osborne-Mendel m i n e r a l mixture was used i n a l l s t u d i e s . T h i s m i n e r a l mixture contained no z i n c , but i t was adequate i n the other m i n e r a l s r e q u i r e d by the r a t . Most of the nonzinc-supplemented diets used i n the v a r i o u s experiments c o n t a i n e d approximately 7 ppm zinc. The l e v e l of mineral mixture used i n the b a s a l d i e t s was 4%, and based on the c h e m i c a l composition of the mixture, the basal diets contained approximately 0.57% calcium and 0.41% phosphorus. In one of the f i r s t experiments, weanling rats were fed 0.25%, 0.5%, and 0.75% zinc for 3 weeks. Animals were s a c r i f i c e d at weekly i n t e r v a l s , and the effects of these levels of zinc on weight gains and on bone c a l c i u m and phosphorus l e v e l s are shown i n Table I. Although none of the levels of dietary zinc had any effect on weight gains at the end of the f i r s t week, l e v e l s of 0.5% and 0.75% z i n c were associated with decreases i n bone calcium and phosphorus levels at the end of the f i r s t week. A n a l y s i s of the data i n d i c a t e d that weight gains of young rats fed 0.75% zinc were markedly lower than the weight gains of the controls at the end of the second week. Table II shows the effects of varying dietary levels of zinc on weight gains and on bone calcium and phosphorus levels of young rats at the end of a 4-week experiment. Increases i n dietary zinc were a s s o c i a t e d w i t h s i g n i f i c a n t l i n e a r decreases i n bone c a l c i u m and phosphorus deposition. The bones taken from animals at the time of s a c r i f i c e and used for the mineral analyses were very soft i n nature and could be e a s i l y squeezed with the fingers. A second step i n the i n v e s t i g a t i o n s i n v o l v e d the a d d i t i o n of calcium and phosphorus supplements to young rats fed 0.75% zinc, and Table I I I shows some of the results when 0.4% levels of calcium and phosphorus were used as supplements. The high l e v e l of dietary zinc a g a i n r e s u l t e d i n marked decreases i n weight gains and i n bone c a l c i u m and phosphorus l e v e l s . Supplementing the high zinc diets w i t h 0.4% c a l c i u m r e s u l t e d i n bone c a l c i u m l e v e l s which were s i g -


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MAGEE

Table

I.

Weeks on Experiment

Influence of Zn on Ca Bioavailability

E f f e c t s o f Z i n c on Growth and Bone M i n e r a l i z a t i o n o f Young Rats Zinc Supplement

Weight Gainl

%


167

Bone Constituent" Ρ

Ca

1

mg/gm dry weight

gm

1

None 0.25 0.50 0.75

39 33 31 27

+ + + +

1 1 1 1

141 132 113 116

+ + + +

4 2 4 4

92 87 85 84

2

None 0.25 0.50 0.75

87 76 67 61

+ + + +

2 4 2 2

151 143 122 114

+ + + +

1 5 8 3

92 + 1 90+1 84 + 2 81 + 2

3

None 0.25 0.50 0.75

134 121 107 88

+ + + +

3 4 2 4

156 145 142 126

+ + + +

6 6 7 7

94 89 89 84

+ + + +

+ + + +

1 2 4 2

1 1 1 1

Note: Sprague-Dawley r a t s a v e r a g i n g 49 gm i n weight i n i t i a l l y . Each v a l u e i s the mean + SEM. *Mean o f 16, 12, and 8 a n i m a l s f o r weeks 1, 2, and 3, r e s p e c t i v e l y . ^Means o f 4 a n i m a l s . Source: Reproduced w i t h p e r m i s s i o n from R e f . 7. C o p y r i g h t 1964 J . N u t r . , American I n s t i t u t e o f N u t r i t i o n .

Table

I I . E f f e c t s o f Z i n c on Growth and Bone M i n e r a l i z a t i o n o f Young Rats

Zinc Supplement

% None 0.25 0.50 0.75

Weight Gain at 4 Weeks

gm 188 + 197 + 161 + 113 +

8 6 6 9

Ca

Bone Constituent Ρ

mg/gm dry weight 146 + 8 144 + 6 131 +10 107 + 9

72 + 70 + 66 + 61 +

Note: Sprague-Dawley r a t s a v e r a g i n g 53 gm i n weight i n i t i a l l y . Each v a l u e i s t h e mean o f 6 animals + SEM. Source: Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1964 J . N u t r . , American I n s t i t u t e o f N u t r i t i o n .


