Is it safe to eat soy?

Soybeans: an ancient and healthful food

Soyfoods can make it easy and fun to plan vegan menus that meet nutrient needs. So it’s a little discouraging to see so many vegans down on soyfoods these days. Some websites and books say that eating too much soy—especially processed soyfoods—can raise your risk for cancer, nutrient deficiency, and thyroid problems.

It can be tempting to take these allegations seriously, since they are usually based on research findings. But they don’t reflect the preponderance of evidence. The selective picking and choosing of individual studies allows people to “prove” just about anything. To get at the truth, health experts examine all the research and base recommendations on the totality of the evidence, not just a few studies.

Furthermore, almost without exception, studies suggesting that soy is harmful have been conducted in rodents or test tubes. This type of research rarely provides meaningful insight into human nutrition. In fact, more often than not, animal studies lead to erroneous conclusions about effects in humans.

In the past 20 years there have been more than 9,000 studies that relate to soyfoods. So what do the data say about soy when we look at all of the research with an emphasis on human studies? The findings are, in fact, pretty reassuring about soy.

There is a lot to say on this subject, so I’m planning a little bit of a soy marathon this week. Today’s column will look at different types of soyfoods and their effect on nutrient status. Tomorrow I’ll write about cognitive function, and then will finish up with a discussion on soy and chronic disease.

Is isolated soy protein safe?

A number of vegan foods, including veggie burgers and other meat analogs, are made from isolated soy protein or soy protein concentrate. While a healthy diet should be based primarily on whole foods, there is no evidence that these isolated proteins are harmful. Claims have been made that humans can’t digest and use the protein in these products, but isolated soy protein is actually well-digested and absorbed.

Studies that might form the basis for misunderstandings have used proteins that were exposed to harsh chemical conditions. These are different from the processes used to isolate soy protein for commercial foods. In fact, the original studies on the protein quality of soy—which looked at the ability of soy to support growth in humans—used isolated soy proteins, not whole soyfoods. These are the studies that established soy as an excellent quality protein, well-digested and utilized by the body.

Are fermented soyfoods better for you?

Fermented foods such as miso, tempeh and natto, were the first soyfoods consumed in Asian countries. They are higher in vitamin K, antioxidants and potentially beneficial bacteria than non-fermented soyfoods. However, non-fermented soyfoods, such as tofu and soymilk, are well-digested and are good sources of protein and other nutrients. Non-fermented soyfoods, like tofu and soymilk, have been consumed in Japan for at least 500 years and in China for more than 1,000 years. At least half of the total soy consumed in both countries today comes from non-fermented foods.

Soyfoods and Mineral Absorption

Like all beans and whole grains, soyfoods are high in phytate—a compound known to inhibit the absorption of minerals such as iron, zinc, and calcium. Despite this, absorption of minerals from soyfoods is good, sometimes surprisingly so. Calcium in soy products like calcium-set tofu and fortified soymilk has high bioavailability and zinc absorption rates are only about 25% lower from soy than from meat (although soyfoods aren’t especially high in zinc).

Recent research--based on a new understanding about the type of iron in soyfoods--shows that absorption of iron from soy may have been underestimated in the past. A recently conducted 10-week study found that replacing animal foods with two to three servings of soyfoods per day had no negative effect on iron or zinc status in women. Vegans should eat a variety of whole foods to meet nutrient needs. But all types of soyfoods can make a contribution to mineral and protein intakes.

I’ll be back later this week with a look at the controversy over soy and cognitive function. and the relationship of soy to chronic disease.