S. J. RITCHEY and L. JANETTE TAPER
Virginia Polytechnic Institute and State University, Department of Human Nutrition and Foods, Blacksburg, VA 24061
The u t i l i z a t i o been studied i n severa
including preadolescent children, young adult women, pregnant women, and elderly adults.
Studies, ranging i n length from 18 to 42 days, were conducted under controlled conditions, including regulation of food and water intake, collection of all excreta, and careful monitoring of a c t i v i - ties of subjects. Experimental diets were comprised of commonly consumed foods with supple- ments of zinc and/or other nutrients to achieve
the objective of the study or to obtain desired intakes of critical nutrients. Apparent absorp- tion of zinc ranged from about 2 percent i n elderly adults to about 60 percent i n one group of children. However, the usual range of apparent absorption was between 20 and 30 percent of intake.
Average zinc excretion through feces, as a per- centage of intake, ranged from 73.0 percent i n preadolescent children to 97.4 percent i n the elderly adults. Urinary zinc was between 0.3 and 0.4 mg/day for most subjects. In the elderly, urinary excretion was about 0.8 mg/day. These studies provide the basis for estimating losses of zinc i n relation to dietary intake and for assessing physiological needs of human subjects.
During the past several years we have investigated the u t i l i z a t i o n of zinc i n several different age groups of human subjects. Work with growing préadolescent children, young adults, pregnant women, and elderly adults has been reported. Our goal through this work has been to provide reasonable estimates of zinc needs of the human at different ages and during normal physiological events. This paper i s not intended to be a compre- hensive review, but focuses on work accomplished at laboratories within this i n s t i t u t i o n .
0097-615 6/ 83/0210-0107$06.00/0
© 1983 American Chemical Society
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
E x p e r i m e n t a l Approach
The balance study technique has been used i n a l l o f these s t u d i e s . Although t h e r e a r e d i f f i c u l t i e s and c e r t a i n problems w i t h balance s t u d i e s , n u t r i t i o n r e s e a r c h e r s have n o t y e t found a r e a s o n a b l e and r e a l i s t i c a l t e r n a t i v e t o e s t i m a t e n u t r i e n t needs of human s u b j e c t s . Continued development and i n c r e a s e d s o p h i s t i - c a t i o n o f methods and a n a l y t i c a l techniques may p r o v i d e a l t e r n a - t i v e s i n t h e f u t u r e , but these a r e n o t c l e a r l y a v a i l a b l e a t the p r e s e n t time.
Throughout our s t u d i e s we employed t h e same g e n e r a l approach and t e c h n i q u e s . We p l a c e d c o n s i d e r a t i o n emphasis on t h e e n l i s t - ment o f c o o p e r a t i v e s u b j e c t s who have an a p p r e c i a t i o n f o r t h e
r e s e a r c h and who understand the need f o r complete adherence t o the g u i d e l i n e s o f t h e s p e c i f i
the need f o r s t r i c t c o n t r o
n a t i o n , an a b s o l u t e e s s e n t i a l f o r s t u d i e s w i t h the t r a c e
m i n e r a l s . We were s u c c e s s f u l i n both of these concerns w i t h minor d e v i a t i o n s .
The g e n e r a l approach t o our s t u d i e s has been d e s c r i b e d i n d e t a i l ( 1 ) . We u t i l i z e d an adjustment p e r i o d r a n g i n g from f i v e to t e n days dependent upon t h e s p e c i f i c experiment. Commonly consumed foods were used t o develop a s e r i e s o f menus ( T a b l e. . I ) » u s u a l l y t h r e e t o s i x , t o a v o i d boredom and because we wanted t o have r e s u l t s as c l o s e l y r e p r e s e n t a t i v e o f the g e n e r a l p o p u l a t i o n as f e a s i b l e . Menus were developed and t e s t e d , key n u t r i e n t s were a n a l y z e d f o r , and m o d i f i c a t i o n s were made as n e c e s s a r y p r i o r to the b e g i n n i n g o f an experiment. Supplements o f n u t r i e n t s were u t i l i z e d t o a c h i e v e the d e s i r e d e x p e r i m e n t a l i n t a k e s and/or t o meet the d a i l y recommended a l l o w a n c e s o f o t h e r e s s e n t i a l n u t r i e n t s
(3) f o r the p a r t i c u l a r age group. U r i n e and f e c a l e x c r e t i o n s were c o l l e c t e d throughout the study. A p p r o p r i a t e composites of food and e x c r e t a were prepared and samples were taken f o r a n a l y t i c a l work. R e t e n t i o n , apparent a b s o r p t i o n , and o t h e r c a l c u l a t i o n s were based on t h e e n t i r e study except t h e adjustment p e r i o d , a l t h o u g h i n some s t u d i e s the c a l c u l a t i o n s were made f o r d i s c r e t e p e r i o d s w i t h i n t h e experiment when t h e r e appeared t o be changes i n n u t r i e n t r e t e n t i o n s as a f u n c t i o n o f an e x p e r i m e n t a l treatment.
