The Role of Phytate in Zinc Bioavailability and Homeostasis
D. OBERLEAS
University of Kentucky, Department of Nutrition and Food Science, Lexington, K Y 40506
Phytate, myo i n o s i t o
is found i n all plant seeds, many roots and tubers.
I t s synthesis follows p o l l i n a t i o n and its content increases with maturity. The greatest concentration is found i n legume seeds, bran and germ of cereal grains. Small amounts are found i n many f r u i t s and vegetables except stem and leafy vegetables. Phytate has been shown to be associated with protein, as magnesium s a l t s i n sesame, and i s water soluble i n corn germ. Phytate complexes with divalent elements with varying degrees of tenacity. The s o l u b i l i t y of
these complexes varies with pH. Zinc phytate i s least soluble at pH 6 and i s less soluble than calcium or other mineral complexes at this pH.
Kinetic synergism of calcium and zinc with phytate causes a complexation less soluble than either separately. Saliva and pancreatic f l u i d secrete large quantities of zinc equivalent to as much as three times the dietary intake which i s also v u l - nerable to phytate complexation. The mechanism of phytate action i n the gastrointestinal tract i s related to complexation and subsequent prevention of absorption and reabsorption of z i n c . The complexa- t i o n can be equated to a phytate:zinc molar r a t i o and the relative hazard may be subsequently estimated from such data.
A compound later to be i d e n t i f i e d as myo-inositol hexakis
(dihydrogen phosphate) was f i r s t isolated from plant seeds by
Pfeffer i n 1872 (_1) (Figure 1). The relative concentration and
widespread d i s t r i b u t i o n i n plant seeds was f i r s t described by
Palladin (2) i n 1895 and identity as an i n o s i t o l compound i n 1897
by Winterstein (_3). Its identity as a hexaphosphate ester of
i n o s i t o l was confirmed early i n this century (4) and synthesis
was accomplished i n 1919 by Posternak 05). The biosynthesis of
146 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 Z I N C
Figure 1. 1,2,3,4,5,6-Hexakis(phosphonooxo)cyclohexane [phytate] (39).
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
p h y t a t e has been s t u d i e d i n c o r n (6^, 7) and peas (<8). P o l l i n a - t i o n was shown to supply the e s s e n t i a l zymogen a c t i v a t o r necessary f o r p h y t a t e f o r m a t i o n and the c o n c e n t r a t i o n i n c r e a s e d t o m a t u r i t y ,
P h y t a t e e x i s t s as d i f f e r e n t complexes i n d i f f e r e n t seeds.
I n corn, phytate i s contained p r i m a r i l y i n the germ i n a water s o l u b l e form ( 9 ) . Though i t seems u n l i k e l y t h a t such a s t r o n g a c i d would be f r e e , complexation w i t h potassium, f o r example, would render water s o l u b i l i t y w i t h o u t being i d e n t i f i a b l e by c u r r e n t methodology. A l s o complexation w i t h p r o t e i n s * which were e i t h e r water s o l u b l e or whose i s o e l e c t r i c p o i n t were above or below the pH of water would render s o l u b i l i t y . I n legumes, p h y t a t e has been shown t o be a s s o c i a t e d w i t h p r o t e i n ( 9 , 10).
T h i s a s s o c i a t i o n i s g r e a t e s t at the i s o e l e c t r i c p o i n t of the p r o t e i n and being r e a d i l y d i s s o c i a t e d at pH's above or below the i s o e l e c t r i c p o i n t (10
i s l e s s w e l l d e f i n e d bu
t i o n s both i n the bran and germ ( 1 2 ) . There appears to be a t l e a s t one f e r r i c i o n a s s o c i a t e d i n the o t h e r w i s e s o l u b l e p h y t a t e complex i n wheat bran ( 1 3 ) . P h y t a t e i n sesame seed appears t o be the most unique and l e a s t s o l u b l e of a l l seeds. I n t h i s case magnesium appears to be the predominant c a t i o n (9^)·
The e s s e n t i a l i t y of z i n c was f i r s t demonstrated i n 1869 by R a u l i n (14) u s i n g A s p e r g i l l u s n i g e r . Attempts by B e r t r a n d and Benzon i n 1922 (15) t o d e p r i v e mice of z i n c were i n c o n c l u s i v e and not u n t i l 1934 (16, 17) was z i n c considered e s s e n t i a l f o r a n i m a l s . These e a r l y s t u d i e s i n c o r p o r a t e d h i g h l y p u r i f i e d d i e t s c o n t a i n i n g l e s s than 2 mg z i n c per kg d i e t . The p r a c t i c a l s i g n i f i c a n c e of z i n c was not r e a l i z e d u n t i l 1955 when Tucker and Salmon (18) demonstrated t h a t supplemental z i n c would prevent or cure p o r c i n e p a r a k e r a t o s i s which was p r e v a l e n t at t h a t time. S h o r t l y t h e r e - a f t e r O ' D e l l and Savage (19) noted t h a t z i n c was l e s s a v a i l a b l e t o the c h i c k from p l a n t p r o t e i n s than animal p r o t e i n s . T h i s was q u i c k l y confirmed by s e v e r a l i n v e s t i g a t o r s (20 - 23).
