By now there is little doubt that the powerful compounds in soybeans protect against numerous health conditions, including heart disease, osteoporosis and various cancers. In fact, there seems to be an almost never-ending stream of research revealing new uses for soy isoflavones, proteins and phytonutrients.

Combine this scientific substantiation with the U.S. Food and Drug Administration's soy health claim, and it's no surprise that an ever-increasing array of soyfoods, powders and supplements continues to flood the market. Often the sheer number and variety of these products can be daunting to consumers, not to mention the questions regarding use, dosage and safety.

Retailers can help shoppers understand the unique advantages and limitations of the various soy products. For example, in addition to antioxidant compounds called isoflavones, soyfoods contain a nearly complete amino acid profile and a high percentage of valuable fiber. Thanks to this combination, soyfoods provide a strong synergistic effect not found in supplements. On the other hand, soy protein powders and isoflavone supplements are convenient and formulated in specific doses. These supplements can help improve health; however, customers shouldn't take isoflavones in excess because some evidence suggests this may lead to toxicity and hormone imbalance.

In the past few years, questions about isoflavone toxicity and other soy-related health concerns have come to light. Here is a roundup of soy's pros and cons to help you educate your customers.

Cancer Prevention: The cells that make up the prostate gland, colon, breasts and uterus contain a large number of estrogen receptors and are therefore quite sensitive to changing estrogen levels. Excess estrogen, caused by hormone imbalances or exposure to outside sources, such as pesticides or hormone-laden foods, can initiate cancer growth in these areas. Enter soy. Because of their mild estrogen-like qualities, soy isoflavones are thought to bind to estrogen receptors, thereby blocking estrogen's effects and reducing hormone-dependent cancer risks.

Free-radical Scavenging: In the human body, the soy isoflavones genistein and daidzein disarm free radicals that would otherwise trigger cellular mutations. These isoflavones also boost some levels of the body's own antioxidants.

In a study published in Nutrition and Cancer (1996), researchers demonstrated that lignans—soy fiber—are also capable of scavenging free radicals. So in addition to speeding up waste transit time, binding to carcinogens in the colon and positively affecting colon bacterial enzymes, lignans may help prevent cancer by neutralizing potentially deadly free radicals.

Bone Protection: A synthetic isoflavone called ipriflavone, which is made from daidzein, has proven to be as effective at protecting bone as its natural cousins daidzein and genistien. In addition, journals including Gynecological Endocrinology (1994) and the American Journal of Medicine (1993), have written that ipriflavone has the ability to relieve osteoporotic fracture pain and restore mass and strength to weakened bone, all with few side effects. Ipriflavone was conceived for those consumers who prefer a supplement to eating soy.

Heart Health: The isoflavones genistein and daidzein found in soyfoods also play an antioxidant role in protecting LDL-cholesterol, which is a key factor in heart health (see story on page 20). Additionally, preliminary research shows that soybean saponins, which resemble cholesterol, lower cholesterol levels by either blocking absorption or increasing excretion of cholesterol from the body. Phytosterol, another substance in soy that is similar to cholesterol, may also lower cholesterol levels. Phytosterols work by competing with dietary cholesterol for absorption.

Hormone Balance: Soy phytoestrogens and protein have been shown to reduce menopausal hot flashes. In one 12-week study published in Obstetrics & Gynecology (1998), Italian researchers found that 60 g of soy in supplement form reduced hot flashes by 45 percent.

Does Soy Have A Dark Side?
Soy Allergy: Like milk and wheat, soy is one of eight foods responsible for the majority of allergies. The most common reactions to soy include acne, asthma, atopic dermatitis, diarrhea, eczema, itching, lethargy, vomiting and shortness of breath. However, the majority of soy allergies arise in infants, who usually outgrow the sensitivity by age 2. Therefore, most adults are safe to eat this health-promoting food.

Hormone-dependent cancers: As a result of a few trials, researchers have suggested that soy isoflavones may aggravate existing hormone-dependent cancers. In a study published in Environmental Health Perspectives (1997), researchers demonstrated that low doses of genistein can stimulate some human breast cancer cells to begin DNA synthesis, a potential red flag in terms of cancer growth. Furthermore, in a trial involving isolated soy protein, researchers found the same cellular changes that occur with breast cancer in premenopausal women.

Despite the estrogen-like changes measured during supplementation with soy protein isolate, there is no evidence that these changes will lead to cancer. To date, the results of such studies seem to be mixed. Cell-culture studies are contradictory, and as of yet, no definitive studies have been performed in women with cancer. Therefore, the best advice is for women with existing cancers to avoid soyfoods or supplements as a form of therapeutic treatment.

Soy and Thyroid: In a handful of articles and studies, researchers have suggested that soy consumption may suppress thyroid function. The thyroid, a gland in the front of the neck, controls metabolism via the release of hormones. Since soy isoflavones can act as hormones in the body, some individuals think they may be capable of disrupting endocrine system function, including the thyroid. Although results of a few studies have added some credence to this theory, researchers do not yet have a clear understanding to what extent, if any, soy interferes with thyroid function.

In a 1991 Japanese study published in Nippon Naibunpi Gakkai Zasshi, researchers revealed that excessive soybean consumption (30 g of soybeans per day for three months) by 14 healthy men led to suppressed thyroid function in half of them, as determined by constipation, fatigue and goiters (thyroid enlargement). Furthermore, a month after the study was completed, all symptoms subsided and thyroids returned to normal.

Although the outcome of this Japanese study may seem clear, many researchers believe such a strong reaction to soy was the result of an already existing tendency toward suppressed thyroid function rather than it being the standard reaction for healthy individuals. As a precaution, people who are susceptible to suppressed thyroid function might do better avoiding soy protein because it seems to have a thyroid-inhibiting effect, which could result in a lack of energy and weight gain.

Copper/Zinc Balance: When excess amounts of copper are ingested and stored, zinc deficiency results.

Soyfoods can be doubly problematic for those with a zinc-copper imbalance because they contribute to high copper levels.

As with most things, the key to solving the zinc/copper issue is to eat soyfoods in moderation. In addition, eating zinc-rich foods such as pumpkinseeds, eggs and some lean meat, as well as taking a zinc supplement, can prevent a copper overload.

When encouraging customers to add soy to their diet, focus on simple soyfoods, such as tofu and soy milk. If recommending a soy protein supplement, suggest one that contains no more than 1 to 2 mg of isoflavones per gram of protein.

Linda Knittel is a freelance writer living in Portland, Ore. She is co-author of The Soy Sensation (McGraw-Hill, 2002).

Natural Foods Merchandiser volume XXIII/number 11/p. 28, 32