Bioavailability dietary phytate

The effect of phytic acid on Ca2+ bioavailability is still in dispute. Some early nutritionists reported rachitogenic properties of dietary phytate based on feeding studies using puppies (16-18) and epidemiological studies on British-born children of Asian immigrants... 

These conclusions were seriously questioned (21-24 and recent results indicate that the bioavailability of Ca2+ is the same from a casein diet as from a high-phytate soy concentrate (25). Oberleas contends that, in the presence of adequate amounts of Ca2+ and vitamin D, dietary phytate is not rachitogenic, even though it may bind substantial amounts of Ca2+ (J 5). This controversy, the relative paucity of available information, and the growing incidence of Ca2+ deficiency prompted us to investigate further the chemical interactions between Ca2+ and phytate and to assess its effect on the bioavailability of Ca2+ administered to mice by gavage. 

From our results on solubility properties of Ca2+-phytate complexes and their bioavailability to mice we conclude that, at least in our experimental system, phytate has no detrimental effect on Ca2+ bioavailability. Indeed, as we have argued elsewhere, adequate levels of dietary phytate may actually be beneficial due to the... 

In conclusion, phytic acid forms soluble complexes with Ca2+ at intestinal pH under a variety of conditions and fails to inhibit Ca2 bioavailability to mice in our experimental system. Despite the hazard in direct extrapolation of results obtained with animals kept on a well-defined dietary regimen to humans consuming a complex diet, many elements of which affect Ca2+ bioavailability, our data demonstrate the need for a reevaluation of the putative antinutritional properties of dietary phytate. Our further contention that adequate levels of dietary phytate may actually be beneficial due to its food preserving properties and its protection against colonic cancer will warrant a prospective epidemiological human study designed to assess the longterm effects of dietary phytate on mineral bioavailability and inflammatory bowel diseases. 

Grases F., Bartolome M.S., Prieto R.M. and March J.G. 2001. Dietary phytate and mineral bioavailability. Journal of Trace Elements in Medicine and Biology 15 221-228. 

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. 

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. 

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. 

Bioavailabilitv of methvlmercurv in food. Measurements of absorption and toxicity have generally been made using aqueous solutions of methylmercury. The absorption and bioavailability of methylmercury in food, specifically fish and bread, may be affected by dietary components. Potential confounders that may affect bioavailability of methylmercury are dietary phytate and other dietary fibrous materials found in bread and the complexation of methylmercury with selenium in fish. 

Many other dietary factors have been reported to affect calcium bioavailability. Phytate, fiber, cellulose, uronic acids, sodium alginate, oxalate, fat (only in the presence of steatorrhea), and alcohol have been reported to decrease calcium bioavailability (15). Lactose and medium chain triglyceride increase it (15). FTuoride also affects calcium retention primarily by stimulating bone formation thereby decreasing calcium excretion (33-38). The effects of fluoride on calcium utilization have been variable (34,38,39). 

Two metabolic balance studies were conducted using healthy adult men to study the effect of phytate on bioavailability of dietary calcium. Dietary treatments were each 15 days in duration. In the first study, a mean daily calcium balance of 208+58 (SD) mg was observed when 2.0 g of phytate from 36 g of whole wheat bran was consumed daily with 1100 mg of calcium, phytate/calcium molar ratio 0.11. Calcium balance was 184+87 mg when 36 g of dephytinized bran was consumed with the same intake of calcium, phytate/calcium molar ratio 0.01. In the second study, calcium intake was 740 mg/day. 

Apparent absorption (intake minus fecal excretion) of calcium decreased when the diet contained muffins with added sodium phytate to increase the molar ratio of phytate/calcium from 0.04 to 0.14 and 0.24. One-half of the men excreted more calcium in feces than was consumed when the high phytate diet was consumed. People consuming diets with molar ratios of phytate/calcium exceeding 0.2 may be at risk of calcium deficiency because of low bioavailability of dietary calcium unless physiological adjustments can be accomplished that maintain homeostasis. 

Our studies do not resolve the question of phytate vs fiber for the effect of wheat bran on dietary calcium bioavailability. Phytate level clearly affected apparent absorption of calcium in HS-II in the presence of an amount of the water insoluble fraction of dephytinized bran equivalent to 12 g of untreated bran and the phytate supplied as sodium phytate. An additional trial using untreated bran and the same amount of fiber as the water insoluble fraction with sodium phytate could resolve the question of fiber vs phytate. In HS-I, the balances were positive when a relatively large amount of bran, 36 g/day, was consumed. Calcium intakes were possibly higher than most men consume, but under the dietary conditions imposed for 15 days, the phytate and fiber of 36 g of bran did not express an adverse effect on calcium balance. 

