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Questions of A More Technical Nature

Soy, Phytate and Types A and AB


QUESTION

How problematic is the zinc-blocking action of phytic acid in soy, especially in the vegetarian diet, which tends to be zinc poor?




ANSWER



Soy (especially the bran or hulls) have been vilified in certain alarmist publications and websites as antinutrient factors. This is due to the presence of phytates, compounds capable of binding calcium, magnesium, zinc and iron in the intestines and preventing their absorption. As with most issues in nutrition, one should seek to evaluate both sides of the controversy, keeping in mind the great advantage our understanding of blood type variation offers to us.

As I always like to remind people during 'this food is good' or 'this food is bad' arguments: One person's food is someone else's poison.

For example, phytates are present in many other plant sources; in wheat hulls at quite high rates, for example. In a normal diet in which soy is being consumed with a calcium supplement added into the diet, or dietary calcium sources are emphasized, or as a partial replacement for meat, fish or in combination with other plant protein sources, and where soy is not the sole dependent source for amino acids, such loss can be looked on as minimal.

Yet, as that famous radio announcer used to say: "Now, for the REST of the story!"

It has been proven that because phytates bind excess iron which can promote free radical DNA damage in the colon it can reduce this local oxidative damage. Phytates in plant fiber are also associated with reductions in the incidence of colon cancer, and directly reduce serum cholesterol and triglycerides, a partial risk factor for athersclerosis: Two areas of long term concern for those who are blood groups A and AB.

Phytates prevent absorption of excess iron in its most reactive form. Excess iron systemically, and at the level of the colon, can lead to an enhanced oxidative stress, which is implicated in heart disease, diabetes, arthritis and, of course, cancer. Additionally, phytates enhance NK cells, and can directly control many types of cancer cell growth.

So in summary, if you were a lab animal that was fed soy husks and nothing else for several weeks, you would in fact see a drop in your mineral levels. If you are an A or AB looking to minimize your risk of certain degenerative diseases by adding a rational amount of soy to your otherwise well-balanced diet, you could probably count on lowering your rate of cardiovascular and malignant disease instead.

Heaney, RP, et al. Soybean phytate content: effect on calcium absroption. Am J. Clin Nutr 53:745-747, 1991.

Erdman, JW, et al. Soy products and the human diet. Am J. Clin Nutr 49:725-737, 1989

Graf, E, et al. Dietary suppression of colonic cancer. Cancer 56:717-718, 1985.

Harland, BF et al. Phytate in foods. Wld Rev Nutr Diet 32:235-259, 1987.




The Ask Dr. D'Adamo internet advice column ran from 1996 to 2009, at which time Dr. D'Adamo's teaching and programming responsibilities no longer allowed him to devote time and resources to directly answering visitor questions. However we've recently reorganized this treasure-trove of material and made it again available to his readership. He occasionally posts new entries. These are marked with a NEW tag.



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