A friend read somewhere that all whole grains, but especially soy products (except fermented ones like tempeh), interfere with absorption of many nutrients. Is this true?

Like all of the World's Healthiest Foods, whole grains and soybeans do far, far more good than harm when it comes to our nourishment. When people read about whole grains or soy products "interfering" with nutrient absorption, they need to realize that whole, natural foods almost never act against our best health interests, no matter what unusual substances they may contain. A particular food might be mismatched to our metabolism, or trigger an allergic reaction, but in terms of providing us with the nutrients we need, it's almost impossible to go wrong with any whole, natural food and that includes all 132 foods we've profiled on our website.

A food like whole wheat contains an impressive variety of nutrients—virtually all of the B vitamins, including B1, B2, B3, B5, B6, biotin and folate, as well as many key recommended minerals, including calcium, magnesium, copper, zinc, manganese, phosphorus, iron, selenium, and zinc. When we eat whole wheat products, we get a certain percentage of each of these 17 nutrients, and many others as well! At the broadest level, consumption of whole grains helps with our absorption of nutrients by providing us with so many nutrients in significant amounts. So in this sense, I would greatly disagree with the information your friend read.

However, I would guess that the article your friend was referring to was focused on substances found in whole grains, and in soyfoods, called phytic acid. Most whole grains and most beans contain phytic acid. While this naturally-occurring substance can work as an antioxidant in plants and has been shown to have some cancer-preventing and lipid-lowering effects in animal studies, it can also bind together with certain minerals and other nutrients to lower their absorption rate from the digestive tract.

A good bit of recent attention has been given to the relationship between phytic acid and iron absorption from soy. This relationship is controversial. In some studies, the naturally-occurring phytates found in soybeans appear to lower absorption of iron found in soybeans by as little as 3-4%. In other studies, this percentage is more like 45-50%. However, since most people don't rely upon soybeans as their primary source of dietary iron—it comes from other foods in the meal plan— any impact of the phytates found in soybeans on iron absorption should not be a major concern for most people.

Soybeans are a good source of protein, and at 29 grams per cup, can provide a significant amount of our daily requirement. (please note that we prefer organic soybeans as they are not grown with genetically modified seeds). Phytic acid is sometimes regarded as interfering with protein digestibility, but the research we've seen suggests otherwise, and we continue to encourage incorporating soybeans into your "Healthiest Way of Eating" partly because of their great protein benefits.

The fermentation process used to produce soy products such as miso, natto and tempeh can have a major impact of the phytic acid level in these foods. Over half—and sometimes more — of the phytic acid can be converted into other substances during fermentation of soy. This process depends, however, upon the specific bacteria used in fermentation, and their ability to make phytase enzymes that can convert phytic acid into other phosphorus-containing substances that will not bind with the nutrients we want to obtain from these soy foods.

For more information on this topic, see:

• Soybeans
• Tofu and Tempeh
• Whole wheat

  References:

  Agte V, Jahagirdar M, and Chiplonkar S. Apparent Absorption of Eight Micronutrients and Phytic Acid From Vegetarian Meals in Ileostomized Human Volunteers..Nutrition. 2005;21(6):678-85.

  Bosscher D, Lu Z, Janssens G, et al. In Vitro Availability of Zinc From Infant Foods With Increasing Phytic Acid Contents. Br J Nutr 2001;86(2):241-7.

  Couzy F, Mansourian R, Labate A, et al. Effect of Dietary Phytic Acid on Zinc Absorption in the Healthy Elderly, As Assessed by Serum Concentration Curve Tests. Br J Nutr. 1998;80(2):177-82.

  Helbig E, de Oliveira AC, Queiroz Kda S, et al. Effect of Soaking Prior to Cooking on the Levels of Phytate and Tannin of the Common Bean (Phaseolus Vulgaris, L.) and the Protein Value. J Nutr Sci Vitaminol (Tokyo). 2003;49(2):81-6.

  Ibrahim SS, Habiba RA, Shatta AA, et al. Effect of Soaking, Germination, Cooking and Fermentation on Antinutritional Factors in Cowpeas. Nahrung. 2002;46(2):92-5.

  Lopez HW, Krespine V, Guy C, et al Prolonged Fermentation of Whole Wheat Sourdough Reduces Phytate Level and Increases Soluble Magnesium. J Agric Food Chem. 2001;49(5):2657-62.

  Makokha AO, Oniang'o RK, Njoroge SM, et al. Effect of Traditional Fermentation and Malting on Phytic Acid and Mineral Availability From Sorghum (Sorghum Bicolor) and Finger Millet (Eleusine Coracana) Grain Varieties Grown in Kenya. Food Nutr Bull. 2002;23(3 Suppl):241-5.

  Vig AP, Walia A. Beneficial Effects of Rhizopus Oligosporus Fermentation on Reduction of Glucosinolates, Fibre and Phytic Acid in Rapeseed (Brassica Napus) Meal. Bioresour Technol 2001;78(3):309-12.

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