What We Still Don't Know About Soy

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Harvard Health

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Harvard Women's Health Watch

August 2001

What We Still Don't Know About Soy

In Asia, soy foods have been a dietary staple for centuries. In the United

States, health guru and breakfast food entrepreneur John Harvey Kellogg

introduced soybean products in the 1920s as healthful substitutes for milk

and meat. Yet for decades most Americans thought of soy primarily as an

unappealing health food store staple or as a mystery meat extender served in

cafeterias and mess halls.

 

Over the last several years, however, American attitudes toward soy have

shifted from dubious to near-devotional. Now supermarket shelves are graced

by thousands of soy products - everything from snack foods and breakfast

cereals to cheese and coffee substitutes. It's estimated that we will spend

more than $3 billion this year on soy foods, not counting supplements made

from concentrated soy compounds.

 

What happened? To a large extent, we became convinced that soy equals

healthy. Soybeans are unique among plant foods in supplying all the

essential amino acids that the human body needs, making soy protein similar

in quality to meat protein -- but with largely unsaturated instead of

saturated fat. In addition, soy contains the isoflavones genistein and

daidzein, plant hormones that seem able to either mimic or counter the

effects of estrogen. They have been proposed as helpful in preventing

several hormone-related diseases.

 

Soy's biggest official boost came with endorsements from the Food and Drug

Administration (FDA), in 1999, and the American Heart Association (AHA),

last year, for its ability to lower cholesterol. That spurred the recent

boom in soy marketing. But it also worries some experts who believe that our

rush to consume large amounts of soy - and, in particular, to take

supplements with large amounts of soy isoflavones - is premature, given the

current evidence on soy's health benefits and possible risks.

 

..Potential Problems

Brain cells. In the 1960s, researchers took dietary information from about

8,000 men of Japanese descent participating in the Honolulu Heart Study. In

tests administered three decades later, when the men were ages 71-93, those

who had eaten the most tofu (more than two servings a week) performed less

well on cognitive tests. The researchers also tested their wives and found a

similar decline in cognitive function among those women who ate the most

tofu. Among men for whom the researchers had brain imaging or autopsy

results, greater tofu consumption was associated with more brain atrophy.

The brains of men who ate a lot of tofu generally resembled those of men

about five years older who had eaten little tofu.

 

 

Reproductive tract. Because of the hormonal effects of soy isoflavones

(which have been found in human amniotic fluid), some scientists worry that

large amounts of soy may cause deleterious changes in the reproductive

tract, particularly during early life. Indeed, animal studies suggest that

exposure to genistein before and just after birth may affect ovarian

follicle development.

 

In May, the National Toxicology Program called for further study of the

health effects of genistein because of credible evidence that hormone-like

chemicals can produce changes in the female reproductive tract and the

prostate gland. The National Institutes of Health is also sponsoring an

infancy-to-adulthood study to detect possible adverse effects of consuming

soy-based formulas in infancy.

 

 

Thyroid. Some physicians have reported cases in which women developed goiter

or symptoms of hypothyroidism while consuming large amounts of soy. In the

laboratory, genistein interferes with a key enzyme in the thyroid gland,

thyroxine peroxidase, making some scientists concerned about the impact of

soy on thyroid function. But researchers tracking hormone levels in

premenopausal and postmenopausal women taking soy isoflavones have found no

clinically significant changes in thyroid function.

 

As a precaution, if you are being evaluated for a thyroid problem or are

receiving thyroid medication, tell your endocrinologist about your use of

soy foods and supplements. If you are taking thyroid hormone to correct

hypothyroidism, soy may interfere with the absorption of the medication in

your intestines. To minimize this impact (thereby allowing you to take the

lowest possible dose), your endocrinologist may advise you not to take your

thyroid medication within 2-3 hours of a meal that contains soy. In infants

who require thyroid hormone because of congenital hypothyroidism, soy

formula can cause a significant absorption problem and should not be

consumed.

 

Allergies. Soy is one of the eight food groups most often responsible for

allergic reactions. It's difficult to avoid soy entirely, since it turns up

in small amounts in many products, such as water-packed tuna, that are not

themselves soy foods. In May, a group of major food manufacturers agreed to

label products containing even tiny amounts of soy and other common

allergens.

 

Evidence on Benefits

Lowering cholesterol. Soy's impact on the risk for heart attack and stroke

has not been directly evaluated. However, last November, the AHA recommended

that people include soy protein foods as part of a low-fat, low-cholesterol

diet in order to promote heart health. The AHA's Nutrition Committee acted

after reviewing evidence from a 1995 meta-analysis of 38 controlled clinical

trials, along with several newer studies assessing soy's impact on lipid

levels. It concluded that consuming 25-50 grams of soy protein daily could

reduce LDL cholesterol by 4%-8%. People with normal cholesterol levels show

slight improvement, and benefits are proportionately greater with moderately

high cholesterol levels. But the impact of dietary soy is decidedly modest -

roughly equivalent to the effect of adding soluble fiber (for example, from

oat bran or pectin) to your diet.

