A SECOND LOOK AT SOY

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This wonder food has come under attack. Is the backlash warranted?

YOU PROBABLY KNOW people like me: I'm the type who gets worked up reading ingredient labels. I pay unfathomable prices for organic blueberries. And I am, of course, fully fluent in soy: I make a great tofu stir-fry, love miso, and can recommend the best soy ice cream-I've tried them all. Soy's a staple in my house, and not just because it tastes good. As you've likely heard, it contains plant chemicals thought to help prevent some of the top killer diseases, including heart disease, osteoporosis, prostate cancer, and breast cancer.

But lately, my miracle food has come under attack. Some research has linked soy to increased risk of kidney stones, thyroid problems, Alzheimer's disease, and even breast cancer. Before I trashed my Tofutti Curies, though, I went digging for the real story. What I found: lots of research, some of it conflicting, some of it confusing. But the take-home message from health experts is that soy is still good for most women--but we shouldn't overdo it. Here's a look at the studies, the warnings, plus advice about how relevant they are to you.

Cancer causer?
United States researchers started tracking soy when they found that Asian women have a much lower incidence of heart disease and breast cancer than do American women. While there are many possible reasons for these health differences, scientists zeroed in on the higher amounts of soy in the typical Asian diet. Soy contains large concentrations of plant chemicals called isoflavones, which fall into a class of compounds known as phytoestrogens ("phyto" simply means "plant"). Isoflavones have a tiny fraction of the power of estradiol, the estrogen that courses through women's bodies. A few studies have shown that soy consumption or isoflavone intake can lower levels of estradiol in women by 20 percent or more. Over years or decades, that could protect women from tumors that feed on estrogen, particularly breast tumors.

Using the same logic, however, other researchers arrived at the opposite conclusion: Because isoflavones behave like estrogen, they could actually stimulate the growth of estrogen-dependent tumors in certain cases. This flip-flop doesn't help when you're in the grocery store trying to decide on dinner.

The problem is that hundreds of studies on isoflavones and breast cancer have turned up dramatically conflicting results. In test-tube and animal studies, isoflavones have been shown both to inhibit and accelerate the growth of breast-cancer cells. However, most--but not all--of the relatively few human studies have concluded that eating soy may decrease breast-cancer risk.

The key may be in the timing. The decision on whether to eat soy--and how much--may depend largely on your life stage. For instance, most researchers now think that soy is most protective before and possibly during puberty, because it may change the structure of the breast tissue to make it more resistant to cancer later in life. During adolescence, breast cells grow rapidly and are more vulnerable to cancer-causing influences. Since the potential for damage is great, the benefits of soy may be the most valuable during this stage.

Soy also gets the green light for women in their childbearing years. Experts such as Claude Hughes, M.D., Ph.D., a consulting professor in the department of obstetrics and gynecology at Duke University Medical Center, believe that soy will help premenopausal women decrease breast-cancer risk in later years. "Once a woman is through puberty, it's reasonable to suppose that the less estrogen her breast is bathed in, the better," he says.

But he says that women who are pregnant or nursing may want to limit soy intake. In studies, rats exposed to soy in utero were more prone to developing tumors. "We know that, in humans, phytoestrogens get into the amniotic fluid," Hughes says. While he acknowledges that animal studies don't always predict what will happen in humans, Hughes advises pregnant and nursing women to play it safe and eat only a couple of servings of soy each week.

For postmenopausal women, a reasonable amount of soy (see "How Much Soy?" on this page) should be fine. But women who have breast cancer, are breast-cancer survivors, or are at high risk for the disease (such as a strong family history) should be extra cautious about soy intake. Experts suspect that the weak estrogenic effects of soy may be active in women whose levels of natural estrogen are low. Indeed, in mice with estrogen levels similar to that of a postmenopausal woman, soy causes estrogen-dependent tumors to grow. Again, there is no evidence that this happens in humans, but the animal studies indicate reason for caution.

For postmenopausal women who have breast cancer or are at high risk, experts say that a modest amount of soy is OK. But keep tabs on how much you eat, particularly if you take soy supplements for hot flashes, cholesterol control, or osteoporosis prevention. These supplements contain heavy doses of isoflavones, so it's easy to get too much. And talk to your doctor if you're not sure about your risk or how much soy to take.

Thyroid threat?
The concerns about soy and thyroid have been overblown, unless you never eat salt or already have a thyroid problem. Soy is only one of many foods containing compounds called goitrogens, which in large amounts can hamper thyroid function. (Goitrogens are also found in cruciferous vegetables such as cauliflower.) A deficiency in iodine, which is essential for thyroid hormone synthesis, can make animals and humans especially vulnerable to these goitrogens.

