An Update on Soy: It's Just So-So
Harvard Health Letters
Harvard Health Letters
The onetime health food champion is an excellent source of protein but it hasn't lived up to its earlier billing.
These days, the notion of a separate category of health foods seems out of date. Those stores with the bulk bins and organic produce are increasingly overshadowed by upscale chains like
Maybe that's why seeing soy in headlines seems like such a throwback. For a long time, soybean-based beverages and foods like soy milk and tofu epitomized health foods: vegetarian, rich in protein, maybe responsible for the lower rates of heart disease and cancer in
But there have also been some nagging doubts about soy. Early research suggested soy protein was "heart healthy" because it could lower LDL (bad) cholesterol levels, but subsequent studies and judgments have dampened that enthusiasm. The results for soy protein have been so unimpressive that the
There have also been worries that the estrogen-like chemicals in soy, called isoflavones, might promote the growth of estrogen-sensitive cells and therefore increase the chance of breast cancer recurrence. Study results reported in 2009 in The Journal of the
With soy back in the news, now seems like a good time to revisit the soy story and its various subplots. An update might help clear up some confusion and provide some guidance on where this standard-bearer of dietary virtue might fit into today's ideas about healthful eating.
Soy is short for soybean. According to
Soybeans are nutritional powerhouses because they contain twice the amount of protein and more oil (healthy unsaturated fat) than other beans, but very little starch. Soybeans are the only plant food that could serve as a person's sole source of protein because they contain all eight essential amino acids. But the relative lack of starch means cooking soybeans doesn't soften them up as much as it does other beans.
Soybeans also have a strong -- too strong for many people -- "beany" flavor that comes in part from the fat content breaking down into smaller pieces. According to McGee, the Chinese and others developed soy milk, which is the watery residue of cooked soybeans, and tofu, which is curdled soy milk, as a way to get around these problems and make soybeans more palatable.
SOY AND ISOFLAVONES
Phytoestrogens are estrogen-like compounds found in plants. The phyto-prefix comes from phyton, the Greek word for leaf. The isoflavones in soybeans are just one of several classes of phytoestrogens. Although phytoestrogens are chemically similar to estrogen and behave like the hormone in some respects, they are far weaker. (Men don't need to worry about feminizing effects from eating soy-based foods.)
The two main types of isoflavones in soybeans are daidzein and genistein. They can be isolated from soy; that's why you can buy isoflavone capsules. But daidzein and genistein are integral parts of soy, so if you drink soy milk or eat tofu or any other kind of food made from soy, you're probably getting some isoflavones, along with the protein and other nutrients.
Soy flour and soybean oil are used to make a lot of different kinds of food, so there are many "hidden" sources of isoflavones in the diet, in addition to the foods that obviously are made from soybeans. For example, doughnuts made with some soy flour may contain a small amount of isoflavones. And black licorice candy contains formononetin, a compound that gets metabolized into daidzein.
We've posted a list of foods and their isoflavone content on our Web site at www.health.harvard.edu/healthextra.
ESTROGEN IMPERSONATOR AND OPPONENT
If high protein content was the only notable attribute of soybeans, they'd be just another, if slightly better, bean. It's their isoflavone (pronounced eye-so-FLAY-vone) content that makes them stand out nutritionally. Isoflavones are structurally similar to estrogen, the female hormone, so the thinking has been that eating a soy-rich diet might have some of the same consequences as an increase in estrogen levels.
If soy has estrogenic effects, it might be good for the bones, the heart, even the brain -- and in men as well as women. Indeed, the notion that a soy-rich diet might make up for a deficit in estrogen is the reason some older women drink soy milk and eat tofu. The hope is that the isoflavones will counteract the drop-off in estrogen levels that causes menopausal symptoms like hot flashes and vaginal dryness.
But how soy and its isoflavones behave in the body is complicated. In some parts -- such as bone, it seems -- isoflavones mimic estrogen, occupying the same receptors and therefore having a similar, if weaker, effect. If soy's isoflavones impersonate estrogen in bone, that's a good thing, because estrogen protects against bone loss by inhibiting osteoclasts, cells that break down bone, and stimulating osteoblasts, cells that build it up. But in other parts of the body -- the breast, for example -- the estrogen-like effects of isoflavones might mean extra cell growth and division and an increase in the risk of a cancer developing.