2 2 1 3

168

N U T R I T I O N A L BIOAVAILABILITY O F C A L C I U M T a b l e I I I . E f f e c t s of C a l c i u m , Phosphorus , and Z i n c Supplements on Growth and Bone M i n e r a l i z a t i o n of Young Rats

Zn

Supplement Ρ Ca

%


0

4 Weeks Weight G a i n gm

1

Bone Constituent^ Ρ Ca m l/& il dry weight

0

0

182 + 8

163 + 9

87 + 4

0.75

0

0

97 + 8

111 + 5

77 + 6

0.75

0.4

0

126 + 9

134 + 8

81 + 1

0.75

0

0.4

106 + 9

109 + 8

75 + 3

0.75

0.4

0.4

154 + 5

158 + 8

88 + 5

Note: Sprague-Dawley r a t s a v e r a g i n g 49 gm i n weight i n i t i a l l y . Êach f i g u r e i s the mean o f 6 a n i m a l s + SEM. Êach f i g u r e i s the mean o f 4 a n i m a l s + SEM. Source: Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1964 J . N u t r . , American I n s t i t u t e of N u t r i t i o n .

n i f i c a n t l y higher than those of rats fed only zinc. Although sup plements of 0.4% phosphorus did not prevent the marked decreases i n bone calcium associated with the addition of zinc to the diet, the combination of phosphorus with calcium resulted i n increases i n bone c a l c i u m l e v e l s g r e a t e r than those observed i n the z i n c - f e d r a t s receiving only calcium supplements. The bone calcium levels of the z i n c - f e d r a t s supplemented w i t h both c a l c i u m and phosphorus were e s s e n t i a l l y the same as the bone c a l c i u m l e v e l s of the c o n t r o l animals. A d d i t i o n a l c a l c i u m appeared to improve bone phosphorus l e v e l s of r a t s fed the high z i n c d i e t , but phosphorus supplements appeared to have l i t t l e e f f e c t on bone phosphorus levels, except i n the presence of additional calcium. Table IV shows data from another experiment i n which the levels of c a l c i u m and phosphorus supplements were 0.4%, 0.8%, and 1.2%. Supplements of calcium resulted i n s i g n i f i c a n t improvements i n bone calcium levels of zinc-fed rats, whereas phosphorus supplements had l i t t l e effect on bone calcium levels i n zinc-fed rats. Increasing the c a l c i u m supplement from 0.4% to 0.8% r e s u l t e d i n a d d i t i o n a l i n c r e a s e s i n bone c a l c i u m d e p o s i t i o n , and a l e v e l of 1.2% added calcium resulted i n the same increase i n bone calcium levels of rats fed 0.75% z i n c as d i d the 0.8% c a l c i u m supplement. Mean bone c a l cium values of zinc-fed rats supplemented with either 0.4% calcium and phosphorus or 0.8% calcium and phosphorus were e s s e n t i a l l y the same as mean bone calcium values of zinc-fed rats supplemented with e i t h e r 0.8% or 1.2% c a l c i u m . A combination supplement of 1.2% c a l c i u m and phosphorus was found to c o m p l e t e l y a l l e v i a t e the de crease i n bone calcium deposition associated with the feeding of a 0.75% l e v e l of zinc. Calcium supplements p a r t i a l l y a l l e v i a t e d the decrease i n bone phosphorus l e v e l associated with the feeding of the


14.

169


M A G E E

T a b l e IV. E f f e c t s of C a l c i u m , Phosphorus, and Z i n c Supplements on Growth and Bone M i n e r a l i z a t i o n of Young Rats

Zn

Supplement Ρ Ca

4 Weeks Weight Gain


%

1

Bone Constituent? Ρ Ca mg/gm dry weight

gm

0

0

0

148 + 5

163 + 5

83 + 2

0.75

0

0

104 + 9

116

73 + 1

0.75

0.4

0

124 + 4

132 + 5

79 + 3

0.75

0

0.4

113 + 3

118

74 + 2

0.75

0.4

0.4

131 + 3

147 + 7

75 + 2

0.75

0.8

0

131 + 2

149 t

80 + 1

0.75

0

0.8

116 + 5

125 + 3

70 + 4

0.75

0.8

0.8

132 + 6

147

80 + 2

0.75

1.2

0

120 + 4

147 + 3

78+1

0.75

0

1.2

119 + 3

126

73 + 2

1.2

115 + 4

157 t

0.75

±

±

±

±

6

4

4

4

4

4

80 + 2

Note: Sprague-Dawley r a t s a v e r a g i n g 47 gm i n weight i n i t i a l l y . 1 E a c h f i g u r e i s the mean of 6 a n i m a l s + SEM. Êach f i g u r e i s the mean o f 4 a n i m a l s + SEM. Source: Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1964 J . N u t r . , American I n s t i t u t e of N u t r i t i o n . 1