P h y t a t e , a s s o c i a t e d w i t h legume seed p r o t e i n s , i s one b a s i c d i f f e r e n c e between p l a n t and animal p r o t e i n s . The a d d i t i o n of p h y t a t e to a c a s e i n - g e l a t i n d i e t f o r c h i c k s was shown t o produce the same symptoms of z i n c d e f i c i e n c y as d i d a soybean p r o t e i n d i e t ( 2 4 ) . T h i s i s i l l u s t r a t e d i n F i g u r e 2. T h i s work was extended to show a c a l c i u m , p h y t a t e , z i n c i n t e r r e l a t i o n s h i p ( 2 5 ) . S u p p o r t i n g o b s e r v a t i o n s have subsequently been made i n many s p e c i e s , i n c l u d i n g swine ( 2 6 ) , dogs ( 2 7 ) , Japanese q u a i l ( 2 8 ) , r a t s ( 2 9 ) , and rainbow t r o u t (30) though f r e q u e n t l y the observers were not aware of the r o l e of p h y t a t e i n d e v e l o p i n g the z i n c d e f i c i e n c y syndrome. A t y p i c a l growth curve f o r r a t s i s shown i n F i g u r e 3.
I n the f i r s t r e p o r t of c l i n i c a l z i n c d e f i c i e n c y i n humans (31, 32) the s i g n i f i c a n t d i e t a r y c o n s i d e r a t i o n , not f u l l y appre- c i a t e d at that time, was t h a t the v i l l a g e p o p u l a t i o n s u b s i s t e d p r i m a r i l y on unleavened whole wheat bread or bread and beans and v e r y l i t t l e animal p r o t e i n was consumed by t h i s p o p u l a t i o n .
148 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 Z I N C
Figure 2. Chick growth at 4 weeks as affected by phytate and supplemental zinc Basal diets contained 9 mg/kg zinc. (Reproduced with permission from Ref. 40.)
Figure 3. Growth curves of rats fed casein-based diets containing phytate and calcium. Each line represents the mean of 11 animals. Basal diets contained 6 mg/kg zinc. Key: , 0.8% Ca, 0% phytate; , 1.6% Ca, 0% phytate;
—, 0.8% Ca, 1% phytate; · · ·, 1.6% Ca, 1% phytate. (Reproduced with permission from Ref. 40.)
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Geophagia was a l s o r e p o r t e d f o r the s u b j e c t s i n I r a n and an a n a l y s i s of a sample of c l a y eaten by some of the s u b j e c t s i n d i c a t e d t h a t the c l a y c o n t a i n e d a very h i g h c o n c e n t r a t i o n of c a l c i u m .
Chemical R e l a t i o n s h i p s
The i n t e r e s t i n the chemical i n t e r r e l a t i o n s h i p s of p h y t a t e i n i t i a l l y c e n t e r e d around i t s complexation w i t h c a l c i u m and subsequent e f f e c t on the a v a i l a b i l i t y of c a l c i u m ( 3 3 ) . T h i s l e d to the c l a s s i c a l s t u d i e s by H o f f - J o r g e n s o n ( 3 4 ) . He determined e i g h t of the twelve d i s s o c i a t i o n constants of p h y t i c a c i d and u s i n g these data c a l c u l a t e d t h | < j s o l u b i l i t y products of penta- c a l c i u m p h y t a t e as between 10 and 10 . A decade l a t e r w i t h the use of n e u t r a l i z a t i o
i t was concluded t h a t p h y t i
which were completely d i s s o c i a t e d i n s o l u t i o n (pk = 1.84), 2 weak a c i d f u n c t i o n s (pk = 6.3) and 4 v e r y weak a c i d protons (pk = 9.7) (34).
