Jury still out on value of soy
By the faculty of Harvard Medical School for The Harvard Medical School Adviser

Q: What's the story with soy protein? I've heard mixed messages about whether it's really good for you.

A: These days, soybased foods and foods enriched with soy protein are widely available in mainstream grocery stores (see sidebar). Although it was once the belle of the health-food ball, soy is looking a bit forlorn these days. Claims that soy protein would protect us from heart disease now seem a little inflated. Studies have shown its effect on LDL ("bad") cholesterol is pretty small. Most studies show a drop that averages less than 5 percent. Large amounts of soy protein -- 50 grams a day, which is about half the amount of protein that Americans typically eat in a day -- have the same modest effect as smaller amounts.

Researchers have looked beyond cholesterol for other ways soy might help the heart. But scientists haven't found much in the way of benefit for other risk factors, such as HDL ("good") cholesterol, triglycerides or blood pressure.

Apart from the protein, many researchers thought soy might have other health benefits. Soy contains substances called isoflavones, which are chemicals that behave like a weakened form of estrogen in the body. In breast tissue, the thinking went, isoflavones would keep estrogen from latching on to receptors that promote some types of cancers. Support for this theory came from observations that women in Asian countries who eat a lot of soy have lower breast cancer rates.

However, when researchers set out to test this idea, they found that isoflavones might actually promote the growth of breast tissue in premenopausal women. Those findings stirred up a lot of Internet chatter, much of it overblown, about soy causing breast cancer. Still, there's enough evidence for the American Cancer Society to warn breast cancer survivors not to consume concentrated sources such as soy powders and isoflavone supplements.

Meanwhile, additional studies concluded that if soy is protective against breast cancer, it's only in women who ate large amounts of it during childhood and adolescence. Adopting a soy-rich diet later in life may be too late.

Hope for soy isoflavones as an alternative and more "natural" way of replacing estrogen in menopause has also become a little shaky. Soy protein and isoflavones in pill form have had little if any benefit for treating hot flashes.

However, a few reports published earlier this year suggest that writing off soy completely would be a mistake. One study compared a diet containing red meat with two other diets, one that substituted soy protein for red meat and another that substituted soy nuts. The soy protein had plenty of good effects, but the soy-nut diet was better at lowering blood sugar and LDL levels. The researchers think that's because soy nuts contain unsaturated fat and more isoflavones than soy protein. Another study found that a diet relying on soy milk, soy yogurt and tofu for protein lowered LDL cholesterol slightly more than a diet based on animal protein.

If you search the Web for information about soy, you'll find all kinds of postings, much of it warning about soy's dire consequences for the thyroid, the brain -- you name it. What you won't find is much of anything resembling solid proof for these claims.

Soy may have come down a peg or two, but working some of it into your diet is OK, particularly if it's replacing red meat. Swapping soy-based foods for meat means trading a source of saturated fat and cholesterol for one with polyunsaturated fat, fiber and some healthful vitamins and minerals. That's always a good nutrition deal. But as with anything, if you overdo it, you might be asking for trouble.

Soy has long been a staple of the Asian diet. It can be a useful part of a healthy American diet, particularly as a replacement for red meat, but it's far from a cure all.

Copyright 2007 the President and Fellows of Harvard Developed by Harvard Health Publications (www.health.harvard.ed u). Distributed by UFS. Submit questions to harvard_ adviser@hms.harva rd.edu.