Young Women Don’t Bone Up on Soy

November 5, 2002 at 1:24 pm

Among the many reported nutritional benefits of diets rich in soy is a strengthening of bone in postmenopausal women. For these Golden Girls, who face an increasing risk of osteoporosis, soy-based foods can provide much-needed assistance in limiting the inevitable loss of bone.

Although soybeans are best known for their oil, their protein is also laced with antioxidant compounds that possess a hormonal alter ego. Those soy-derived estrogens have been linked to reduced risk of serum cholesterol–and, more recently, to cutting bone loss in older women. United Soybean Board

If soy’s good for older women, it should similarly benefit women in their 20s–fortifying their bodies’ structural scaffolding during peak bone-building years. Or so reasoned John Anderson and his colleagues at the University of North Carolina.

In fact, Anderson says, “We saw zero effect.”

Although data from the yearlong trial surprised the researchers, Anderson acknowledges that in retrospect the findings make sense.

The agents in soy attributed with bone-enhancing power are known as phytoestrogens. In vertebrates, these plant chemicals emulate the female sex hormones known as estrogens, which play an integral role in building and maintaining bone. At menopause, a woman’s production of estrogens diminishes and bone loss usually commences. Dietary supplements of phytoestrogens can substitute for waning natural estrogens and slow the loss of bone.

However, the bodies of younger women make copious amounts of estrogens. So, bones may have all of the estrogen that they need–rendering superfluous any extra hormonal action provided by soy.

That’s not to say that soy offers no benefits to young women. This legume is a rich source of high-quality protein. Also, Anderson says, studies have shown it to lower cholesterol concentrations, reduce risk of breast cancer, and provide some other health benefits.

A new age limit

Since 1998, two studies have reported bone benefits from soy in women at or after menopause. Both tests supplemented the women’s diets with about 90 milligrams of phytoestrogens per day. So, Anderson’s team provided soy protein fortified with 90 mg of phytoestrogens to 15 of the 28 women in their study. The phytoestrogens were principally a combination of genistein, daidzein, and glycitein. The remaining women received soy protein containing negligible amounts of phytoestrogen.

All the recruits arrived monthly to pick up a 30-day supply of soy. Each had her choice of a powder to be sprinkled into food or a chocolate-flavored drink. No surprise, most chose the chocolate formulation. Until the end of the trial, neither the volunteers nor the scientists knew whether an individual’s soy supplements had been fortified with phytoestrogen.

All the women received scans to measure bone mass and density before the trial, 6 months into it, and again at the end.

And when that trial ended, all women had the same bone mass and density they had started with.

Throughout life, people’s bones undergo a constant remodeling–with some portions breaking down as others reform. When the activity of bone-forming cells exceeds that of the cells tearing bone down, bone mass increases. As women enter menopause, the activity of their bone-demolition cells tends to dominate handily–leading to bone loss. Says Anderson, “The primary effect of phytoestrogens seems to be to keep us from losing bone.”

He says future studies may focus on men. Although their bone-remodeling cells also have estrogen receptors, males make little estrogen. So, he reckons, their bones may respond to soy as bones of postmenopausal women do.

People who are interested in increasing their phytoestrogen intake need to keep in mind that the researchers used commercially available soy protein already fortified with more than its normal complement of these compounds. In contrast, tofu has only low concentrations of phytoestrogens.