Zinc bioavailability

In natural waters, dissolved zinc speciates into the toxic aquo ion [Zn(H20)6]2+, other dissolved chemical species, and various inorganic and organic complexes zinc complexes are readily transported. Aquo ions and other toxic species are most harmful to aquatic life under conditions of low pH, low alkalinity, low dissolved oxygen, and elevated temperatures. Most of the zinc introduced into aquatic environments is eventually partitioned into the sediments. Zinc bioavailability from sediments is enhanced under conditions of high dissolved oxygen, low salinity, low pH, and high levels of inorganic oxides and humic substances. 

Other workers (115-124 for example) have also centered their efforts on the role of phytic acid on zinc and iron bioavailabiliy from both soy and wheat products. It has been suggested (120) that the phytate-to-zinc molar ratio could be used to predict zinc bioavailability in high-phytate foods. Several groups (115, 117), including ours (113), 1 least partially supporT this hypothesis. However, recent work from our laboratory (112) involving soy protein of similar phytate-to-zinc molar ratios clearly demonstrates that zinc bioavailability is also altered by food processing. In this study, zinc from neutralized soy concentrates and isolates was shown to be less available to the rat than was the corresponding acid-precipitated products. This is unfortunate as alkaline conditions are commonly utilized for soy and other plant proteins to obtain beneficial functional properties. 

Lock K, Janssen CR. 2003. Influence of ageing on zinc bioavailability in soils. Environ Pollut 126 371-374. 

KIES ET AL. Zinc Bioavailability from Vegetarian Diets... 

Zinc intakes of 6 Swedish vegans (individuals who consume no food of animal origin at all) using chemical analyses of diets from a duplicate portion sampling technique approach were included in a study by Abdulla et al. (12). Dietary fiber Intake of male subjects was 62+9 g/day and that of female subjects was 43 9 g/day. Zinc Intake of male subjects was 13 2.3 mg/day and that of female subjects was 6.5 1.3 mg/day, so as in the study of lacto-ovo-vegetarians by Anderson et al., subjects were found to have relatively high Intakes of fiber and relatively low Intakes of zinc with the zinc that was supplied being from foods believed to exhibit low zinc bioavailability. 

Little information is available on the effect of oxalic acid on zinc bioavailability. In one study, Welch et al. ( fed weanling rats zinc-deficient diets with and without 0.75% sodium oxalate. The rats were dosed orally with zinc-labeled spinach leaves or zinc-labeled zinc sulfate. Dietary oxalate enhanced the availability of radioactive zinc from zinc sulfate, but had no effect on zinc from spinach leaves. Absorption and retention of zinc was greater from spinach leaves than from zinc sulfate. 

OBERLEAS Role of Phytatc in Zinc Bioavailability and Homeostasis 153... 

Phytic acid and dietary fiber in foods have been implicated as important determinants of dietary zinc bioavailability (, 6,7). For rats, the molar ratio of phytic acid to zinc is a good predictor of bloavailabillty of dietary zinc when semlpurified type diets are fed in which ZnSOij and either sodium phytate or... 

We conducted a human metabolic balance study to test the concept of dietary phytate/zinc molar ratio as a predictor of zinc bioavailability to humans. Using unaltered or enzymatically dephytinlzed wheat bran with ordinary foods we attained phytate zinc molar ratios of about 1 and 12 with relatively high intakes of dietary fiber in the menus and found no difference in zinc balance. Retrospectively, the result may be qualified on the basis of the magnitude of the zinc Intake and possible adaptive or homestatic responses over the period of the study. A second study was then conducted and a wider range of phytate/zinc molar ratio was provided than in the first study. We will briefly outline the first study, give a progress report on the second study and, with some information on phytate intakes obtained by our laboratory, discuss the nutritional implication. 

Zinc Bioavailability from Processed Soybean Products... 

Rackis and coworkers (, ) noted from their survey of the literature that for experimental animals, zinc bioavailability from soy protein isolates was low compared to zinc availability from soybean meal, casein or other animal protein diets. They... 

Oberleas (.9) first suggested that the molar ratio of phytate to zinc might be useful for prediction of the zinc bioavailability from phytate-rich foods. Molar ratios of greater than 20 1 seemed to be indicative of poorly available zinc. O Dell (10), Morris and Ellis (11) and Davies and Olpin (12) have all pointed out the importance of the calcium content of the diet to the phytate to zinc molar ratio. Higher dietary calcium clearly depresses zinc bioavailability at phytate to zinc molar ratios of less than 20 1 in diets fed to rats. 

Besides the effect of dietary calcium level, the amount of iron, and perhaps the level of other metals, may affect zinc bioavailability. Solomons and Jacob (13) have shown in human subjects that increasing the iron/zinc ratio from 0 1 to 3 1 in solutions containing 25 mg of zinc and corresponding amounts of iron as ferrous sulfate produced a progressive decrease in the plasma zinc response. They further reported that the chemical form of iron was an important determinant of the interaction. Solomons (14) extensively reviewed both inhibitory factors and enhancers of zinc bioavailability found in foods. 

