Previous studies have reported conflicting results concerning soy isoflavones (40 to 99 mg/d doses) and bone health for postmenopausal women. However, no long term studies looking at the potential benefits in men have been reported, said the researchers.
"This is the first study to evaluate the effect of soy supplementation on BMD in older men," wrote lead author Katherine Newton from.
Limiting bone loss could ease the burden of osteoporosis, estimated to affect about 75m people in Europe, the USA and Japan. According to the International Osteoporosis Foundation, the total direct cost of osteoporotic fractures is €31.7bn in Europe, and 17.5bn in the US (2002 figure). The total annual cost of osteoporosis in the UK alone is over £1.7bn (€2.5bn), equivalent to £5m (€7.3m) each day.
Newton and her colleagues from the University of Washington, Group Health Cooperative and the Fred Hutchinson Cancer Research Center performed a controlled, parallel-arm, double-blinded trial with 145 participants (average age 68, average BMI 29 kg per sq m, 123 men).
The study was part of a larger trial looking at the effects of soy isoflavones on the colon health, and only participants with colonic polyps were recruited.
The participants were randomly assigned to receive a soy beverage every day for 12 months, one drink contained 83 milligrams of isoflavones (45.6 mg genistein, 31.7 mg daidzein), while the other contained only three milligrams. Both drinks were obtained from the Solae Company. Bone mineral density (BMD) at the hip and spine was measured at the start and end of the study using dual-energy X-ray absorptiometry.
Compliance, defined as those who consumed 80 per cent or more of the packets, varied between the groups, said the researchers, with about 73 per cent of women in isoflavone-rich drink group, but only 44 per cent of the women in the control group consumed this amount. For men, compliance was 62 per cent for the isoflavone-rich drink group and 82.5 per cent for the control soy drink group.
Improvements in BMD was observed for both sexes, particularly at the spine, with the isoflavone-rich drink associated with a 1 per cent greater increase in men, relative to the control group. This result was not statistically significant, said the researchers.
In women, spinal BMD increased by a significant 2.5 per cent, compared to the control soy drink group. "By comparison, percent change in hip BMD was similar in the [isoflavone-rich drink] groups, and was not different between men and women," said Newton.
"Supplementation of intact soy protein providing 83 mg isoflavones daily tended to increase both hip and spine BMD in men and women, with a borderline statistically significant effect on spine BMD among women at 12 months," said the researchers.
The study does have several limitations of note, including the small number of women participants. No markers of bone metabolism and resorption were measured. Also, all of the participants had been diagnosed with polyps in the colon.
"We have no reason to believe that the bone response to soy isoflavones of individuals with adenomatous polyps would differ from that of individuals without polyps, but this remains a possibility," wrote Newton.
This study does not close the door on the potential benefits for men and women, and the researchers said that, ultimately, larger and longer trials including both men and women are needed to answer the question.
Such clarification may not be too far away, however, with large clinical trails currently underway on both sides of the Atlantic.
"If positive effects on BMD are found, trials with fracture outcomes might be warranted in the future," said the researchers.
"Currently there is insufficient evidence to recommend soy isoflavone supplementation for osteoporosis or fracture prevention," they said.
Source: Maturitas October 2006, Volume 55, Issue 3, Pages 270-277 "Soy protein and bone mineral density in older men and women: A randomized trial" Authors: K.M. Newton, A.Z. LaCroix, L. Levy, S.S. Li, P. Qu, J.D. Potter and J.W. Lampe