Phytate forms s a l t s / c o m p l e x e s w i t h most heavy m e t a l s . The r e l a t i v e s o l u b i l i t y of the complex i s dependent on pH as w e l l as on the presence of a secondary c a t i o n , of which o n l y c a l c i u m has been s t u d i e d (29, 3 5 ) . Of g r e a t e s t i n t e r e s t are the c a t i o n s which are complexed most t i g h t l y at the p h y s i o l o g i c a l range of p Hfs . Complexes of s i n g l e d i v a l e n ^ | c a t i o n s , a ^ p H 7 ^ , formed i n| |
the f o l l o w i n g d e c r e a s i n g o r d e r : Cu > Zn > Co > Mn > Fe > Ca (36, 3 7 ) . I t i s i n t e r e s t i n g t h a t c a l c i u m , which has u n t i l recent y e a r s r e c e i v e d the most a t t e n t i o n , i s at the lower end of t h i s s e r i e s . F e r r i c phytate i s known to be l e a s t s o l u b l e i n d i l u t e a c i d . However as a t e r t i a r y component i n the complexation of z i n c (26, 29) and o t h e r d i v a l e n t elements ( 3 5 ) , c a l c i u m has been shown t o promote a synergism t h a t i s unique i n chemical k i n e t i c s . T h i s was f i r s t s t u d i e d i n an i n v i t r o model w h i l e s e a r c h i n g f o r a mechanism t o e x p l a i n the e x p e r i m e n t a l o b s e r v a t i o n s of c a l c i u m and/or phytate as c a u s a t i v e agents i n z i n c d e f i c i e n c y ( 3 8 ) .
F i g u r e 4 i l l u s t r a t e s the q u a n t i t y of p r e c i p i t a t e formed when equimolar c o n c e n t r a t i o n s of p h y t a t e , z i n c and/or c a l c i u m (1:1 or 2:1) a r e mixed i n an open v e s s e l and pH's a d j u s t e d f i r s t to l e s s than 3, than c a r e f u l l y to the a p p r o p r i a t e pH. The pH range between 3 and 9 was s e l e c t e d t o encompass the p h y s i o l o g i c a l l y important range. The r e s u l t s i n d i c a t e t h a t 1) c a l c i u m and p h y t a t e , i n equimolar c o n c e n t r a t i o n , a r e q u i t e s o l u b l e at a l l pH's under these c o n d i t i o n s ; 2) z i n c p h y t a t e i s l e s s s o l u b l e than c a l c i u m phytate and a t pH 6 i s l e s s s o l u b l e than c a l c i u m a t t w i c e the molar c o n c e n t r a t i o n ; 3) z i n c , c a l c i u m and p h y t a t e i n a l l combina- t i o n s t e s t e d was l e s s s o l u b l e than e i t h e r z i n c or c a l c i u m p h y t a t e or the sum of these alone a t pH 6. The pH 6 i s v e r y important p h y s i o l o g i c a l l y because t h i s i s the approximate pH of the duodenum and upper jejunum, an area of the g a s t r o i n t e s t i n a l t r a c t i n which z i n c must be absorbed. At pH 6 and a 2:1:1 c a l c i u m : z i n c : p h y t a t e molar r a t i o , 98% of the z i n c was i n the p r e c i p i t a t e (28, 38, 39).
150 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 Z I N C
Figure 4. Quantity of precipitate formed at varying molar ratios and pH's. Key:
• , Ca.phytate (1:1); Δ , Ca.phytate (2:1); O , Zn:phytate (1:1); X , Ca:Zn:phytate (1:1:1); · , Ca:Zn:phytate (2:1:1). (Reproduced with permission from Ref. 40.)
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Z i n c i s a t r a c e element and both c a l c i u m and p h y t a t e are p r e s e n t i n many foods i n macro q u a n t i t i e s ; t h e r e f o r e , an i n v i t r o model was developed to study the q u a n t i t y of z i n c a v a i l a b l e f o r a b s o r p t i o n w i t h these more p h y s i o l o g i c a l r a t i o s . The r e s u l t s , F i g u r e 5, i n d i c a t e t h a t as the r a t i o of c a l c i u m : p h y t a t e i n c r e a s e s t h e r e i s a decrease i n the uncomplexed z i n c i n s o l u t i o n which would be a v a i l a b l e f o r a b s o r p t i o n (28, 38, 4 0 ) . EDTA added i n t o t h i s same model i n c r e a s e d the s o l u b l e z i n c (38) i n d i c a t i n g t h a t s o l u b l e and absorbable c h e l a t i n g compounds may compete w i t h p h y t a t e and make some z i n c a v a i l a b l e f o r a b s o r p t i o n or reabsorp- t i o n . The i m p l i c a t i o n s of the s t u d i e s d e s c r i b e d above are t h a t the i n t e r a c t i o n of phytate w i t h z i n c and c a l c i u m i n v o l v e s a c h e m i c a l r a t h e r than a p h y s i o l o g i c a l r e a c t i o n .
