Soy Protein and Bone Health

Soy Protein and Bone Health in Women

Soy, a protein-rich legume, has been prominently featured in the traditional cuisines of Far Eastern cultures for thousands of years. In addition to its high protein content, soy also contains the other two macronutrients, carbohydrates and fats, as well as many vitamins, minerals and other micronutrients such as calcium, folic acid, and iron. Dietary sources of soy protein include soybeans, tofu, beans curd, tempeh, miso, and soymilk. Since it represents a vegetable source of high quality complete protein, containing all of the essential amino acids, soy is particularly valued by vegetarians, who must derive all their dietary protein from vegetables rather than from animal products.(DGPL)

Soy has yet to become as integrated into American cuisine as it has always been in the Far East, but its sales have gradually increased over the last ten years, to approximately $3 billion and almost one third of Americans consume soy in one form or another at least once a week. A cholesterol-free, high protein, high fiber, complex carbohydrate-rich food source that is low in saturated fat and sodium, soy is such a healthful food that researchers have theorized that it may be one explanation for the comparative lower incidence of certain medical problems in the Orient.(Rubin)

Numerous ongoing studies are endeavoring to determine the beneficial value of soy protein on such varied medical conditions as heart disease, cancer, and on various symptoms associated with menopause. Researchers have expressed particularly strong interest in the possibility that soy protein may reduce post- menopausal calcium loss in bones and concurrent estrogen deficiency-related reduction of bone density in women. So far, the results are somewhat positive, but ultimately inconclusive, in light of several recent studies.

Possible Bone Health Benefits:

Osteoporosis is an age-related loss of bone density that affects as many as 24 million Americans of both genders, but the highest incidence is among post- menopausal women. Anecdotal evidence of the relatively low incidence of osteoporosis in Asian cultures inspired medical researchers to examine the possible role of soy protein in the diets of comparatively healthy populations in China and Japan, where soy has been relied upon for dietary protein instead of primarily animal proteins.(DGPL)

Soy proteins contain high levels of isoflavones, which are molecularly similar to human estrogens, and relatively concentrated in soybeans. Scientists at the University of North Carolina at Chapel Hill suggest that bean plants produce isoflavones as protection against parasitic organisms and oxidation.(ScienceDaily) The process of oxidation has also been implicated extensively by literature on human aging. These factors suggest a possible mechanism through which diets rich in soy protein might counteract menopause-related osteoporosis in women. The theory is simply that isoflavones constitute a natural form of "estrogen replacement," which is commonly prescribed to treat symptoms of menopause, such as hot flashes and particularly, calcium loss and decreased bone density associated with osteoporosis.

The implications are potentially significant, owing to the numerous side effects cancer, most notably,) and other idiosyncratic medical contraindications often associated with traditional forms of synthetic hormone replacement therapy.(Boyles)

Indeed, several short-term studies do suggest that isoflavone-rich diets correspond to slower bone loss in menopausal and post-menopausal women, as measured in one study, by higher spinal density in women eating high soy diets.

According to the author of one study conducted by the Division of Nutritional

Sciences at the University of Illinois at Champaign-Urbana, soy-rich diets corresponded with measurably stronger, denser bones that were also higher in mineral content, in menopausal women compared to a control group. Similarly, researchers at the Department of Human Nutrition and Dietetics at the University of Illinois at Chicago concluded that soy protein is effective in preventing the increase of serum cholesterol typically associated with decreasing ovarian hormones. The results of the DHND Chicago study mirrored the findings of researchers at DNS Champaign-

Urbana in that high soy diets corresponded to better bone quality, in addition to reduction of calcium loss. (GGPL)

The Journal of Medicinal Foods reports that the results of thirteen different studies indicated that high soy diets were effective in significantly reducing the incidence of hot flashes in menopausal women, which are closely linked to and often concurrent with osteoporosis, and also traditionally treated by hormone replacement therapy.(Messina, Hughes)

However, enthusiasm over the potential beneficial medical uses of high soy diets as an alternative to hormone replacement therapy for osteoporosis has been somewhat tempered by the (unexpected) contradictory results of another recent study investigating its effects on younger women.

Contradictory Studies:

In a study reported by researchers at the University of North Carolina at Chapel Hill's School of Public Health and Medicine, high soy diets apparently had no beneficial effect on the relative density or mineral content of the bones of twenty eight female subjects in their early twenties.(ScienceDaily) The twelve-month study consisted of one group of subjects on a diet high in soy protein, while another group consumed only isoflavone-deficient soy, pursuant to a double-blind protocol in which neither the subjects nor researchers knew which group received which soy supplement. Other variables, such as body mass index and physical activity levels were also monitored and incorporated into the data.

Whole-body, dual-energy X-ray absorptiometric measurements were taken at the onset of the study, and halfway through, and again, at the conclusion of the study.

Contrary to expectations, the results revealed absolutely no benefit or measurable difference in relative bone density between the two groups of subjects. This was particularly surprising in light of earlier studies suggesting that soy rich diets at the time of puberty corresponded to lower adult rates of breast cancer owing to beneficial changes in breast tissue structure during their developmental stage.(Rubin)

According to the author of that study, the results contradict neither any previous data concerning the lifelong benefits of reduced cancer risks among women consuming high soy diets during puberty, nor the data related exclusively to menopausal and post-menopausal women.(Boyles) The working assumption among researchers is simply that naturally high estrogen levels of estrogen in young women outweighs any benefit conferred by the isoflavones in soy.(Boyles)