D i e t a r y P r o t e i n and Z i n c
Z i n c r e c e i v e d a t t e n t i o n i n our i n s t i t u t i o n as e a r l y as 1956 i n s t u d i e s w i t h préadolescent c h i l d r e n . A s e r i e s o f experiments were done under c o m p l e t e l y c o n t r o l l e d c o n d i t i o n s (1) w i t h t h e g o a l of d e f i n i n g n u t r i e n t needs o f t h e growing c h i l d . S e v e r a l i n v e s t i g a t o r s , r e p r e s e n t i n g a g r i c u l t u r a l experiment s t a t i o n s i n the southern r e g i o n and working under t h e r e g i o n a l r e s e a r c h concept, p a r t i c i p a t e d i n these s t u d i e s . Engel e t a l . (4) and
7. RiTCHEY A N D T A P E R Zinc Utilization by Humans 109
T a b l e I . Example menu p a t t e r n used i n human s t u d i e s .3
B r e a k f a s t
Orange j u i c e 122 Bran f l a k e s 28 Toast 23 M a r g a r i n e 5
C o f f e e 2 M i l k 122 Sugar 7
Lunch
Tuna s a l a d 60 L e t t u c e 26 C r a c k e r s 11 Apple dumpling 75
Punch 244 Mayonnaise 14 Dinner
Turkey b r e a s t 90 P o t a t o e s 100 Green beans 100 Cranberry j e l l y 40
R o l l 26 Margarine 10 F r u i t c o c k t a i l 100
C o f f e e 2 Sugar 7 Punch, f r u i t 244
Graham c r a c k e r s 28
Apple j u i c e 122 Bran f l a k e s 28 Hard cooked egg 44
Toast 23 Margarine 10
2 Sugar 7
Cottage cheese 90 Tomato wedges 75 L e t t u c e 26 C r a c k e r s 11 Cantaloupe 100 Lemonade 244
Baked f i s h 90 Baked p o t a t o 100
Peas 50 C a r r o t s 50 R o l l 26 Margarine 10 Sherbet 100 C o f f e e 2 Sugar 7 Lemonade 244 Graham c r a c k e r s 28
^ i s t o f menu items and w e i g h t s ( g ) . Analyzed v a l u e s f o r key n u t r i e n t s were done p r i o r t o and throughout t h e experiment.
Z i n c c o n t e n t s were 6.10 and 5.98 mg, r e s p e c t i v e l y . The mean p r o t e i n content was 56 g and the mean energy c o n t e n t was 2000 k c a l s . Adapted from ( 2 ) .
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
P r i c e e t a l . (5) measured t h e r e t e n t i o n o f z i n c and o t h e r minor elements i n s t u d i e s i n which t h e major v a r i a b l e s were source and l e v e l o f d i e t a r y p r o t e i n . Comparison o f p r o t e i n sources from p l a n t and mixed sources i n d i c a t e d t h a t t h e apparent a b s o r p t i o n was somewhat lower from the d i e t s w i t h o n l y p l a n t p r o t e i n s than t h a t from mixed sources. A lower l e v e l o f p r o t e i n (25 g/day) caused a lower r e t e n t i o n of z i n c than t h e moderate p r o t e i n (46 g/day) d i e t , a l t h o u g h c o n c l u s i o n s were c o m p l i c a t e d because o f o t h e r v a r i a b l e s .