Wheat bran has been the fiber source most commonly used to study effects of dietary fiber on calcium absorption in controlled laboratory studies. However, wheat bran and other forms of fiber as they occur in food products present several disadvantages in terms of definition and by concurrently altering intakes of other substances or materials known or suspected of having an adverse effect on the bioavailability of calcium such as phytates and oxalates (5,13,17,22-28). Several studies have been conducted which have sought to separate or compare the effects of phytate and fiber... 

Dietary Reference Intake (DRI) of Cu, 17-18% of the DRI of K, P, and Fe, and between 5 and 13% of the DRI of Zn, Mg, and Mn (Table 5.1). Potatoes are generally not rich in Ca, but can be a valuable source of trace elements, such as Se and I, if fertilized appropriately (Eurola et al., 1989 Poggi et al., 2000 Turakainen et al., 2004 Broadley et al., 2006). Moreover, since potato tubers have relatively high concentrations of organic compounds that stimulate the absorption of mineral micronutrients by humans, such as ascorbate (vitamin C), protein cysteine and various organic and amino acids (USDA, 2006), and low concentrations of compounds that limit their absorption, such as phytate (0.11-0.27% dry matter Frossard et al., 2000 Phillippy et al., 2004) and oxalate (0.03% dry matter Bushway et al., 1984), the bioavailability of mineral elements in potatoes is potentially high. 

Manganese is a nutritionally important trace element for chicks. Dietary energy and protein sources contain very little bioavailable Mn, and these feed ingredients reduce the biopotency of inorganic Mn supplements. This adverse effect is exerted primarily in the intestine as a result of reduced Mn absorption and is mediated by the fiber and/or ash components of the feedstuffs. Gut absorption efficiencies are higher when a phytate-and fiber-free casein-dextrose diet is fed than when a corn-soybean meal diet is fed. Dietary interrelationships exist between Mn and Co and between Mn and Fe. Cobalt increases Mn absorption and may precipitate Mn toxicosis. Excess dietary Mn reduces Fe utilization, but excess Fe does not affect Mn utilization. Eimeria acervulina infection increases Mn absorption. 

In a series of manganese tolerance tests, three dietary components —cellulose, pectin, and phytate—were found to reduce plasma uptake of manganese. Although the amount of manganese administered (40-50 mg) in these tolerance tests was much larger than that typically consumed (0.9 to 7.0 mg per day) (55,56), the results were similar to those obtained in bioavailability studies with other trace elements. Thus it appears that diets high In fiber and phytates also reduce the bloavailabllity of manganese. 

The relative vulnerability of the secreted zinc to phytate complexatlon has only recently been demonstrated. The injection p zinc deficient rats intraperitoneally with a tracer dose of zlnc allows a portion of this zinc to be in equilibrium with the endogenous metabolic pool. This zinc then is secreted thru the saliva, pancreatic fluid, and bile. Those animals maintained on the phytate containing soy protein contained 2-4 times the radioactivity of the animals fed a casein protein diet (Table II). Therefore, not only does phytate affect the bioavailability of dietary zinc but also the reabsorption of endogenous zinc and thus has a net effect on zinc homeostasis. Since this total phytate effect cannot be measured ly labeling only the dietary pool, the expression of the net effect as the phytateizinc molar ratio is the most sensitive and accurate method of estimating the relative risk of zinc deficiency in any individual or population. 

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... 

Apparently the zinc of these water insoluble complexes dissociates in the digestive tract or the complex is not strong enough to prevent the intestine from extracting the zinc. However, when phytate is present in the diet in 12 to 15-fold molar excess over zinc rats exhibit signs of decreased bioavailability of dietary zinc ( , 0). The metabolic balance does not measure true absorption, but a measure can be obtained over time of the amount of bioavailable nutrient under a given dietary regimen. Decreased bioavailability would be indicated by a decrease in apparent... 

It has been reported frequently that fibre and phytate have an adverse effect on the bioavailability of zinc in foods (3). The cereals in this study were therefore analysed for phytate (15) and dietary fibre (16). The results are given in Table 4. 

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). 

This book is based on the symposium that was designed to assess the current perspective and future direction of research on the nutritional bioavailability of zinc. Inhibitors suspected of interfering with the absorption of zinc are some factors that infiuence bioavailability. Phytates, dietary fibers, proteins, nonenzymatic browning products, and certain micronutrients are among these substances. These inhibitors are covered in various chapters. 

Dietary fiber and phytate. Dietary fiber and phytate are known as potential inhibitors of the absorption of divalent cations however, the literature regarding the effect of dietary fiber and phytate on the bioavailability of minerals is contradictory. Data by Yannai and Sachs (1993) indicate that phytate does not affect methylmercury absorption. Yannai and Sachs (1993) compared the absorption by rats of mercury found intrinsically in experimental fish meal with and without added phytate and found no significant differences in the absorption of Hg (93 5%) between 2 experimental fish meal diets (containing 1.4 mol Hg/kg diet), with or without added sodium phytate. The authors speculated that phytate might be preferentially bound to zinc, iron, and copper, which were present at much higher concentrations in the diet. 

---