With the l a r g e d i f f e r e n t i a l between the m o l e c u l a r weight of phytate (660) and the atomi
percentage comparisons
molar r a t i o reduces both of these components to a common denomi n a t o r which r e p r e s e n t s a m e a n i n g f u l , c h e m i c a l comparison. I t i s apparent t h a t some means was needed to express t h i s r e l a t i o n s h i p on a chemical b a s i s t h a t a l s o has p h y s i o l o g i c a l i m p l i c a t i o n s . The use of p h y t a t e : z i n c molar r a t i o was t e s t e d on d a t a from s e v e r a l experiments i n which the d i e t a r y model was i n d e n t i c a l except f o r the c o n c e n t r a t i o n s of p h y t a t e and z i n c . S i n c e growth r a t e d e p r e s s i o n i s an e a r l y , s e n s i t i v e and e a s i l y measured p a r a - meter, i t was q u i t e s u i t a b l e f o r the dependent v a r i a b l e . The d a t a , Table I , shows the i n v e r s e r e l a t i o n s h i p between d e c r e a s i n g p h y t a t e : z i n c molar r a t i o and average growth r a t e of these r a t s w i t h i n 4 weeks. The s e v e r i t y of c l i n i c a l symptoms are d i r e c t l y
Table I . VARIABILITY OF ZINC DEFICIENCY
D i e t a r y V a r i e n t P h y t a t e / Z i n c Gain/Wk + SD D i e t P h y t a t e Z i n c
(%) (mg/kg) (molar) (4 Weeks)
1 1.0 2* 495.5 8.8 + 3.0 (24)
2 1.0 15 66.1 12.5 + 4.1 (27)
3 0.4 15 26.1 15.3 + 4.6 (18)
4 0.4 70 5.7 37.5 + 7.0 (47)
5 0.4 125 3.2 48.5 + 4.2 (30)
*
P r o t e i n EDTA washed.
From O b e r l e a s , D. ( 3 9 ) .
Figure 5. Uptake of
65 Zn by rat jejunal strips in vitro as affected by calcium and phytate. Data represent activity/mg tissue nitrogen as a percent of the control of each replicate (7 replicates). (Reproduced with permission from Ref. 40.)
to d H S H δ > 2 δ > < > > 2 5 Η ο m Ν 2 ο
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
r e l a t e d to the p h y t a t e : z i n c molar r a t i o ( 3 9 ) . The d i e t s u t i l i z e d i n these experiments contained 1.6 percent c a l c i u m which accentuates the experimental e f f e c t of phytate but i s c o n s i d e r a b l y h i g h e r than the t y p i c a l human i n t a k e . Three l a b o r a - t o r i e s have subsequently and independently confirmed the v a l i d i t y of the p h y t a t e : z i n c molar r a t i o u t i l i z i n g d i e t a r y models w i t h more reasonable c a l c i u m l e v e l s ( 4 ^ , _42, 1*2)· ^ 1 tnree l a b o r a - t o r i e s have concluded t h a t w i t h moderate c a l c i u m i n t a k e , molar r a t i o s l e s s than 10 are l i k e l y t o p r o v i d e adequate a v a i l a b l e z i n c and molar r a t i o s g r e a t e r than 10 are a s s o c i a t e d w i t h symptoms of z i n c d e f i c i e n c y ; namely, growth r a t e d e p r e s s i o n . Though the d e t e r m i n a t i o n of t h i s c r i t i c a l molar r a t i o was made i n r a t s , i t r e p r e s e n t s an e x p r e s s i o n of a chemical r e l a t i o n s h i p and thus a p p l i e s to a l l monogastric s p e c i e s f o r s i m i l a r d i e t a r y p h y t a t e : - z i n c molar r a t i o s . Th
w i t h the need f o r a mola
l e v e l s of c a l c i u m i n t a k e to p r o v i d e adequate a v a i l a b l e z i n c . Homeostatic F l u x e s of Z i n c
F r e q u e n t l y ignored i n many s t u d i e s are the r e l a t i v e l y l a r g e q u a n t i t i e s of z i n c which are r e c y c l e d i n t o the g a s t r o i n t e s t i n a l t r a c t . The most w i d e l y s t u d i e d are the r e l a t i v e l y l a r g e s e c r e - t i o n s of z i n c i n the p a n c r e a t i c f l u i d (44, 45, 4 6 ) . The pan- c r e a t i c c o n t r i b u t i o n to the duodenum may be more than t h r e e (3) times the d i e t a r y i n t a k e (44, 4 5 ) . Some c o n t r i b u t i o n i s a l s o made t h r u the b i l e (44, 4 6 ) . Z i n c i s a l s o s e c r e t e d i n s i z e a b l e q u a n t i t i e s i n the s a l i v a ( 4 7 ) . With such a l a r g e c o n t r i b u t i o n of z i n c to the t o t a l g a s t r o i n t e s t i n a l p o o l , much of the s e c r e t e d z i n c must be reabsorbed t o prevent the body from e x p e r i e n c i n g a p e r p e t u a l z i n c d e f i c i t .