I n a r e c e n t study (6) w i t h young a d u l t s , the e f f e c t o f l e v e l of d i e t a r y p r o t e i n on z i n c u t i l i z a t i o n was examined. Two l e v e l s of p r o t e i n , 49.2 and 94.9 g/day, and two l e v e l s o f z i n c , 9.78 and 19.20 mg/day, were v a r i a b l e s i n a 24-day study. Z i n c r e t e n t i o n s were n o t s i g n i f i c a n t l y a f f e c t e d by the l e v e l o f p r o t e i n . However, u r i n a r y e x c r e t i o n o f z i n s i g n i f i c a n t l ( < 0.05) h i g h e f o s u b j e c t s consuming t h
d u r i n g t h e l a s t 6-day p e r i o q u e s t i o appro p r i a t e l e n g t h o f human b a l a n c e s t u d i e s . The v e r y h i g h f e c a l e x c r e t i o n s f o r s u b j e c t s on the higher p r o t e i n , h i g h z i n c t r e a t - ment may have been a o d d i t y . These e x c r e t i o n s r e s u l t e d i n a n e g a t i v e n e t r e t e n t i o n and may be m i s l e a d i n g i n our e v a l u a t i o n of these d a t a .
Reports from other l a b o r a t o r i e s have c o n f l i c t e d w i t h regard to the e f f e c t s o f p r o t e i n on z i n c u t i l i z a t i o n . Greger and
Snedeker (7) r e p o r t e d t h a t apparent a b s o r p t i o n o f z i n c was a f f e c t e d by the l e v e l o f d i e t a r y p r o t e i n , phosphorus i n t a k e and the i n t e r a c t i o n between p r o t e i n and phosphorus. Apparent r e t e n - t i o n o f z i n c was h i g h e s t w i t h t h e h i g h p r o t e i n , moderate
phosphorus d i e t compared t o other t r e a t m e n t s , i n c l u d i n g low p r o - t e i n and e i t h e r low o r moderate phosphorus v a r i a b l e s . U r i n a r y z i n c was h i g h e r i n s u b j e c t s consuming t h e h i g h p r o t e i n d i e t s . Based on a l e n g t h y m e t a b o l i c study w i t h men, Sandstead e t a l . (8) have r e p o r t e d t h a t t h e d i e t a r y requirement f o r z i n c i n c r e a s e d a s the l e v e l o f p r o t e i n i n t a k e i n c r e a s e d . A l l o f these i n v e s t i g a - t i o n s suggest t h a t a h i g h e r amount o f p r o t e i n i n the d i e t r e s u l t s i n poorer u t i l i z a t i o n and/or i n c r e a s e d m e t a b o l i c need f o r z i n c . The s i t u a t i o n may prove t o be s i m i l a r t o t h a t f o r other m i n e r a l s , such as c a l c i u m .
O r a l C o n t r a c e p t i v e Use and Pregnancy i n A d u l t Women
A l t e r a t i o n s i n blood l e v e l s of z i n c d u r i n g pregnancy and d u r i n g t h e use o f o r a l c o n t r a c e p t i v e agents a r e w e l l documented
(9, 1 0 ) . However, the u t i l i z a t i o n and need f o r z i n c d u r i n g pregnancy and when women a r e u s i n g hormonal c o n t r a c e p t i v e agents have r e c e i v e d l i t t l e a t t e n t i o n . U t i l i z a t i o n o f z i n c d u r i n g t h e use o f o r a l c o n t r a c e p t i v e s was n o t a f f e c t e d (11). C o n t r a c e p t i v e u s e r s were compared t o c o n t r o l s (non-users) i n an 18-day balance study. R e t e n t i o n s and f e c a l e x c r e t i o n s were s i m i l a r f o r t h e two groups - b o t h groups were i n n e g a t i v e balance on a mean z i n c
7. RiTCHEY A N D T A P E R Zinc Utilization by Humans 111 i n t a k e of 9.12 mg/day. Apparent a b s o r p t i o n s c o u l d not be c a l c u - l a t e d s i n c e f e c a l e x c r e t i o n s exceeded i n t a k e . U r i n e e x c r e t i o n s were not d i f f e r e n t f o r the treatments.
