Plant Chat: Mark Messina Answers Top Questions on Soy

Is soy healthy, or is consuming soy dangerous? Learn about the facts on soy with soy expert Mark Messina in this interview.

As a plant-based dietitian, it seems that I can’t go through a single day without someone asking me questions about the safety of soy. Is it really safe? Does it cause cancer? Does it feminize men? The questions keep rolling in, and there is just so much confusion on this plant food. That’s why I sat down with one of the nation’s leading soy experts, Mark Messina, PhD, in order to answer some of your most pressing questions about the safety of consuming soy. Read on in our interview.

Answering Today’s Top Questions on Soy

Sharon: Why are consumers so afraid of soy? There is so much negative buzz surrounding this food!

Mark: There are many reasons, some legitimate but most definitely not.

In the late 1990s, research raised questions about the safety of soyfoods because of their rich isoflavone or phytoestrogen content. For well over a decade now, there has been speculation, based on studies in mice, that isoflavones might stimulate the growth of estrogen-sensitive breast tumors. Understandably, that made many women fearful of soy. There has also been a mistaken equating of soy phytoestrogens with the hormone estrogen. That led to concerns about soy consumption by children in particular, and men because of concern about feminizing effects.

However, extensive human research has shown isoflavones and estrogen often act very differently, which is not at all surprising since small differences in chemical structure can result in huge differences in physiological effects. A good illustration of this point is the differing effects of dietary cholesterol and phytosterols, which have almost identical chemical structures.

However, cholesterol raises blood cholesterol levels whereas phytosterols lower it. Clinical research shows that neither soy nor isoflavones lower testosterone or adversely affect sperm or semen. In fact, Italian researchers suggested that isoflavones could be a treatment for low sperm count.

In regard to breast cancer, the human research (clinical and epidemiologic or population studies) supports the safety of soyfoods for breast cancer patients. Not surprisingly, both the American Institute for Cancer Research and the American Cancer Society have concluded breast cancer patients can safely consume soyfoods. In fact, a recently published analysis involving nearly 10,000 breast cancer patients found that those women consuming higher amounts of soy (1-2 servings per day) after diagnosis of their disease were about 25% less likely to have a recurrence of their cancer over the approximately 7 year period the study participants were followed than were women who consumed little soy.

Now to the nonsense on the internet which alleges that soy is responsible for all manner of harm. These claims are not based on an objective evaluation of the science. When there is as much research conducted on a food as there is on soy, it is easy to scare the public by citing the results of studies that don’t represent the findings overall. Or to rely on the results of animal studies while ignoring human research. Some of the allegations are so plainly false it is absolutely painful to see them repeated in blog post after blog post.

The one that is most irksome is this notion that Asians consume less than a tablespoon of soy per day and that all of it is fermented. There are probably a hundred studies that provide intake data based on the results from validated food frequency questionnaires that include as many as 9 questions on the types and amounts of soyfoods consumed. Some of these surveys include 50,000 participants. Using the same erroneous method for coming up with this tablespoon figure about soy intake would also indicate that Americans on average eat only a tablespoon of dairy per day. In regard to the fermented nonsense (discounting soy sauce which is a condiment,  not a food), ethnic Chinese (China, Hong Kong, Singapore) tend to consume almost no fermented soyfoods whereas in Japan about half of the soy consumed is fermented (natto, miso) and half not (tofu).

I also think part of the “anti-soy” mentality has to do with a bias against the food industry. This association is particularly ironic because the research budget for the soyfood industry absolutely pales in comparison to that which exists for so many other foods — meat, milk, eggs, to name just a few. And for those who are adamant about the harmful effects of genetically modified foods, soy is an easy target because most soybeans grown in the US are genetically modified.

However, nearly all soy grown in the US is used for animal feed. It is quite easy to purchase non-genetically modified soyfoods so there is no reason to avoid soy irrespective of views on genetic modification.

Sharon: How can moderate soy intake contribute to a healthy diet?

Mark: There are several benchmarks one can use as a basis for developing soy intake recommendations. These include Asian soy intake and the results from clinical and epidemiologic studies. It isn’t really possible to talk about “Asian intake” per se because the amount of soy consumed in Asian countries differs quite markedly. Even within China,  there are huge differences among geographical regions. In contrast, Japan is much more homogeneous.

