Soy and soy foods are rich in protein, fiber, phytochemicals and other nutrients. Soy and soy foods are popular in the United States, however there are some myths that effect consumers' opinion of soy. I asked Dr. Mark Messina, adjunct associate professor of nutrition at Loma Linda University and an expert on soy and nutrition, to help me dispel the myths surrounding soy.
Myth: Soy Blocks Absorption of Minerals
Soy foods are often accused of blocking absorption of minerals in your digestive tract, especially calcium, iron and zinc. How did this myth start? Perhaps because soy is often used as a substitute for other mineral-rich foods. According to Dr. Messina, "soy foods are frequently used in place of animal foods, many of which are good sources of iron and zinc, and in the case of dairy foods, calcium. Consequently, questions have arisen about the impact of soy on mineral status. The consumption of relatively little red meat is able to meet daily iron and zinc requirements, so questions about the effects of soy on the status of these two minerals pertains mostly to those eating a predominately plant-based diet." So, people who eat any animal-based foods don't have to worry about soy affecting their mineral status.
But what soy and vegetarians and vegans? Dr. Messina explains how people who eat only plant-based diets may need to choose foods with more zinc. Not because soy interferes with absorption, but because non-meat diets may tend to be lower in zinc. "Soybeans, like other legumes and whole grains, are high in phytate, which reduces the absorption of some minerals, especially iron, calcium and zinc. Zinc absorption from soy foods is only modestly lower than that from other sources, but because soybeans contain relatively little zinc, unfortified soy foods are not particularly good sources of this mineral. Since zinc status is difficult to assess, vegetarians are advised to identify good plant sources of zinc in their diet or to take a zinc supplement."
On the other hand, soy is rich in iron -- and unlike the iron found in other plant-based foods -- soy contains a form of iron that is easily absorbed. Dr. Messina continues, "until recently, the prevailing thinking was that the iron in soy foods and essentially all plant foods was poorly absorbed. However, new research using improved methodology indicates that iron absorption from soy may be higher than previously thought because most of the iron in soy is in the form of ferritin. Although there is debate about the bioavailability of ferritin, some research shows it to be highly available. Furthermore, and more importantly, clinical studies show that incorporating soy into the diet does not negatively impact iron status."
Still not convinced? Dr. Messina described a study conducted by researchers from Iowa State University, that was designed specifically to examine the effect of soy foods on mineral status and provides very reassuring results. "In this study, young premenopausal women consumed daily either two or three servings of soy foods or non-soy foods matched for type of food; i.e., soy burgers in place of hamburgers, soy milk in place of dairy milk. Results showed there were no statistically significant effects of soy on urinary and serum zinc, serum hemoglobin and iron, and transferrin saturation." Dr. Messina also discussed the affect of soy on calcium. "In addition to phytate, as noted previously, soybeans contain oxalate, which like phytate, is an inhibitor of calcium absorption. Nevertheless, calcium absorption from soy is actually quite good despite the presence of these two compounds. However, the more relevant and more complex issue is the bioavailability of calcium from calcium-fortified products, such as soy milk. Absorption depends to some extent on the type of supplemental calcium used.Calcium absorption from soy milk fortified with calcium carbonate is similar to the absorption of calcium from cow's milk. In contrast, calcium absorption from soy milk fortified with tricalcium phosphate is about 25 percent lower than from dairy milk. However, because of the high amounts of calcium added, the amount of calcium available to the body from both types of calcium-fortified soy milk is similar to that from cow's milk."
Dr. Messina has one tip for drinking calcium-fortified soy milk. "Some research indicates that even with vigorous shaking, the calcium in soy milk comes out of the solution. However, while some sedimentation occurs in certain soy milks, with mild shaking, this sediment is re-suspended for the majority of soy milk purchased in the United States."
Mark Messina, Ph.D. is a Health and Wellness Expert for The United Soybean Board.
Source
Email interview with Mark Messina, PhD. April 2009. The Soy Connection.