A r e c e n t study (12) focused on the z i n c needs d u r i n g the second t r i m e s t e r of pregnancy. T h i s study was d i f f e r e n t from most i n our l a b o r a t o r y because we p e r m i t t e d s u b j e c t s to consume
t h e i r normal d i e t s , thus the b a l a n c e and e x c r e t i o n d a t a r e p r e s e n t a range of i n t a k e s . We found z i n c i n t a k e s f a l l i n g a t two l e v e l s , 6.7 and 22.0 mg/day. The h i g h e r i n t a k e s r e s u l t e d from supplements of z i n c . We measured r e t e n t i o n s i n two weeks d u r i n g the 4.5 month to the 5.5 month of pregnancy. The study d e s i g n r e s u l t e d
i n v a r y i n g l e v e l s of i n t a k e and r e t e n t i o n s , p a r t i c u l a r l y f o r the lower, non-supplemented group of women. Both groups of s u b j e c t s were i n n e t p o s i t i v e b a l a n c e , a l t h o u g h the lower l e v e l was mar- g i n a l a t b e s t when a l l p o t e n t i a z i n
f o r i n e s t i m a t i n g r e t e n t i o n s
from 9.6 to 23.79 p e r c e n t w i t h no e f f e c t r e s u l t i n g from l e v e l of d i e t a r y i n t a k e .
Taper e t a l . (13) i n v e s t i g a t e d the e f f e c t s of z i n c i n t a k e on copper u t i l i z a t i o n i n a d u l t women. The experiment i n v o l v e d f e e d - i n g t h r e e l e v e l s of z i n c , r a n g i n g from 8.0 to 24.0 mg/day. Copper r e t e n t i o n was not a f f e c t e d by z i n c i n t a k e . U r i n e l o s s e s of z i n c were from 0.4 to 0.6 mg/day and f e c a l e x c r e t i o n s p a r a l l e l e d the
i n t a k e . P e r c e n t apparent a b s o r p t i o n s of z i n c were 7.75, 9.13, and 11.50 percent when i n t a k e s were 8.0, 16.0 and 24.0 mg/day.
U t i l i z a t i o n i n Growing C h i l d r e n
I n a d d i t i o n to the study of Engel e t a l . (4) and P r i c e e t a l . ( 5 ) , two o t h e r experiments have been conducted i n our l a b o r a - t o r i e s . Examining the e f f e c t of z i n c supplementation on p r o t e i n u t i l i z a t i o n i n the growing c h i l d , Meiners e t a l . (14) f e d z i n c l e v e l s of e i t h e r 5.5 mg/day, the amount p r e s e n t i n d i e t s t y p i c a l of low-income p o p u l a t i o n s i n the Southern r e g i o n , or 10.5 mg/day.
R e t e n t i o n s were b o r d e r l i n e e q u i l i b r i u m f o r s u b j e c t s on the lower i n t a k e , but were p o s i t i v e f o r those c h i l d r e n consuming 10.5 mg/
day. U r i n e e x c r e t i o n s were i n the range of 0.45 to 0.79 mg/day.
Losses were h i g h e r than f o r most a d u l t s t u d i e s i n our l a b o r a t o r y . The apparent a b s o r p t i o n s ranged from 7.1 t o 21.2 p e r c e n t .
Préadolescent g i r l s were p r o v i d e d z i n c l e v e l s r a n g i n g from 5.61 t o 14.61 mg/day to e v a l u a t e the needs f o r z i n c by t h i s age group (15). P e r c e n t apparent a b s o r p t i o n ranged from 26.1 to 43.7 p e r c e n t w i t h most s u b j e c t s i n the more narrow range of 25 t o 30 p e r c e n t .
Losses o f Z i n c
The c o n s i d e r a t i o n of z i n c u t i l i z a t i o n i s i n l a r g e measure an assessment of z i n c l o s s e s or e x c r e t i o n under any g i v e n s e t of c i r c u m s t a n c e s , i n c l u d i n g d i e t a r y v a r i a b l e s , p h y s i o l o g i c a l s t a t e ,
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
p a t h o l o g i c a l i n s u l t , s t r e s s , and other c o n d i t i o n s which may a f f e c t u t i l i z a t i o n . Major r o u t e s o f z i n c e x c r e t i o n a r e f e c a l and u r i n a r y , w i t h p o t e n t i a l l o s s e s through sweat, and p r o b a b l y some s l i g h t l o s s e s through o t h e r r o u t e s . S t u d i e s from our l a b o r a - t o r i e s and from o t h e r s a r e now such t h a t we can b e g i n t o have some g e n e r a l n o t i o n o f l e v e l s o f e x c r e t i o n by human s u b j e c t s and can p r e d i c t i n a g e n e r a l f a s h i o n the u t i l i z a t i o n o f z i n c .
