Soy and mineral absorption (1)

Groups opposed to soy suggest that “vegetarians who consume tofu and bean curd as a substitute for meat and dairy products risk severe mineral deficiencies.”  This allegedly is due to phytates in soy preventing the body from absorbing minerals.  As part of evaluating these anti-soy claims, I have been looking at studies on foods that inhibit mineral absorption.

Nuts actually contain two inhibitors of iron absorption: Phytates and tannins (polyphenols). One study compared the effect of “nut meals” made from walnuts, almonds, hazelnuts, and peanuts to “bread meal.” (Peanuts, of course, are not nuts, they are legumes, as are soybeans.) The study volunteers variously had a glass of “full-cream milk” for breakfast with either two slices of white bread and margarine or a nut butter made from one of the nuts and honey. Next, the breakfasts were repeated, this time with vitamin C added to the glass of milk. Finally, one breakfast used sprouted peanuts in the nut butter.

Rarely do studies explain their food choices. Here, perhaps white bread was used because it is lower in fiber than a whole-grain bread. Fiber affects iron absorption. White bread is also low in nutrients (e.g., vitamin C and A) and nutrients can enhance mineral absorption. Of course, a breakfast low in fiber and nutrients with added sugar, trans-fats, and dairy sounds like a fairly standard American breakfast offering. Perhaps that is why white bread was paired with margarine and a glass of milk.

Fortunately, this study did not conclude that white bread with margarine is healthier than nuts with honey even though the nuts definitely inhibited iron absorption. Instead, it showed that if nuts were eaten with a little vitamin C (25-50 mg) they only slightly reduced iron absorption. In some instances, vitamin C overcame inhibition entirely, confirming my conclusion that nuts and legumes should always be eaten with fruits and vegetables.

One breakfast used sprouted peanuts. Sprouting improved iron absorption but the effect was slight and did not come anywhere near the benefit of adding vitamin C.

Interestingly, some white bread breakfasts inhibited iron absorption more than other white bread, margarine, and milk breakfasts.  Homemade white bread did better than store bought bread. “The reason for these differences is not clear.”  So, apparently if you are a big white bread fan you might want to bake your own.

In the end, the researchers concluded that the phytates were not the cause of the reduced iron absorption. This suggests (but does not prove) that soy phytates will not reduce mineral absorption if eaten with in the context of a balanced meal. Certainly, nothing in this study suggests that soy phytates are likely to cause a severe mineral deficiency as long as they are eaten with fruits and vegetables.

White Bread, 1964, oil on canvas by James Rosenquist Creative Commons some rights reserved by Cliff1066

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