Despite Promising Soy Research Results, Cancer Experts Urge Caution

American Institute for Cancer Research
Friday, 29 October 1999

According to experts at the American Institute for Cancer Research (AICR), many key questions remain about the role of soy in cancer prevention. Amid promising research results, the Institute advised against adding large amounts of soy or soy supplements to the diet, saying that it is still too early for scientists to make specific dietary recommendations.

"A balanced diet that includes soy is certainly a good idea," said soyfoods expert Mark Messina, Ph.D., "but it should also include plenty of fruits, vegetables, whole grains and beans.

"In general, loading up on any one food or nutrient is not advisable. Many different plant foods appear to offer health benefits, and the best way to take advantage of that fact is to eat a large variety, all the time," he added. Dr. Messina is a former researcher with the Diet and Cancer Branch of the National Cancer Institute.

Recent reports in the popular media moved AICR to make this announcement. Chief among these is the Food and Drug Administration's decision to allow food marketers to make claims about soy protein's role in lowering risk of coronary heart disease.

AICR stressed that the new FDA-approved health claim does not address cancer risk. "More laboratory tests and clinical trials need to be completed before we can speak to that question responsibly," said Dr. Messina.

"As of now, the data on soy and prostate cancer, for instance, are very encouraging but limited. The data on soy and breast cancer are more mixed, especially for women who have or have had breast cancer. It's simply not possible as yet to draw any conclusions about soy consumption and cancer prevention, but further research is certainly warranted," he added.

To date, soy has displayed anti-carcinogenic activity in lab tests involving cancers of the breast, prostate and colon. In some preliminary trials with human subjects, soy and isolated soy components showed the potential to inhibit these same cancers. Some clinical trials, however, suggested possible risks involved with increased soy consumption.

In one much-discussed human study, a soy product was found to stimulate DNA synthesis in the breast cells of premenopausal women. Increased DNA synthesis is traditionally regarded as an indicator of increased cancer risk.

Results of individual clinical trials should not be regarded in isolation, Dr. Messina cautioned. The premenopausal women in the breast cancer trial, for example, were studied over a very short period of time (two weeks.) In addition, the study focused on DNA synthesis, which is only one of several cancer risk indicators.

"Nevertheless," Dr. Messina continued, "this study and others raise legitimate questions about soy consumption and possible risks. More research is needed, particularly into the effects of soy on breast tissue. It's just too early to say anything for certain."

Experts Trace Progress of Research To Date

Soy has been making headlines and sparking debate ever since several laboratory tests showed its potential to inhibit the development of breast cancer, said Dr. Ritva Butrum, Vice President of Research at AICR.

Subsequent studies found the rates of several chronic diseases to be lower in Asian countries, where soy and soy products are a dietary staple. This observation spurred more research into soy's potential to combat cancer and other "Western" diseases like heart disease, stroke, and diabetes.

Laboratory studies have revealed that soy contains a class of plant chemicals called isoflavones. In cell cultures and animal tests, isoflavones have been shown to produce weak estrogen-like effects.

A popular hypothesis suggests that these compounds "stand in" for estrogen at certain sites in the body, providing beneficial effects. At other sites, they may act as anti-estrogens, preventing full expression of the hormone's effects on tissues.

If proven, this theory may help to explain why high-soy diets have been specifically linked to lower risk of hormone-related cancers like those of the prostate and breast. It would also explain why soy seems to show health benefits at several sites in the body.

Further laboratory studies have revealed that soy contains several plant chemicals that also impede or prevent the development of cancer. Among these additional compounds are protease inhibitors, which may slow the rate of cancer division in cells; saponins, which may prevent cancer cells from multiplying; and plant sterols, which also seem to block estrogen.

Clinical Trials Necessary to Gauge Possible Benefits, Risks

"At this time," said Dr. Butrum, "laboratory tests on cell cultures and animal subjects are giving way to clinical trials. We're starting to see if the anti-cancer effects observed in a laboratory can and do occur in the human body." Dr. Butrum said that clinical trials are vital to understanding the real-world benefits - and risks - of important laboratory findings. Scientists use them to determine the efficacy, proper dosage, and reaction time of cancer-fighting agents within the complex human body.

Human trials, said Dr. Messina, tend to be the most "media-friendly" phase of research, and frequently end up on the six o'clock news. He cautioned, however, that despite their importance to the progress of research, individual human trials do not constitute "proof" of anything. It is only when all available research on any given subject is compiled, repeated, and verified that consensus can be reached.

"This is the case when possible risks arise, as in those breast cancer trials, but it's equally true when the clinical studies point to potential benefits."

Preliminary clinical trials have suggested, for example, that soy and/or isoflavones may increase bone density in postmenopausal women. The sample sizes of these studies were relatively small, however. More research into this area will be needed to determine if soy may actually help reduce the risk of osteoporosis.

Dr. Anna Wu, of the University of Southern California, agrees that more work is needed before any dietary recommendations can be made. At AICR's 9th Annual Research Conference in September, she said, "Right now there does seem to be a link between high-soy diets and lower risk of cancers of the breast, prostate and colon, but until the next generation of studies come in, some very important questions remain. What are the effective doses of exposure? Is cancer protection afforded only if soy is present in the diet early in life? Do the processed soy items now hitting the Western marketplace offer an anti-cancer benefit, or is it strictly the traditional soy foods that do so?"

Until these and other standing questions can be addressed, AICR recommends that people wishing to reduce their cancer risk eat a balanced, healthy diet. A large variety of fruits, vegetables, whole grains and beans - including soy - is encouraged, and the Institute urges people to moderate their consumption of fatty, salted and smoked foods.

For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org