The significance of soy may depend on your cultural background. In farming areas, soybeans are one of North America’s largest cash crops. The annual harvest of more than 75 million acres goes to such commercial uses as feed products for livestock and, more recently, fuels (i.e., biodiesel).1 For people of Asian descent or those choosing a vegetarian diet, soy is better known as the source of tofu, soy milk, and edamame. From 1992 to 2008, soy foods sales increased from $300 million to $4 billion.1

Background

The soybean is a legume mostly grown in the United States, Brazil, Argentina, and China.2 The soybean plant is covered with fine brown or gray hairs. The leaves dry and fall before the seed is ready for harvest. The seed is held within hairy pods that grow in cluster of three to five.2 The bean itself is pea-sized, dusky yellow-gold, and very hard. All parts of the bean are used in a seemingly endless variety of products.

Science

The soybean is the only plant considered a complete protein. Complete proteins are those that contain adequate amounts of the eight essential amino acids for cellular repair and metabolism.3 This makes soy an inexpensive animal-protein replacement. Soy also contains a significant amount of omega-3 fatty acids and isoflavones, both known for their antioxidant and cellular-protective attributes.4

Soybeans have been the subject of extensive medical research, primarily in the areas of cardiovascular health, endocrinology, and oncology.

Studies have had mixed findings with regard to cardiovascular benefits of soy. In an American Heart Association review of a decade of clinical trials, at least three different meta-analyses failed to show impressive changes in either blood cholesterol levels or BP readings.3 While there were single trials that found small but statistically significant reductions, the overall reductions in LDL averaged only 3%.3 No significant reduction in BP was noted in the review, although there were isolated trials with marginal decreases.3

Soy has earned recognition as a nonhormonal method for combating menopausal symptoms. Since the results of the Women’s Health Initiative were made public in 2002, millions of women sought “natural” alternatives to estrogen.5 The isoflavone component of soy produces a mild estrogenic effect in humans.3 The public seized this information, and the popularity of soy-based products for women soared. However, after years of study, nearly 20 clinical trials have failed to show a statistically significant improvement in menopausal symptoms.3

Osteoporosis is one consequence of low estrogen levels in postmenopausal women. Because of the nature of osteoporosis and the length of time required to determine bone matrix changes, adequate trials are still lacking, but preliminary results show a trend of increasing and maintaining bone mineral density with soy supplementation.3 In one study, women in the placebo group lost 12% of their bone mass over the three-year trial, while those in the treatment arm who received an average of four servings of soy foods per day gained 9% bone mass.1

Researchers are torn between hoping for positive results attributable to the anti-oxidative effect of soy and the potential for worsening of estrogen-receptive cancers, especially in women.3,6 However, Asian population studies show lower rates of breast and endometrial cancers.6 Multiple factors (e.g., length exposure to soy, age, and family history) make it difficult to definitively state whether soy plays a positive or negative role in estrogenic cancers.3,6

Studies of the use of soy in men with prostate cancer have shown impressive results. Since soy is both estrogenic and anti-androgenic, its potential for preventing or slowing prostate cancer is logical. In vitro studies have shown that soy prevents the development and growth of prostate cancer cells. Multiple studies have verified a decrease in prostate specific antigen velocity and doubling time, both measures of the efficacy of anti-cancer therapy.7