The best source of iodine is table salt, so the vast majority of people in this country get far more iodine than they need. Other sources include shellfish, saltwater fish, and kelp. "For healthy adults who consume sufficient amounts of iodine, I don't think there's a concern at all," says Mark Messina, Ph.D., adjunct associate professor of nutrition at Loma Linda University. "If you have a compromised thyroid function or if you consume inadequate amounts of iodine, you may need to be careful." But then, it's important to get treated for a thyroid condition, no matter how much soy you eat. Thyroid function is checked with a simple blood test that all women should have every five years starting at age 35.

Kidney stone scare?
Recent headlines warning that eating soy foods might increase the risk of kidney stones were also misleading. Kidney stones form when oxalate binds to calcium in the urine and creates tiny crystals that stick together. The bad press for soybeans is a result of a study that found that soybeans are high in oxalate. But soy's oxalate content is no higher than that of other legumes, such as peanuts and beans. Linda Massey, Ph.D., R.D., a professor of human nutrition at Washington State University in Spokane, who headed the oxalate study, says other foods measure higher in oxalate: Parsley ranks number one, spinach and rhubarb tie for second, and legumes come in third. People with a personal or family history of kidney stones should limit their soy intake, just as they should be careful about eating too much of these other foods.

Alzheimer's alert?
A study published last year in the Journal of the American College of Nutrition caused a stir: It showed a correlation between tofu consumption and cognitive decline in older men, and suggested that soy might make them more vulnerable to Alzheimer's. These findings run counter to virtually all other research in this area, and several experts have raised questions about the study's weaknesses. According to Clare Hasler, Ph.D., founding director of the Functional Foods for Health Program at the University of Illinois at Urbana-Champaign, several studies since have shown that soy may improve cognitive function. Researchers are continuing to study the effects of soy on the brain, but for now it appears not to be a concern.

Toss tofu in--or out?
Clearly, many cautions surrounding soy are unfounded or meant for people with special conditions. But soy can have powerful effects, and, as with any food, should be eaten in moderation. Expert recommendations vary, with some suggesting very small amounts and others touting the benefits of 80 or more milligrams of isoflavones per day. Consuming between 30 and 40 milligrams each day (about as much as the Japanese get) is probably a good bet, and most likely that's more than you're eating now.

But if, like me, you do eat a lot of soy, count isoflavones. The amount is generally not listed on food labels and can vary widely. As a conservative rule of thumb, when you're buying whole foods, count 2 milligrams of isoflavones for every gram of soy protein. Scan the chart on page 60 for rough estimates of the level of isoflavones in one 3-ounce serving of specific soy products.

Keep in mind that soy specialty products like protein drink mixes and meal-replacement bars may be pumped up with extra isoflavones. "You can buy soy milk that contains a modest amount of isoflavones," says Bill Helferich, Ph.D., associate professor of nutrition at the University of Illinois. "But you can also buy a product that looks like an instant breakfast drink with soy in it, and it's enhanced with isoflavones from an extract. So it really is no longer a food, it's a supplement with pharmacological doses."

The best advice remains to eat a varied diet--including soy, if it's to your liking. No need to toss your Tofutti Cuties. Just don't eat them all in one day.

how much soy?
We took a look at a few common soy products to see how isoflavone amounts stacked up. For healthy people, the closest there is to an expert recommendation is to eat like the Japanese--which means about 30 to 40 milligrams per day. Be aware, though, that isoflavone content can vary even within one brand, so use this chart only as a guideline; check the labels and, if they don't list isoflavone content, estimate 2 milligrams of isoflavones for every gram of soy protein listed.

MILLIGRAMS OF ISOFLAVONES PER 3.5-OUNCE SERVING

soy sauce 2
soy breakfast sausage 4
soy noodles 9
soy milk 10
immature soybeans
(edamame, green soybeans) 14
soy hot dogs 15
tofu yogurt 16
tofu 28
tempeh burger 29
soy cheese 31
miso 43
tempeh 44
soybean chips 54
mature soybeans, boiled 55
soy protein isolate
(found in shake mixes and protein bars) 97
soy protein concentrate
(found in textured vegetable protein) 102
instant soy beverage powder 110
Isoflavone counts are rounded. Table adapted from the U.S. Department of Agriculture-Iowa State University Database on the Isoflavone Content of Foods, 1999.

skip soy?
Talk to your doctor about the safety of eating large amounts of soy if you:

have or have had breast cancer
are at very high risk for breast cancer
have a personal or family history of kidney stones
are pregnant or nursing
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By Lynn Prowitt-Smith

Lynn Prowitt-Smith is a health writer who has also contributed to Fitness and Newsweek.

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