high l e v e l of z i n c , but phosphorus supplements had no apparent b e n e f i c i a l e f f e c t on bone phosphorus d e p o s i t i o n i n the z i n c - f e d r a t s . A combination of e i t h e r 0.8% c a l c i u m and phosphorus or 1.2% calcium and phosphorus was associated with bone phosphorus l e v e l s approaching those observed i n the non-zinc supplemented rats. The e f f e c t of z i n c t o x i c i t y on the c a l c i u m balance of young r a t s i s presented i n Table V. Marked i n c r e a s e s i n the f e c a l ex cretion of calcium was noted i n the zinc-fed rats, and the o v e r a l l effect of zinc t o x i c i t y was a substantial decrease i n the apparent retention of calcium. The effects of zinc on calcium retention were noted as early as one week of the experimental period. Table VI shows the e f f e c t of 0.75% d i e t a r y z i n c on the phos phorus balance i n young rats. A decrease In the apparent retention of phosphorus was noted i n the zinc-fed rats as early as the end of the f i r s t week. P o s s i b l y a more s i g n i f i c a n t o b s e r v a t i o n was the apparent movement of phosphorus excretion from the urine, the normal pathway for phosphorus excretion, to the feces l a rats fed the high zinc diet. Such a s h i f t i n g of the phosphorus excretion to the f e c a l pathway i n animals fed a high zinc diet should r e s u l t i n an increase


170

N U T R I T I O N A L

B I O A V A I L A B I L I T Y

O F

C A L C I U M

Table V. E f f e c t of Zinc Toxicity on Calcium Retention i n Young Rats


Weeks on Experiment

Zinc Supplement

"~'~~Ca Intake

Fecal Ca

Urinary Ca

Ca Retention

mg/day

mg/day

%

mg/day

mg/day

1

0 0.75

48.5 45.7

16.5 31.1

0.1 0.3

31.9 14.3

2

0 0.75

56.2 51.3

21.1 45.0

0.1 0.2

35.0 6.1

3

0 0.75

58.5 54.2

27.4 40.3

0.2 0.3

30.9 13.6

4

0 0.75

67.4 57.9

34.1 48.5

0.3 0.3

33.0 9.1

Note: Each f i g u r e i s the mean o f 4 a n i m a l s . Source: R e p r i n t e d w i t h p e r m i s s i o n from R e f . 7. J . N u t r . , American I n s t i t u t e o f N u t r i t i o n .

Table VI.

Weeks on Experiment

E f f e c t of Z i n c T o x i c i t y on Young Rats

C o p y r i g h t 1964

Phosphorus

YecâT

Retention i n

Zinc Supplement

P Intake

%

mg/day

mg/day

mg/day

mg/day

Ca

Urinary Ρ

Ρ Retention

1

0 0.75

63.5 58.0

8.0 21.0

14.8 7.5

40.7 29.5

2

0 0.75

73.7 65.1

8.6 28.6

20.4 14.8

44.7 21.7

3

0 0.75

76.9 68.8

11.7 21.6

23.6 14.1

41.6 33.1

4

0 0.75

88.4 73.5

14.9 26.3

26.7 14.4

46.8 32.8

Note: Each f i g u r e i s the mean o f 4 a n i m a l s . Source: Reproduced w i t h p e r m i s s i o n from R e f . 7. J . N u t r . , American I n s t i t u t e o f N u t r i t i o n .

C o p y r i g h t 1964



14.