Our laboratories have been concerned with the role that unit food processing operations play in the bioavailability of zinc from complete diets. Soybean foods have served as models for the evaluation of processing effects upon both endogenous and added zinc. Below are described results from both rat bioassays and in vitro tests for zinc bioavailability. Prediction of zinc bioavailability from soy-containing diets is far more complex than an analysis of phytate and zinc molar ratios. 

As pointed out earlier in this review, increasing the level of dietary calcium decreases the zinc bioavailability from phytate-containing foods. Presumably the mechanism is through the formation of chemical complexes containing zinc, phytate and calcium which are insoluble at intestinal pH and nonabsorbable (24). Recently, our laboratories used slope ratio techniques to compare the bioavailability of zinc contained in calcium sulfate-and in magnesium chloride-precipitated soybean curd (Tofu) to that of zinc added as the carbonate to egg white diets by slope ratio techniques (25). Total dietary calcium level in all diets was adjusted to 0.7% with calcium carbonate. The results (not shown) indicated that the relative availability of zinc from both tofu preparations was 51% as measured by weight gain and 36-39% for bone zinc. These results are similar to those reported for full fat soy flour (16) in Table I. 

Calcium Effects on Zinc Bioavailability for the Rat and the Human. It should be pointed out at this juncture that the nutrient requirement of calcium for the rat is much higher than for man. In fact, the molar ratio of calcium to zinc in excess of 660 1 is recommended for rat diets, while for man the ratio is between 80 1 and 160 1. To feed rats molar ratios of calcium and zinc similar to human requirements would necessitate either a very calcium deficient diet or one containing zinc at a level well in excess of the requirement. Neither choice is nutritionally suitable for demonstrating an effect of phytate on zinc availability. 

We would have to question the validity of the use of dietary calcium levels of up to 1.75% calcium in rat studies to study zinc bioavailability. These levels would never be approached in human diets without use of excessive calcium supplementation. 

Our work with soy products indicates that extrinsic zinc is more available than intrinsic zinc. The intrinsic and extrinsic zinc pools do not completely mix in all soy products. Phytic acid is an inhibitor of zinc bioavailability and this inhibition is aggravated by higher levels of dietary calcium and perhaps magnesium. Neutralization of soy isolates and concentates, with subsequent drying, reduces zinc utilization for rats. 

The prediction of zinc bioavailability from complex food systems is not a simple matter. Animal bioassays and in vitro tests help us to identify factors that may enhance or inhibit zinc utilization from the diet. With simple model food systems, we can demonstrate the negative effects of phytic acid, calcium and other factors on zinc bioavailability. However, the interaction of these factors in complex food systems, and their effect on zinc status for man is not well understood at this time. 

Simple predictors, such as the phytate to zinc molar ratio, will not be accurate for human diets. The aggravating effect of calcium and perhaps of magnesium seen in rat and in vivo tests may or may not be of practical significance to man. We can, however, develop a list of factors that have been shown to affect zinc bioavailability from foods. These are listed in Table VII. 

Zinc Bioavailability from Cereal-Based Foods... 

Table 4. Zinc bioavailability and phytate and neutral detergent fibre...

Almost all the evidence showing that phytate decreases zinc absorption in man and animals is based on pure phytate added to the diet. The effect of natural phytate is variable (18). It has, however, been reported that phytate in bran affected zinc bioavailability in the same way as sodium phytate (19). Dietary fibre in the rural Iranian diet was considered to be the main cause of zinc deficiency in Iran (20). However, the addition of 26 g of fibre from various sources to the American diet did not have any significant effect on the zinc requirements of male adults (21). Similarly, Indian men consuming a diet containing only 10.8 mg of zinc were reported to be in balance in spite of a dietary fibre intake of 50 g per day (22). Moreover, the presence of fibre and phytate in soy flour did not affect the bioavailability of zinc added as zinc carbonate, to the diet of rats (17), although others (23) have reported that the bioavailability of zinc in breakfast cereals depends mainly on their phytate-zinc molar ratio. Our results indicate that there is some, as yet, undetermined difference in the phytate or the fibre of cereals which affects the bioavailability of zinc. It may be some component of dietary fibre (24) or the intrinsic differences in the protein-phytate-mineral complex (10). 

To determine if any component of the dietary fibre in the cereals investigated here was correlated with zinc bioavailability, hemicellulose, cellulose and lignin were determined by the method of Mongeau and Brassard (16). The results are summarized in Table 5. 

The phytate and neutral detergent fibre in cereal grains and soy products, which make up infant cereals or breakfast cereals, have unknown intrinsic differences which have varying effects on zinc bioavailability. Consequently, the concept of a common phytate/zinc molar ratio below which there is no adverse effect on the bioavailability of zinc cannot be supported from our results. 

Methods for studying zinc bioavailability in humans include metabolic balance studies, radioisotopic techniques, stable isotope techniques, circulating zinc response, and perfusion techniques. These methods of bioavailability, along with other zinc-related studies, also are covered in detail. 

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