What makes soy even harder to figure out is that isoflavones can also have anti-estrogen effects. For example, isoflavones may dock on receptors that the hormone would otherwise occupy and activate, thereby competing with and thwarting estrogen's effects. Isoflavones may also suppress estrogen levels by inhibiting enzymes involved in the hormone's production. Whether soy has these anti-estrogen effects or estrogen-like ones may hinge on how much of the hormone is present to begin with. And soy may have some effects beyond estrogen's purview, such as inhibiting the growth of blood vessels that can supply cancer.
All of these ins and outs -- and others not mentioned here -- have made it very difficult to pin down the health effects of soy and its isoflavones. Literally thousands of studies have been done. Soy and isoflavones may be one of the most-researched topics in all of nutrition. The abundance of research is a mixed blessing: it's easy to get lost in a maze of inconsistency and nuance. Even so, a few generalizations can be made.
The following is a brief overview of some of the research on soy and isoflavones:
SOY AND CANCER
Studies of colon, lung, prostate, and ovarian cancer suggest that soy does have anticancer properties. The caveat is that many of these results are from case-control studies, which aren't the most reliable type of study. The relationship between soy and breast cancer has been difficult to sort out. Several important studies failed to show any kind of protective effect, but studies conducted in
Some researchers have speculated that a certain amount of soy may need to be consumed before breast cancer protection kicks in. How much? The threshold might be 10 milligrams of isoflavones a day, which could be achieved -- and then some -- by consuming a standard, 80-gram serving of tofu every day.
Worries about soy promoting breast cancer recurrence came mainly from animal and other experiments. The previously mentioned JAMA study included about 5,000 women in
SOY AND HEART DISEASE
Soy's career as a health food got a big boost in 1995 when a meta-analysis published in
But what if consuming soy meant losing weight? The low-carbohydrate, high-protein "Eco-Atkins" diet replaces animal protein with plant-based sources, including soy. A small study published in 2009 found that people lost weight (about 9 pounds) on the Eco-Atkins and lowered their LDL and blood pressure.
SOY AND MENOPAUSAL SYMPTOMS
Interest in soy as a natural alternative to hormone therapy pills (estrogen alone or estrogen and progestin) started to increase after a large, government-sponsored trial showed that the pills increased the risk of developing breast cancer and, under certain circumstances, the risk of blood clots, heart attack, and strokes.
In 2009, researchers at
SOY AND BONE
Several short-term studies (six months or less) showed that soy-rich diets might combat the bone loss that affects many women after menopause because of a dip in estrogen levels. But in 2009 and 2010, results from larger and longer randomized trials didn't show much, if any, bone benefit from soy or isoflavone tablets.
SOY AND THE BRAIN
In 2000, a study linked tofu consumption in middle age to poor cognition in older age. The research results since have meandered between showing brain benefits and not. But as for harm, there doesn't seem to any reason to worry, the tofu study notwithstanding.
SOY AND THYROID
Some studies have hinted at soy isoflavones reducing thyroid hormone levels. But it's now clear that people with normal thyroid function who aren't taking thyroid medicine don't have to worry about soy suppressing their thyroid levels. A handful of case reports have indicated that soy might interfere with the absorption of thyroid medicine. The greatest concern is with babies born with thyroid problems who are given thyroid medication shortly after consuming soy-based infant formula. Still, the general recommendation is that people on thyroid hormone replacement therapy who have a lot of soy in their diet should consume soy at least two hours after they take thyroid hormone.
THE BOTTOM LINE
Because of its reputation as a health food, soy seems to look at us reprovingly. Tofu wags its finger and says, hey, you really should be eating more of me.
But soy has emerged from the thousands of studies a bit humbled. The claim that it's especially good for the heart and lowering LDL cholesterol now looks like it was overblown. As brain food, it's just another "well, maybe" among many others, and some studies have cast doubt on whether it's good for bone. The research into whether the isoflavones in soy will help alleviate menopausal symptoms is a work in progress. A bright spot is the number of studies suggesting that a soy-rich diet could be protective against some kinds of cancer, but one must be careful about making too much out of them.
On the other hand, soy is an excellent source of protein, which is important to vegetarians and vegans who need plant-based protein. And, yes, there are many nutritionally valid reasons to steer our eating in a vegetarian direction. But it now looks like soy is just another food choice, and there are many perfectly good ways to maintain a healthful diet without it. - Harvard Health Letter
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