Soy protein enriched with isoflavones appear to have no effect on bone mineral content and bone mineral density in young women, according to a new study. Researchers say the finding will disappoint nutritionists hoping to document benefits from diets containing the nutrients, not to mention the soy industry, which has been placing increasing emphasis on the benefits of soy isoflavones.
A report on the study, conducted at the University of North Carolina at Chapel Hill, appears in the current (October) issue of the Journal of the American College of Nutrition. Isoflavones are chemicals made by plants, possibly to protect them against oxidation and organisms that might attack them, and soy beans are an especially good source.
"We had seen earlier reports that isoflavones had beneficial effects on the bones of women undergoing menopause or who already were postmenopausal," said Dr John J.B. Anderson, professor of nutrition at the UNC schools of public health and medicine.
"We felt that if this were true for older women, it might also be true for healthy young women and could help protect their bones over their lifetimes."
The study involved 28 young women in their early 20s. One group of 15 volunteers took soy protein supplements enriched with isoflavones for a year. The second group consumed an isoflavone-deficient soy protein diet, along with other foods over the same period. No one knew until the study ended who was in which group.
Researchers assessed the bone mineral content and density of each subject using dual-energy X-ray absorptiometric measurements at the beginning of the project, after six months and after a year, Anderson said. In other words, they scanned the young women's whole bodies, spines and upper thigh bones to determine how dense their bones were and if those measurements changed over time. The team also controlled for each woman's body mass index - a measure of how heavy she was for her height - along with her physical activity level and diet.
"We found no changes in bone mineral densities and contents after 12 months in either the isoflavone-treated group or the isoflavone deficient group," he said. "Other variables also remained essentially constant over the year, including normal menstrual patterns in both groups."
Isoflavones such as genistein are structurally similar to human oestrogens and for that reason have some oestrogen-like properties, Anderson said. Scientists and drug companies have become increasingly interested in them over the past few years since the naturally occurring chemicals seem to produce positive effects in bone without the negative impact, such as cancer, that oestrogens are believed to have on reproductive organs in some women. The value of oestrogens remains in dispute.
"We were surprised that we saw zero effect," he said. "We think we found nothing because our young women were menstruating normally and therefore had normal oestrogen levels. The natural oestrogen levels may simply have overwhelmed any possible effect of the isoflavones."
Repeating the small but intensive UNC study to confirm or refute the results should help clarify if the new findings are valid, the scientist said.
"A lot of women are taking these plant products because they are afraid of taking oestrogens due to the possible cancer risks," Anderson said. "We need more data to be able to say that, yes, there's a benefit, or no there's not. Right now, we have a lot more questions than answers, but at least we don't think soy isoflavones are harmful in any way."
Support for the work came from the United Soybean Board of Chesterfield, Mo., and Protein Technologies International of St. Louis. The UNC researchers used the same soy isoflavones previously used in two studies at the University of Illinois and Iowa State University reporting beneficial effects of isoflavones in menopausal and post-menopausal women.