The r e l a t i v e v u l n e r a b i l i t y of the s e c r e t e d z i n c t o phytate complexation has only r e c e n t l y been demonstrated. The i n j e c t i o n g|i z i n c d e f i c i e n t r a t s i n t r a p e r i t o n e a l l y w i t h a t r a c e r dose of
z i n c a l l o w s a p o r t i o n of t h i s z i n c t o be i n e q u i l i b r i u m w i t h the endogenous m e t a b o l i c p o o l . T h i s z i n c then i s s e c r e t e d t h r u the s a l i v a , p a n c r e a t i c f l u i d , and b i l e . Those animals maintained on the phytate c o n t a i n i n g soy p r o t e i n contained 2-4 times the r a d i o a c t i v i t y of the animals f e d a c a s e i n p r o t e i n d i e t (Table I I ) . T h e r e f o r e , not only does phytate a f f e c t the b i o a v a i l a b i l i t y of d i e t a r y z i n c but a l s o the r e a b s o r p t i o n of endogenous z i n c and thus has a net e f f e c t on z i n c homeostasis. Since t h i s t o t a l p h y t a t e e f f e c t cannot be measured by l a b e l i n g o n l y the d i e t a r y p o o l , the e x p r e s s i o n of the net e f f e c t as the p h y t a t e : z i n c molar r a t i o i s the most s e n s i t i v e and a c c u r a t e method of e s t i m a t i n g the r e l a t i v e r i s k of z i n c d e f i c i e n c y i n any i n d i v i d u a l o r p o p u l a t i o n .
154 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 Z I N C
TABLE I I . RATIO OF ZINC EXCRETED BY RATS ON SOY PROTEIN DIET VS CASEIN PROTEIN DIET FOLLOWING INTRAPERITONEAL INJECTION
Days 1 3 5 7 9 11 13
1.6 % ; Ca 1.5 1.4 1.8 3 . 9 3 . 3 2 . 8 2 . 8 0 . 8 % ; Ca 1.3 1.6 1.6 2 . 3 3 . 3 2 . 5 2 . 5 Mechanism of A c t i o n
I n order t o f o r m u l a t e a mechanism of a c t i o n of p h y t a t e on z i n c homeostasis, c e r t a i
p r o c e s s must occur i n th
not absorbed except f o r s m a l l amounts by b i r d s ; 2) c a l c i u m must be a t e r t i a r y component i n the t o t a l process but there must be some r e a c t i o n w i t h o u t e x c e s s i v e amounts of c a l c i u m ; 3) c e r t a i n c h e l a t i n g compounds such as EDTA must be capable of competing w i t h the process and make some z i n c a v a i l a b l e f o r a b s o r p t i o n o r
r e a b s o r p t i o n and 4 ) There must be some e x p l a n a t i o n f o r the d a t a which i n d i c a t e s t h a t 40% o r more of the d i e t a r y p o o l may be a v a i l a b l e f o r a b s o r p t i o n (4j3, 4J)). A l l these c o n d i t i o n s are s a t i s f i e d by the f o l l o w i n g f o r m u l a which i s an e x p r e s s i o n of the
"Law of Mass A c t i o n " :
Zn*"*" + P h y t a t e - — — Zn(Ca)Phytate EDTA
One must r e c o g n i z e t h a t the t o t a l z i n c p o o l a v a i l a b l e f o r s a t i s - f y i n g the c o n d i t i o n s of t h i s r e a c t i o n must i n c l u d e the endogenous s e c r e t e d z i n c which may r e p r e s e n t 3-4 times the d i e t a r y i n t a k e even when an adequate l e v e l of z i n c i s consumed. Whereas p h y t a t e i s a p o l y f u n c t i o n a l compound and the s o l u b i l i t y product of such a mixed s a l t i s d i f f i c u l t t o c a l c u l a t e , the concept of e x p r e s s i n g the phytate and z i n c as molar r a t i o s i s s i m p l e t o c a l c u l a t e and u s e f u l i n the d e t e r m i n a t i o n of r e l a t i v e z i n c s t a t u s ( 5 0 ) . P o t e n t i a l P o p u l a t i o n Hazards