MAGEE

171


i n the amount of phosphorus i n the lower r e g i o n of the g a s t r o i n t e s t i n a l t r a c t and subsequently provide a favorable environment f o r the f o r m a t i o n of i n s o l u b l e calcium-phosphate s a l t s . I f t h i s does occur, as these data suggest, the subsequent l o s s of c a l c i u m from the system or the decrease i n the b i o a v a i l a b i l i t y due to zinc t o x i c i t y may be the result of phosphorus status i n the system. Zinc supplementation may exert a major influence i n f a c i l i t a t i n g a s h i f t i n the phosphorus excretion pathway. Table VII shows the calcium balance of zinc-fed and non-zincfed rats supplemented with 0.8% calcium and/or phosphorus. Marked increases i n f e c a l calcium and corresponding decreases i n apparent c a l c i u m r e t e n t i o n s i n the z i n c - f e d r a t s c o u l d be r e v e r s e d w i t h c a l c i u m supplementation. Phosphorus supplements appeared to be a s s o c i a t e d w i t h i n c r e a s e s i n c a l c i u m r e t e n t i o n i n the absence of z i n c , but decreases i n c a l c i u m r e t e n t i o n i n the presence of z i n c without calcium supplementation. Decreases i n f e c a l calcium were noted i n animals fed c a l c i u m supplements i n the presence of phos phorus or zinc. High levels of zinc were associated with increases i n f e c a l calcium excretion i n the absence of extra calcium or i n the presence of extra phosphorus. Calcium supplementation was generally a s s o c i a t e d w i t h a decrease i n the u r i n a r y e x c r e t i o n of c a l c i u m , while zinc and phosphorus supplements were generally associated with an increase In urinary calcium excretion. Table V I I .

E f f e c t s of Calcium, Phosphorus, and Z i n c Supplements on C a l c i u m R e t e n t i o n i n Young Rats

Ca Intake

Diet

mg/day Basal

+ 0.8% Ca + 0.8% Ρ + 0.8% Ca + 0.8% Ρ + + + +

0.75% Zn 0.75% Zn 0.75% Zn 0.75% Zn + 0.8%

+ 0.8% Ca + 0.8% Ρ + 0.8% Ca Ρ

Fecal Ca

Urinary Ca

Ca Retention

percentage of intake

66.9 288.1 82.0 292.2

58.7 49.9 43.0 30.7

0.7 0.4 1.9 0.7

40.6 49.7 55.1 68.4

61.4 208.2 79.4

91.0 58.1 107.0

1.3 0.2 2.4

8.1 41.5 -9.4

232.6

56.8

0.5

42.7

Note: Each f i g u r e i s the mean o f 4 a n i m a l s . Source: : Reproduced w i t h p e r m i s s i o n from Ref. 8. J . N u t r . , American I n s t i t u t e o f N u t r i t i o n .

Copyright

1964

Table V I I I shows the phosphorus balance of z i n c - f e d and nonzinc-fed rats supplemented with 0.8% calcium and/or phosphorus. A d i e t a r y l e v e l of 0.75% z i n c was a s s o c i a t e d w i t h decreases i n the apparent r e t e n t i o n of phosphorus, and these data support the findings of Sadasivan (2-4). Calcium supplements were a s s o c i a t e d w i t h s l i g h t i n c r e a s e s i n the r e t e n t i o n of phosphorus, w h i l e phos-



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NUTRITIONAL

BIOAVAILABILITY O F C A L C I U M

phorus s u p p l e m e n t s were a s s o c i a t e d w i t h d e c r e a s e s i n p h o s p h o r u s retention. C a l c i u m and z i n c supplements were a s s o c i a t e d w i t h i n c r e a s e s i n f e c a l phosphorus e x c r e t i o n , and p h o s p h o r u s s u p p l e m e n t s were a s s o c i a t e d w i t h i n c r e a s e s i n the u r i n a r y e x c r e t i o n of phos phorus. I n t e r p r e t a t i o n o f s i g n i f i c a n t Ca χ Zn and Ca χ Ρ χ Zn i n t e r a c t i o n s s u g g e s t e d a c o m b i n a t i o n of c a l c i u m p l u s z i n c r e s u l t e d i n a d d i t i o n a l i n c r e a s e s i n f e c a l phosphorus l e v e l s , w h i l e the p r e s e n c e of phosphorus appeared to moderate the e f f e c t s of c a l c i u m or z i n c on f e c a l phosphorus e x c r e t i o n . A s i g n i f i c a n t Ρ χ Zn i n t e r a c t i o n s u g g e s t e d t h a t z i n c r e s u l t e d i n an i n c r e a s e i n u r i n a r y phos phorus e x c r e t i o n i n t h i s p a r t i c u l a r e x p e r i m e n t i n the absence of p h o s p h o r u s and a d e c r e a s e i n u r i n e p h o s p h o r u s e x c r e t i o n i n t h e p r e s e n c e of phosphorus. Table V I I I .