 

In the human studies reviewed by the AHA, soy protein that contained

isoflavones lowered cholesterol significantly more than soy protein without

isoflavones. And isoflavones separated from soy protein did not lower

cholesterol. It may be that the cholesterol-lowering effect of soy products

is due to a synergy among soy protein, isoflavones, and other components of

soy.

 

Easing menopausal symptoms. Because soy isoflavones have certain estrogenic

effects, women and researchers have looked to soy as a possible alternative

to standard hormone replacement therapy. Greater intake of dietary soy has

been proposed to explain why Japanese women report fewer hot flashes than

American women. Within Japan, researchers found that women with the highest

consumption of soy products and the highest intake of isoflavones had fewer

hot flashes than those who ate little soy.

 

But when soy is given to Western women in controlled studies to prevent hot

flashes, the benefits have been slight. For example, in a trial of

postmenopausal women in Italy, those taking soy protein reported 45% fewer

moderate or severe hot flashes after 12 weeks of treatment, but women taking

a non-soy protein powder also improved, by 30%. In comparison, hormone

replacement therapy has been shown to reduce hot flashes by at least 70%.

 

Protecting bone. Two prospective studies have found some evidence that soy

consumption might help prevent the bone loss associated with menopause. In

one, perimenopausal women still having regular periods but experiencing 10

or more hot flashes a week were assigned to groups receiving supplements,

variously, of isoflavone-rich soy, isoflavone-poor soy, or a whey protein

control. After six months, women receiving whey had significant losses in

bone mass and density. Those receiving isoflavone-poor soy had

non-significant bone loss, while those receiving isoflavone-rich soy showed

a small improvement. In another six-month study, postmenopausal women showed

an increase in bone mineral content and density in their lumbar spine (but

not in the thigh) after taking an isoflavone-rich soy preparation. Milk

protein and soy protein with a low concentration of isoflavones gave no

protection.

 

If the level of protection documented in these studies continued for many

years, women taking isoflavone-containing soy products might be at lower

risk for osteoporosis. However, the researchers caution that six months is a

relatively short period of time in terms of bone turnover. Longer-term

trials are underway.

Soy and Breast Cancer: Promise or Peril?

Asian nations with high soy consumption have lower rates of breast cancer,

which suggests that components in soy may somehow protect breast tissue. But

population-based differences are far from proof of a cause-and-effect

relationship. Many other features of a particular culture, including other

dietary components, could account for such patterns. Studies undertaken in

China and Japan have not identified any consistent relationship between soy

intake and breast cancer risk. For now, it's unclear whether adding soy to

the diet might reduce a woman's risk of developing breast cancer, and

there's no evidence that soy is of any particular benefit for women who

already have breast cancer.

 

Still, there are reasons for the research interest in this connection. Soy

isoflavones resemble tamoxifen and raloxifene, selective estrogen receptor

modulators already used for breast cancer prevention. Like these

medications, isoflavones attach to estrogen receptors but act differently

from estrogen in some tissues. In addition to hormonal influences,

isoflavones also have anti-tumor activity and block the formation of blood

vessels in chemically induced breast tumors in animals. Researchers are

investigating the possibility that they may act against non-hormonal cancers

as well as the hormonally influenced cancers of the breast and prostate.

 

Soy's effects on breast tissue are not well understood. Its impact on breast

cancer may depend on when in a woman's life she consumes it. There is the

possibility, bolstered by some animal studies, that soy must be consumed

early in life to exert a protective effect on breast tissue. Animal studies

also suggest that prepubertal exposure to the soy component genistein is

protective. And when premenopausal women drink isoflavone-containing

soymilk, it significantly lowers their circulating levels of both estrogen

and progesterone, potentially protecting them from estrogen's

cell-stimulating effects. On the other hand, one small, randomized study

found that soy consumption (in the form of textured vegetable protein) was

associated with increased breast tissue proliferation in a group of

premenopausal women with benign or malignant breast disease. In

postmenopausal women with little estrogen production, isoflavones with even

a weak estrogen-like effect may potentially stimulate the growth of

estrogen-dependent breast tumors.

 

Women receiving tamoxifen as treatment for breast cancer often wonder

whether it would be valuable or safe to add soy foods or supplements to

their daily regimen. Unfortunately, no definitive studies are available,

according to researchers at Harvard Medical School. Some experts caution

that anything with desirable estrogenic effects can also have unwanted

estrogenic effects, e.g., increased cell reproduction. This may be

especially true of concentrated soy supplements. Others researchers believe

that breast cancer patients are probably fine if they follow an Asian diet

and lifestyle in which soy foods are substituted for some meat protein, and

lots of vegetables and exercise are included. Certainly before starting

isoflavone supplements or changing your diet to incorporate large amounts of

soy, you should discuss it with your oncologist

 

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