The e a r l i e s t t e s t s of the p h y t a t e : z i n c molar r a t i o concept i n humans was p r o v i d e d by d a t a s u p p l i e d by R e i n h o l d ( 5 1 , 5 2 ) . He a n a l y z e d a l a r g e number of samples of Middle E a s t e r n bread f o r p h y t a t e and z i n c . These breads were of two t y p e s ; " B a z a r i and Sangak" which are leavened breads s o l d i n the urban areas and
"Tanok" which i s an unleavened bread consumed i n the v i l l a g e s of the M i d d l e E a s t (Table I I I ) . F o r many of the v i l l a g e r s , "Tanok",
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
c o n s t i t u t e s over 90% of t h e i r d i e t . I t i s from these v i l l a g e p o p u l a t i o n s t h a t c l i n i c a l z i n c d e f i c i e n c y was f i r s t d e s c r i b e d (31) and i s e a s i l y diagnosed ( 5 3 ) . A t a z i n c c o n c e n t r a t i o n of 30 mg/kg ( R e i n h o l d , p e r s o n a l communication) t h e unleavened "Tanok"
has a p h y t a t e : z i n c molar r a t i o of about 23 whereas the leavened bread had molar r a t i o s of about 10-12. The l a t t e r would imply an element of r i s k but not t o t h e extent of e x h i b i t i n g c l i n i c a l symptoms. Since the "tanok" bread c o n s t i t u t e s the major p o r t i o n of the v i l l a g e d i e t s , i f an e s t i m a t e of consumption of bread were 1 kg/day, c l i n i c a l z i n c d e f i c i e n c y was p r e v a l e n t a t a molar r a t i o of 23 i n s p i t e of a z i n c i n t a k e of 30 mg/day. The l a t t e r would be t w i c e the c u r r e n t Recommended D i e t a r y Allowance (RDA) i n t h e U n i t e d S t a t e s .
TABLE I I I . PHYTAT
B a z a r i Sangak Tanok
Samples (N) 56 P h y t a t e (mg/100 g) 326 P h y t a t e (m moles) 4.94 Z i n c Estimated a t 30 mg/kg P h y t a t e / Z i n c (molar) 10.8
50 126 388 684 5.88 10.36 f o r a l l breads (0.46 m moles)
12.8 22.6 From: Oberleas ( 3 9 ) .
A c u r r e n t study of d i e t a r y r e c a l l s from EFNEP s u b j e c t s on s e l f - s e l e c t e d d i e t s from t h r e e areas of Kentucky i n d i c a t e t h a t from a s y s t e m a t i c a l l y s e l e c t e d sample of 1294 s u b j e c t s a p p r o x i - mately 800 had p h y t a t e : z i n c molar r a t i o s g r e a t e r than 10 and about 600 had r a t i o s g r e a t e r than 15. A s m a l l group of s u b j e c t s had molar r a t i o s g r e a t e r than 15 and z i n c i n t a k e s g r e a t e r than 15 mg/day.
Beyond t h i s , one has but t o look a t t y p i c a l d i e t s consumed throughout the w o r l d and imagine t h a t z i n c d e f i c i e n c y , although not always e x p r e s s i n g i t s e l f w i t h o v e r t c l i n i c a l symptoms, may a f f e c t a very l a r g e segment of t h e world's p o p u l a t i o n . P a r t i - c u l a r l y v u l n e r a b l e are the poorer segments of t h e p o p u l a t i o n which must depend on c e r e a l g r a i n s and legume seeds as the major source of p r o t e i n and c a l o r i e s . W i t h i n the U n i t e d S t a t e s , t h e most " a t r i s k " p o p u l a t i o n may be represented by a d o l e s c e n t s who f r e q u e n t l y i n c l u d e c e r e a l s , peanut b u t t e r and p a s t a as major d i e t a r y components, "vegans", o r those s u b s t i t u t i n g , soybean based s y n t h e t i c meats f o r animal p r o t e i n .