E f f e c t s of C a l c i u m , P h o s p h o r u s , and Z i n c on P h o s p h o r u s R e t e n t i o n i n Y o u n g R a t s -

ρ

Diet

Intake

mg/day Basal + 0.8% + 0.8% 0.8%

Ca Ρ Ca + 0.8%

+ 0.75% Zn + 0.75% Zn + 0.75% Zn 0.75% Zn + 0.8%

+ 0.8% + 0.8% + 0.8% Ρ

Ρ

Ca Ρ Ca

Fecal Ρ

Supplements

Urinary Ρ

Ρ Retention

percentage of intake

71.4 75.2 176.4 190.0

20.0 57.5 12.4 26.9

28.3 3.6 55.1 38.6

51.7 51.7 32.4 34.5

60.7 74.1 164.4

57.2 60.2 29.3

28.6 12.5 50.5

14.2 27.6 20.2

187.4

38.4

32.5

29.1

E a c h f i g u r e i s t h e mean o f 4 an ima1 s. Note : R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 8. Source : J . N u t r . , American I n s t i t u t e of N u t r i t i o n .

Copyright

1964

The d a t a p r e s e n t e d i n t h i s p a p e r i n d i c a t e t h a t e x c e s s l e v e l s (0.75%) o f d i e t a r y z i n c r e s u l t i n d e c r e a s e s i n the b i o a v a i l a b i l i t y o f c a l c i u m and p h o s p h o r u s i n r a t s and i n t e r f e r e w i t h n o r m a l bone mineralization. H i g h d i e t a r y l e v e l s of c a l c i u m or z i n c appeared to cause a s h i f t i n the e x c r e t i o n of phosphorus from the u r i n e to the f e c e s , w h i l e the p r e s e n c e of e x t r a phosphorus tended to keep the pathway of phosphorus e x c r e t i o n v i a the u r i n e . The p r e s e n c e o f l a r g e a m o u n t s o f p h o s p h o r u s i n t h e i n t e s t i n a l t r a c t due t o high i n t a k e s o f z i n c would i n c r e a s e the p o s s i b i l i t y of the f o r m a t i o n of i n s o l u b l e phosphate s a l t s w i t h v a r i o u s c a t i o n s , i n c l u d i n g c a l c i u m , w h i c h may be p r e s e n t . A s h i f t i n p h o s p h o r u s e x c r e t i o n f r o m the f e c e s to the u r i n e , however, c o u l d r e s u l t i n an e n v i r o n m e n t a l con d i t i o n w i t h i n the system w h i c h would tend to i n c r e a s e the b i o a v a i l a b i l i t y of c a t i o n s to the a n i m a l . The a d v e r s e e f f e c t o f z i n c t o x i c i t y on c a l c i u m and p h o s p h o r u s s t a t u s o f y o u n g r a t s c o u l d be a l l e v i a t e d w i t h c a l c i u m and/or phosphorus supplements.


14.

MACÏI I

Influence of Zn on Ca

Bioavailability

173

Acknowledgments


E x p e r i m e n t s r e v i e w e d i n t h i s paper were conducted i n c o n n e c t i o n w i t h p r o j e c t s f i n a n c e d by the N o r t h C a r o l i n a A g r i c u l t u r a l R e s e a r c h S e r v i c e , North C a r o l i n a State U n i v e r s i t y , R a l e i g h , North C a r o l i n a . P a p e r No. 9305 o f t h e J o u r n a l S e r i e s o f t h e N o r t h Carolina A g r i c u l t u r a l Research Service. The u s e o f t r a d e names i n t h i s p u b l i c a t i o n d e e s n o t i m p l y e n d o r s e m e n t by t h e N o r t h C a r o l i n a A g r i c u l t u r a l R e s e a r c h S e r v i c e o f p r o d u c t s named, nor c r i t i c i s m of s i m i l a r ones n o t m e n t i o n e d . The a u t h o r i s i n d e b t e d t o M r s . B e t t y Weaver f o r a s s i s t a n c e w i t h the p r e p a r a t i o n of t h i s paper.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Sutton, W. R.; Nelson, V. E. Proc. Soc. Exptl. Biol. Med. 1937, 36, 211-213. Sadasivan, V. Biochem. J. 1951, 48, 527-530. Sadasivan, V. Biochem. J. 1951, 49, 186-191. Sadasivan, V. Biochem. J. 1952, 52, 452-455. Whiting, F.; Bezeau, L. M. Canadian J. Animal Sci., 1958, 38, 109. Thompson, Α.; Hansard, S. L.; Bell, M. C. J. Animal Sci. 1959, 18, 187. Stewart, A. K.; Magee, A. C. J. Nutrition 1964, 82, 287-295. Magee, A. C.; Fu, S. C. Nutr. Reports Inter. 1979, 19, 343351.

RECEIVED October 15, 1984


Nutritional Bioavailability of Calcium - ACS Publications - American

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