Calcium absorption phytate study

Calcium Absorption. Phytates in cereal grains have also been reported to interfere with the absorption of calcium. However, a long-term study indicated a retention of calcium in subjects that consume large amounts of bread made with high extraction of flour (19). 

The brown or whole meal bread diets employed by previous investigators were often variable in calcium and phytate intakes, not only between individuals, but by the same individual subjected to different diet treatments. Nevertheless an estimate of the molar ratio of phytate/calcium in the brown or whole meal bread diets used by McCance and Widdowson (UO), Walker et al. (11) and Reinhold et al. (2, 12) is 0.25 or greater. These investigators observed either negative or less positive calcium balance and apparent absorption when the brown bread diets were consumed compared to white bread diets with phytate/calcium molar ratios less than 0.05. Our results support their findings. Reinhold et al. (2) and McCance and Widdowson (33) used sodium phytate in some studies as well as whole wheat bread and observed similar results. 

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... 

Zinc Is a trace element and both calcium and phytate are present In many foods In macro quantities therefore, an In vitro model was developed to study the quantity of zinc available for absorption with these more physiological ratios. The results, Figure 5, Indicate that as the ratio of calclum phytate Increases there Is a decrease In the uncomplexed zinc In solution which would be available for absorption (28, 38, 40). EUTA added Into this same model Increased the soluble zinc (38) Indicating that soluble and absorbable chelating compounds may compete with phytate and make some zinc available for absorption or reabsorp-tlon. The Implications of the studies described above are that the Interaction of phytate with zinc and calcium Involves a chemical rather than a physiological reaction. 

Figure 1. Dietary phytate/calcium molar ratio and apparent absorption of calcium. Each triangle is the mean for one individual for 3 consecutive 5-day menu cyles. See text for study details, HS-II.

The behavior of minerals is often influenced by the presence of other food constituents. The recent interest in the beneficial effect of dietary fiber has led to studies of the role fiber plays in the absorption of minerals. It has been shown (Toma and Curtis 1986) that mineral absorption is decreased by fiber. A study of the behavior of iron, zinc, and calcium showed that interactions occur with phytate, which is present in fiber. Phytates can form insoluble complexes with iron and zinc and may interfere with the... 

The complexity of food effects on zinc absorption is illustrated by the studies of Sandstrom et al.(25,37,38) in which phytate, protein, calcium, zinc and other factors appear to have variable effects on zinc absorption. Although It appears certain that food interferes with zinc absorption, the effects of individual food substances are unknown and difficult to predict. Regardless of individual effects, zinc administration with food will complicate efforts to measure the effects of other variables on intestinal absorption. 

Evidence that dietary fiber interfered with absorption of bivalent metals by the intestine was first ob -tained in connection with studies of human zinc deficiency in Iran. Human subjects who consumed purified phytate exhibited smaller feeal losses of zinc and calcium than they did when they ate equivalent amounts of phytate in the form of unleavened wheaten whole meal flat breads that are the staple food in rural Iran... 

Early studies of the isolated ingredient may be helpful to identify biomarkers and to help in the design of more complex products. In addition to identifying any toxic or untoward reactions, preclinical studies may help identify appropriate levels of use. New ingredients should be evaluated in the matrix expected to be used in the final product in order to determine absorption, utilization, and activity when other ingredients are present. For example, soy formulas contain phytate, which can bind zinc, iron, and other divalent cations and make them unavailable. To allow for this, soy formulas are fortified with zinc, iron, and calcium. They also provide relatively large amounts of iron to allow for decreased absorption. 

Several human studies have demonstrated that diets rich in NSP may reduce the bioavailability of minerals, such as iron, calcium, and zinc. Nevertheless, this effect is more likely due to the presence of phytate, which inhibits the absorption of those minerals, than the NSP content of the diet. 

The possibility of an adaptative increase in the fraction of Mg absorbed as Mg intake is lowered is controversial. In fact, experimental studies indicate that fractional intestinal absorption of Mg is directly proportional to dietary Mg intake. Because only soluble Mg is absorbed, all the factors increasing Mg solubility increase its absorption while formation of insoluble complexes in the intestine may decrease Mg absorption. Most well-controlled studies indicate that high calcium intake does not affect intestinal Mg absorption in humans. In contrast, dietary phytate in excess impairs Mg... 

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