Generally, those consuming a traditional diet in Japan and Shanghai (lots of data are available from this city because of the existence of two large cohort studies comprised of Shanghai residents) consume about one to two servings per day. In Asian epidemiologic studies, higher amounts of soy, say two to three servings per day are often associated with better health. In clinical studies in which benefits are observed, participants in the soy group usually consume the equivalent of two to four servings per day.

Given the above, for adults I consider moderate intake to be one to two servings per day. If you split the difference and assume a serving provides 7 grams of protein, then roughly 10 grams of dietary protein would come from soy at this intake level. Ten grams represents about 15% of the total protein of the average American. I recommend limiting soyfoods to four servings daily simply because the dietetic principles of variation and moderation dictate not placing undue emphasis on any single food.

Sharon: What are the nutritional properties of soy that hold promise?

Mark: Soyfoods are good sources of protein and have an excellent fatty acid profile (low in saturated fat, moderate in monounsaturated fat, high in polyunsaturated fat including both the essential omega-6 and omega-3 fatty acids). That alone,  in my opinion,  is sufficient to warrant a larger role for soy in US diets than currently is the case. Soyfoods are also good sources of potassium, a problem nutrient, and of B-vitamins. Most soymilks are fortified with calcium and vitamin D, and some tofus are made using calcium, so these products are good sources of those nutrients.

Sharon: Overall, which areas of health show the most promising benefits?

Mark: I entered this field because of my interest in the role that soy may have in preventing cancer. That is still my hope, that research will definitively show that soy reduces cancer risk. There are intriguing data indicating soy intake during childhood and/or adolescence reduces breast cancer risk. And there is substantial evidence that soy reduces risk of developing both prostate cancer and lung cancer (among non-smokers). Interestingly, there is also evidence that soy benefits patients with these cancers. However, it is nearly impossible to reach definitive conclusions about diet and cancer relationships because such data require conducting large, long-term clinical trials with cancer as an endpoint. These types of diet studies are nearly impossible to conduct.

There is solid evidence that soyfoods reduce risk of coronary heart disease through multiple mechanisms. Soyfoods provide heart-healthy fat, soy protein lowers blood cholesterol (and possibly blood pressure accordingly to recently published meta-analyses) and the isoflavones appear to improve arterial health, at least in postmenopausal women. Isoflavones also alleviate hot flashes.

One new area that holds promise is skin health. There are preliminary published data showing a reduction in wrinkles in response to soy consumption. However, the existing research is much too limited to reach any conclusions. More importantly, are very impressive recent data showing a reduction in wrinkles in response to isoflavone intake but so far these data have only been presented not published. Until the findings go through peer-review, the evidence remains speculative.

Sharon: Is there any fear that people can consume soy at levels that might be problematic, especially considering the use of soy protein isolates, etc?

Mark: Not in my view. A wide variety of soy products are found in a large number of commonly consumed non-soy foods. However, soy is primarily added to these foods for functional,  not nutritional reasons. That is to say, to increase water retention or shelf life for example. Consequently, the amounts added are very small and nutritionally irrelevant. Estimates are that Americans currently consume only about one to two grams of soy protein per day. That is negligible given average total protein intake is about 85 grams per day. So unless one is actually consuming soyfoods, whether they be traditional (e.g., tofu) or modern (soy burgers), it is hard to see how soy intake could be excessive.

Having said this, there are now juices, cereals and energy bars that are made using or fortified with soy protein so that might be a way of consuming substantial amounts of soy without consuming “soyfoods.” But unless one was also consuming soyfoods the amount consumed wouldn’t be worrisome. Besides, the same situation exists for proteins like milk proteins (casein and whey), which are also used extensively in the above-mentioned products but in this case, most Americans already consume dairy products.

For some of my favorite soy recipes, check out the following:

Vegan Stir Fry with Chinese Long Beans with Tofu
Spicy Sesame Grilled Tofu
Sesame Tempeh Grain Bowl
Buffalo Cauliflower Tofu Skewers
Crunchy Mandarin Tofu Salad
Asparagus Dill Tofu Quiche