As a n a l y t i c a l techniques f o r e s t i m a t i n g p h y t a t e become a v a i l a b l e , computer data bases can be updated t o c o n t a i n a c c u r a t e e s t i m a t e s of the phytate and z i n c c o n c e n t r a t i o n s of foods. When
156 n u t r i t i o n a l bioavailability o f zinc
good data bases become e s t a b l i s h e d a simple and n o n i n v a s i v e t e c h nique of c a l c u l a t i n g p h y t a t e : z i n c molar r a t i o from d i e t a r y r e c a l l s w i l l p r o v i d e a s e n s i t i v e and a c c u r a t e e s t i m a t e of the r e l a t i v e r i s k of z i n c d e f i c i e n c y f o r any p o p u l a t i o n . S i n c e t h i s r e p r e s e n t s a p h y s i c a l constant the molar r a t i o of 10 o r l e s s r e p r e s e n t s and adequate standard p r o v i d e d some minimal i n t a k e of a p p r o x i m a t e l y 5 mg of z i n c / d a y i s consumed. L i k e w i s e , the RDA f o r z i n c , c u r r e n t l y a t a somewhat a r b i t r a r y 15 mg/day needs t o be m o d i f i e d t o r e f l e c t the e f f e c t of p h y t a t e on z i n c homeostasis as expressed by the p h y t a t e : z i n c molar r a t i o . Z i n c from almost any m i n e r a l source i s r e a d i l y a v a i l a b l e f o r a b s o r p t i o n ( 5 4 ) . There
f o r e , i n the absence of p h y t a t e , z i n c d e f i c i e n c y r e p r e s e n t s l i t t l e more than an academic c u r i o s i t y ; the e f f e c t e x h i b i t e d by phytate makes z i n c d e f i c i e n c y a r e a l and p r e v a l e n t problem.
Acknowledgment
T h i s was supported i n p a r t by the S c i e n c e and E d u c a t i o n A d m i n i s t r a t i o n of the U.S. Department of A g r i c u l t u r e under Grant No. 5901-0410-8-0076-0 from the Competive Research Grants O f f i c e .
Literature Cited
1. Pfeffer, E. Pringsheims Jb. Wiss. Bot. 1872, 8, 429,475.
2. Palladin, W. Z. Biol. 1895, 31, 199.
3. Winterstein, E. Ber. dtsch. chem. Ges. 1897, II, 2299.
4. Suzuki, U . ; Yoshimura, K . ; Takaishi, M. C o l l . Agric. B u l l . , Tokyo Imp. Univ. 1907, 7, 503.
5. Posternak, S. Compt. Rend. 1919, 169, 138-40.
6. De Turk, E.E.; Holbert, J.R.; Hawk, B.W. J. Agric. Res.
1933, 46, 121-41.
7. Earley, E.B.; De Turk, E.E. J. Am. Soc. Agron. 1944, 36, 803-14.
8. Fowler, H.D. J. Sci. Food Agric. 1957, 8, 333-41.
9. O'Dell, B . L . ; deBoland, A.R.; Koirtyohann, S.H. J. Agric.
Food Chem. 1972, 20, 718-21.
10. Fontaine, T.D.; Pons, W.A., J r . ; Irving, G.W., J r . 1946, 164, 487-507.
11. Smith, A.K.; Rackis, J.J. J. Am. Chem. Soc. 1957, 79, 633-7.
12. deBoland, A.R.; Garner, G.B.; O'Dell, B.L. J. Agric. Food Chem. 1975, 23, 1186-9.
13. Morris, E.R.; Ellis, R. J. Nutr. 1976, 106, 753-60.
14. Raulin, J. Ann. Sci. Nat. Bot. Biol. Vegetale 1869, 11, 93.
15. Bertrand, G.; Benzon Β. Compt. Rend. 1922, 175, 289-92.
16. Todd, W.R.; Elvehjem, C.A.; Hart E.B. Am. J. Physiol. 1934, 107, 146-56.
17. Stirn, F . E . ; Elvehjem, C.A.; Hart E.B. J. B i o l . Chem. 1935, 109, 347-59.
18. Tucker, H.F.; Salmon, W.D. Proc. Soc. Exp. Biol. Med. 1955, 88, 613-6.
In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
19. O'Dell, B . L . ; Savage, J.E. Poul. Sci. 1957, 36, 459-60.
20. Morrison, A.B.; Sarett, H.P. J. Nutr. 1958, 65, 267-80.
21. Kratzer, F.H.; Allred, J.B.; Davis, P.N.; Marshall, B.J.;
Vohra, P.N. J. Nutr. 1959, 68, 313-22.
