Soy provides protein, healthy fats and nutrients. While soy and soyfoods are popular, consumers have a number of concerns about soy.
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 persist, so I asked Dr. Messina to address some of them.
Will Soy Destroy Your Pancreas?
Trypsin inhibitors and protease inhibitors found in soybeans (and other foods such as legumes and whole-grains) could potentially affect the activity of protein-digesting enzymes in the foods you eat, but only in very large amounts.
While raw soybeans contain a large amount of these inhibitors, we don't eat them when they're raw. We cook them or process them into tofu, tempeh or soy milk. The processing and cooking removes almost all of the inhibitors, and even if it didn't, laboratory research suggests it wouldn't matter.
Messina explains, "Research in the male miniature swine -- which is considered to be the ideal animal for studying the effects of protease inhibitors because their digestive physiology and anatomy is similar to humans -- found there were essentially no adverse effects in response to the consumption of diets containing very high amounts of trypsin inhibitors."
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?
This idea began with a study published in 2000 called the Honolulu-Asia Aging Study (HAAS), which was designed to determine if westernization of the diet of Japanese men living in Hawaii increased heart disease risk. Near the end of this 30-year study, a cognitive function element to the design was added. That part of the study found that higher tofu intake was associated with poor cognitive test performance.
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 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."
Messina suggests that 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," he said.
Another study showed no effect 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.
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.
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?
Some people worry about the possibility that phytoestrogens in soy foods and certain infant formulas might be bad for boys. They fear the phytoestrogens may negatively effect male hormones. Dr. Messina assures that's not the case, "The isoflavones in soy foods, which are classified as phytoestrogens, have raised concerns about feminization.
However, isoflavones are different from the hormone estrogen. Studies show quite clearly that the two molecules - isoflavones and estrogen - exert different physiological effects. For example, estrogen raises levels of triglycerides, which increases risk of heart disease, and raises levels of HDL-cholesterol, which lowers risk. In contrast, isoflavones have no effect on triglycerides or HDL."
Nor do isoflavones have any detrimental effects on male hormones.
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?
According to 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," he added, "questions have arisen about the impact of soy on mineral status. The consumption of relatively little red meat is enough 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."
Messina explains that 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," Messina said.
"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," Messina said.
"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," he added.
Does Soy Damage Your Thyroid
According to Messina, concerns about the anti-thyroid effects of soy are based primarily on in vitro and animal studies involving the soybean isoflavones (phytoestrogens). Several cases of goiter were attributed to the use of soy infant formula but this problem was eliminated in the mid-1960s by fortifying the formula with iodine.
Can we always believe animal studies as they might relate to humans?
Messina says, "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."
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," he said, "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,"
Messina added that 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?
Soy contains proteins called lectins that were discovered over a century ago. Lectins can cause red blood cells to agglutinate, or clump together, so they are also given the name haemagglutinins. As such, some people worry that eating soy and soyfoods might cause blood cells to clump together in the body.
According to Messina, research shows the lectins in soy don't cause any health problems. He says that a study done by Dr. Irvin E. Liener, from the University of Minnesota, says that unlike a number of other lectins, soybean lectins were found to be essentially harmless.
Messina says 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.
How did the concern about clumping blood cells begin?
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 -- not 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.
Email interview with Mark Messina, PhD. April 2009. The Soy Connection.