According to United Soybean spokesperson Dr. Mark Messina, eating soy reduces heart disease risk by lowering cholesterol and blood pressure. Eating soy may also reduce the risk of breast and prostate cancer when consumed during childhood and most studies show that the isoflavones in soybeans alleviate menopause-related hot flushes.
But, despite the known health benefits of eating soy, the concerns, so I asked Dr. Messina to address some of them.
Will Soy Destroy Your Pancreas?
Messina also points out that if these protease inhibitors were so active, they would prevent your body from absorbing the proteins found in soy. But, in fact, your body absorbs soy proteins very well. "Throughout the 1980s, Vernon Young and colleagues from the Massachusetts Institute of Technology showed in long-term human-feeding studies that the quality of soy protein is quite high - essentially similar to that of animal protein."
Does Soy Cause Dementia?
The problem is that nutrition and diet studies can be tricky to interpret. It's difficult to track all the foods people eat every day, so results aren't always accurate. Most of us don't remember what we ate an hour ago -- much less a day or a week ago. And often, the study designs don't take other foods and factors into consideration that can skew the results. According to Dr. Messina, the HAAS had several design weaknesses. For example, the intake of only 26 foods was assessed. "Nowadays, it is common for epidemiologic studies of this type to assess the intake of at least 100 foods," Dr. Messina says. "By assessing the intake of so few foods, it is very difficult to control for potentially confounding variables. Perhaps tofu intake was associated with cognition because of dietary habits common to tofu consumers, and not because of tofu consumption per se. Also, the intake questions pertaining to tofu differed from one time point to the next so the authors had to devise a somewhat convoluted method for classifying men based on different intake responses."
Another study showed no affect of soy on brain function at all. "In contrast to the Hawaiian and Indonesian studies, a study from Hong Kong found that isoflavone intake was unrelated to cognitive function.
Is it possible that soyfoods actually provide cognitive benefits? Maybe for some women. Dr. Messina explains that clinical trials that have examined the impact of isoflavone-rich products on cognitive function actually suggest that, at least in younger postmenopausal women, isoflavones favorably affect several aspects of cognitive function, but its' really too early to tell. Dr. Messina says, "At this point, it is not possible to draw conclusions about the impact of soy or isoflavones on cognitive function although generally speaking, intervention studies -- which are suggestive of benefit -- carry more scientific weight than epidemiologic studies, which show mixed results."
Is Soy Bad for Boys?
Nor do isoflavones have any detrimental effects on male hormones. Dr. Messina explains, "A meta-analysis examined the relationship between soy/isoflavone intake and reproductive hormone levels in men. The study included 36 treatment groups and found no effects on total and free testosterone levels. By the way, three clinical studies also show no effects on sperm or semen."
Does Soy Block Absorption of Minerals?
But what about 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."
Does Soy Damage Your Thyroid
Can we always believe animal studies as they might relate to humans? Messina tells us, "In rats, isoflavones inhibit by about half, the activity of an enzyme called thyroid peroxidase (TPO), which is required for the synthesis of thyroid hormone. However, not only are rats much more sensitive to possible goitrogens than humans, but even though TPO was inhibited in this study, thyroid function remained normal in the rats."
Dr. Messina believes human studies are necessary to really understand how soy (or any other foods) effect the human body. In 2006, according to Messina, a review of 14 clinical trials concluded there was little evidence that soy foods or isoflavones had an adverse affect on thyroid function in healthy human subjects. "Studies published subsequent to this review have also found no effect on thyroid function. Thus, quite clearly in individuals with adequate iodine intake and normal thyroid function, the evidence indicates soy foods even in very large amounts do not adversely affect thyroid function. The only study that raised concern was published in a Japanese journal in 1991. The study was never repeated and was poorly designed."
Does Soy Make Your Red Blood Cells Clump Together?
According to Dr. Messina, when you eat soy and soyfoods, the lectins are destroyed by your digestive system -- they don't do anything to your blood cells. Actually, most of the lectins are destroyed by heat, cooking or processing long before you eat soyfoods.
Where did the concern about clumping blood cells come from? Dr. Messina says the origins are from research studies that were performed by exposing cells to lectins in a lab dish or by injecting soy lectins into the body and not actually feeding soy to lab animals. As Dr. Messina concludes, "Citing reports of soybean lectin toxicity that involve the effect of lectins on cells in the lab and/or other than oral administration have little relevance to in vivo(in the body) findings."
Mark Messina, Ph.D. is a Health and Wellness Expert for The United Soybean Board.
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Email interview with Mark Messina, PhD. April 2009. The Soy Connection.