22. Ziegler, T.R., Leach, R.M., J r . ; Norris, L.C.; Scott, M.L.
Poul. Sci. 1961, 40, 1584-93.
23. Smith, W.H.; Plumlee, M.P.; Beeson, W.M. J. Anim. Sci. 1692, 21, 399-405.
24. O'Dell, B.L.; Savage, J.E. Proc. Soc. Exp. B i o l . Med. 1960, 103, 304-6.
25. O'Dell, B.L.; Yohe, J.M.; Savage, J.E. Poul. Sci. 1964, 43, 415-19.
26. Oberleas, D.; Muhrer, M.E.; O'Dell, B.L. J. Anim. Sci. 1962, 21, 57-61.
27. Robertson, B.T.; Burns M.J Am J. Vet Res 1963 24 997-1002.
28. Fox, M.R.S.; Harrison 116, 256-61.
29. Oberleas, D.; Muhrer, M.E.; O'Dell, B.L. J. Nutr. 1966, 90, 56-62.
30. Ketola, H.G. J. Nutr. 1979, 109, 965-9.
31. Prasad, A.S.; Halsted, J.A.; Nadimi, M. Am. J. Med. 1961, 31, 532-46.
32. Prasad, A.S.; Miale, Α., J r ; , Farid, Z . ; Sandstead, H.H.;
Schulert, A.R.; Darby, W.J. ΑΜΑ Arch. Int. Med. 1963, 111, 407-28.
33. McCance, R.A.; Widdowson, E.M. Biochem. J. 1935, 29, 2694- 99.
34. Barre', R.; Courtois, J.E.; Wormser, G. Bull. Soc. Chim.
B i o l . 1954, 36, 455-74.
35. Oberleas, D.; Moody, N. Trace Element Metabolism in Man and Animals-IV. (J. McC. Howell, J.M. Gawthorne, and C.L. White, eds.), Australian Academy of Science, Canberra, ACT., 1981, pp. 129-132.
36. Maddariah, V.T.; Kurnick, A.A.; Reid, B.L. Proc. Soc. Exp.
B i o l . Med. 1964, 115, 391-3.
37. Vohra, P.; Gray, G.A.; Kratzer, F.H. Proc. Soc. Exp. B i o l . Med. 1965, 120, 447-9.
38. Oberleas, D. Ph.D. Dissertation, U. of Missouri, 1964, pp.
143-54.
39. Oberleas, D. Proc. Western Hemisphere Nutr. Congress-IV (P.L. White and N. Selvey, eds.) Publishing Sciences Group, Inc., Acton, MA, 1975, pp. 156-61.
40. Oberleas, D.; Muhrer, M.E.; O'Dell, B.L. Zinc Metabolism (A.S. Prasad, ed.) C.C. Thomas, Springfield, IL. 1966, pp. 225-38.
41. Davies, N.T.; Olpin, S.E. Br. J. Nutr. 1979, 41, 591-603.
42. Morris, E . ; Ellis, R. J. Nutr. 1980, 110, 1037-45.
43. Lo, G.S.; Settle, S.L.; Steinke, F . H . ; Hopkins, D.T. J.
Nutr. 1981, 111, 2223-35.
158 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 OF ZINC
44.
Pekas, J.C. Am. J. Physiol.
1966, 211, 407-13.45.
Miller, J.K.; Cragle, R.G. J. Dairy Sci.
1965, 48, 370-3.46. Sullivan, J.F.; Williams, R.V.; Wisecarver, J.; Etzel, K . ; Jetton, M.M.; Magee, D.F. Proc. Soc. Exp. B i o l . Med. 1981,
166, 39-43.
47. Freeland-Graves, J.H.; Hendrickson, P . J . ; Ebangit, M.L.;
Snowden, J.V. Am. J. Clin. Nutr.
1981, 34, 312-321.48. O'Dell, B . L . ; Burpo, C.E.; Savage, J.E. J. Nutr. 1972, 102,
653-60.
49. Evans, G.W.; Johnson, P.E. Am. Clin. Nutr. 1977, 30,
873-78.
50. Oberleas, D.; Harland, B.F. J. Am. Diet. Assoc. 1981, 79,
433-6.
51.
Reinhold, J.G. Am. J. Clin. Nutr.
1971, 24, 1204-06.52.
Reinhold, J.G. Ecol Food Nutr
19721,
187-8253. Ronaghy, H.A. Pahlav
54.
Edwards, H.M., J r
, , RECEIVEDOctober
13,1982In Nutritional Bioavailability of Zinc; Inglett, G.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1983.