U r i n a r y l o s s e s o f z i n c r e p r e s e n t s m a l l , and a p p a r e n t l y f a i r l y c o n s t a n t amounts. U s i n g t h e d a t a from s e v e r a l s t u d i e s i n our l a b o r a t o r y , we made c a l c u l a t i o n s o f average l o s s e s by groups of s u b j e c t s (Table I I ) . U r i n e e x c r e t i o n was u s u a l l y i n the range of 0.3 t o 0.4 mg/day w i t h c o n s i d e r a b l e v a r i a b i l i t y around those average v a l u e s . However, we have a reasonable b a s i s f o r suggest- i n g t h a t u r i n a r y l o s s e s w i l l be c o n s i s t e n t l y w i t h i n t h e 0.3 t o 0.5 mg/day range when s u b j e c t consumin commonl d foods From our d a t a , e l d e r l
g e n e r a l i z a t i o n , but t h e r t h i s p o o l o f d a t a .
F e c a l e x c r e t i o n s were c a l c u l a t e d as a percentage o f i n t a k e s i n c e t h i s l o s s almost always i n our e x p e r i e n c e c l o s e l y f o l l o w s i n t a k e (Table I I ) . Percentages o f i n t a k e e x c r e t e d through f e c e s ranged from 73.0 i n t h e préadolescent c h i l d t o 97.4 i n t h e e l d e r l y a d u l t s . D i e t a r y f a c t o r s may have more impact on the l o s s o f z i n c through t h e g a s t r o i n t e s t i n a l t r a c t , thus a wide range o f e x c r e - t i o n s from study t o study might be a n t i c i p a t e d . I t i s i n t e r e s t - i n g t h a t t h e lowest percentages o c c u r i n those groups t h a t would have t h e h i g h e r p h y s i o l o g i c a l needs f o r z i n c , préadolescent c h i l d r e n and pregnant women.
Losses o f z i n c through sweat can be s i g n i f i c a n t , p a r t i c u l a r l y i n h o t , humid c l i m a t e s . Data from a few s t u d i e s , i n c l u d i n g those of Jacob e t a l . (16) and R i t c h e y e t a l . (15), suggest t h a t
l o s s e s through t h i s r o u t e a r e i n t h e range from 0.5 t o 1.0 mg/day, perhaps somewhat h i g h e r i n t r o p i c a l a r e a s .
Summary
I n most o f t h e i n d i v i d u a l s s t u d i e d i n our l a b o r a t o r y , z i n c i s absorbed w i t h i n t h e range o f 20 t o 30 p e r c e n t of i n t a k e , a l t h o u g h t h e r e a r e s i g n i f i c a n t d e v i a t i o n s from t h a t a p p a r e n t l y normal response. L o s s e s , i n c l u d i n g e x c r e t i o n s and sweat l o s s , from commonly consumed d i e t s can be p r e d i c t e d i n a g e n e r a l way a t the p r e s e n t time, thus u t i l i z a t i o n o f z i n c by human s u b j e c t s can be e s t i m a t e d under most circumstances. A d d i t i o n a l i n f o r m a t i o n and r e f i n e m e n t s a r e needed f o r some p o p u l a t i o n groups, such as the e l d e r l y and pregnant women.
7. R I T C H E Y A N D T A P E R Zinc Utilization by Humans 113
Table I I . E x c r e t i o n s o f z i n c through u r i n e and f e c e s by groups o f s u b j e c t s .
Feces No. of U r i n e (mg/day) (% of i n t a k e )
Group Subj e c t s Mean Range Mean Range Ref.
Préadolescent 70 0.38 0.20-0.79 73.0 56.3-92.9 (15) c h i l d r e n
A d u l t females 57 0.37 0.27-0.57 93.2 82.3-104.7 (6,11, 13) Pregnant 24 0.30 0.25-0.34 82.9 76.2-90.5 (12)
women
E l d e r l y 10 0.78 0.46-1.09 97.4 96.3-98.5 (2) a d u l t s
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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