DEAR DR. BLONZ: Soy is present sometimes in the dishes I enjoy. At a recent meal, I was told that soy contains enzyme inhibitors and phytic acid, which block the absorption of essential nutrients. Is this true? -- C.K., via email
DEAR C.K.: "Mission-critical" for any plant is the ability to reproduce its next generation. Plants that have withstood the test of time developed genes to adapt to various assaults to their propagation. Examples of these threats include poor growing conditions, rival plants competing for resources, and bugs or critters using them for food. Plants' defenses against such threats can be physical, such as tough barriers to protect critical structures, but they can also be phytochemical, using substances to dissuade would-be diners.
Soy, like other plants, has several defenses, one of which is its trypsin inhibitor. Trypsin is a protein-digesting enzyme that helps disassemble proteins in the small intestine. Animals eating soy plants are affected by this element, and it's made worse if soy is their primary source of sustenance.
An important perspective is that the trypsin inhibitor is active only in raw soy; it is inactivated when the soy is boiled or otherwise cooked. Another key consideration is that human trypsin is more resistant to soy's inhibitor than trypsin in other mammalian species. The bottom line is that this "antinutrient" concern would only be an issue if raw soy were the major protein in an otherwise low-protein diet. (To be clear, low-protein eating is not common in our Western diet.)
While on the topic, an additional issue with soy, and several other grains and legumes, is that it contains phytic acid, another antinutrient. Phytic acid binds with certain nutrients -- including iron -- creating a compound that is too large to be absorbed. Fermentation degrades phytic acid to blunt this effect; this occurs in baking, as yeast acts on phytate-containing flour. The binding activity of phytic acid is also blunted when soybeans are fermented into products such as natto or tempeh. (For a table of major foods with phytic acid, visit b.link/ve7uyx.)
Ironically, phytic acid (also known by its chemical names "inositol 6-phosphate" and "myo-inositol") is marketed as an ingredient in dietary supplements and is being investigated for specific beneficial effects.
Soy's antinutrients should not be a serious concern. That said, soy should not stand alone as the main source of protein in your diet. This consideration is not unique to soy; I always encourage an assortment of healthful foods as a primary theme. It gives us a better shot at providing the body with the variety it needs for health.
Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to email@example.com. Due to the volume of mail, personal replies cannot be provided.