Soy Extracts Don't Improve Bone Density in Older Women

by Harvard Women's Health Watch

Soy once looked like the kinder, gentler alternative to hormone therapy for menopausal symptoms and risk reduction.

Many population studies suggested that women who regularly ate soy products had not only fewer hot flashes but also a lower risk of developing heart disease and osteoporosis. This apparent power of soy has been attributed to its estrogen-like effects.

Scientists seeking to isolate the chemical components in soy that are responsible for its seeming benefits have homed in on soy protein and isoflavones. Studies with isoflavones suggest that the compounds act preferentially in bone. And a few studies have shown that isoflavones increase bone mineral density in perimenopausal women. But findings have been inconsistent or lacking in older women -- in particular, women 10 years or so past menopause, the time when fracture risk starts rising dramatically.

Researchers at the University of Connecticut have studied the effects of isoflavones on bone loss in women over age 60.

They divided 97 healthy women into four groups and assigned each for a year to one of the following daily regimens: 18 grams (g) of soy protein (about the amount in one cup of tofu) plus 105 milligrams (mg) isoflavone; 18 g of animal protein (about the amount in a 3-ounce serving of salmon) plus 105 mg isoflavone; 18 g of soy protein plus placebo; or 18 g of animal protein plus placebo. The protein (soy or animal) was consumed as a powder mixed into food and beverages; the isoflavone or placebo was in the form of three 35-mg pills daily. All the women were told to get at least 1,200 to 1,500 mg of calcium per day.

At the beginning of the study, the women underwent bone density testing with dual energy x-ray absorptiometry (DXA) and blood tests for enzymes that indicate bone remodeling activity. During the study their blood was tested for isoflavones to make sure they were taking the pills and powder as directed.

After a year, average bone mineral density had declined equally in all four groups, and bone remodeling enzymes were equally active in all four groups. The researchers concluded that adding soy protein or isoflavone supplements to the diet doesn't build bone or prevent bone loss in older postmenopausal women. Results were published in The American Journal of Clinical Nutrition (July 2009).

Soy is a good source of protein, but women over age 60 with osteoporosis should talk to their clinicians about other options, including bisphosphonates and synthetic parathyroid hormone (teriparatide, or Forteo).

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