Soy proteins and isoflavones affect bone mineral density in older women: a randomized controlled trial

Abstract

Background: Soy foods contain several components (isoflavones and amino acids) that potentially affect bone. Few long-term, large clinical trials of soy as a means of improving bone mineral density (BMD) in late postmenopausal women have been conducted.

Objective: Our goal was to evaluate the long-term effect of dietary soy protein and/or soy isoflavone consumption on skeletal health in late postmenopausal women.

Design: We conducted a randomized, double-blind, placebo-controlled clinical trial in 131 healthy ambulatory women aged >60 y. Ninety-seven women completed the trial. After a 1-mo baseline period, subjects were randomly assigned into 1 of 4 intervention groups: soy protein (18 g) + isoflavone tablets (105 mg isoflavone aglycone equivalents), soy protein + placebo tablets, control protein + isoflavone tablets, and control protein + placebo tablets.

Results: Consumption of protein powder and isoflavone pills did not differ between groups, and compliance with the study powder and pills was 80-90%. No significant differences in BMD were observed between groups from baseline to 1 y after the intervention or in BMD change between equol and non-equol producers. However, there were significant negative correlations between total dietary protein (per kg) and markers of bone turnover (P < 0.05).

Conclusions: Because soy protein and isoflavones (either alone or together) did not affect BMD, they should not be considered as effective interventions for preserving skeletal health in older women. The negative correlation between dietary protein and bone turnover suggests that increasing protein intakes may suppress skeletal turnover. This trial was registered at ClinicalTrials.gov as NCT00668447.

Figures

FIGURE 1

Inclusion and follow-up of the study population. Analysis n = 97.

FIGURE 2

Mean (±SEM) changes in markers of bone turnover [urinary N-telopeptide crosslinks of collagen normalized for creatinine (NTX/CRT) and bone-specific alkaline phosphatase (BAP)] expressed as a percentage change from baseline in the 97 women who completed the study. There were no significant differences from baseline or between the 4 groups by ANOVA.

FIGURE 3

Mean (±SEM) percentage change in bone mineral density (BMD) between equol (n = 25) and non-equol (n = 26) producers among women who received the isoflavone tablets. There were no significant differences between equol and non-equol producers at any of the bone sites by an independent t test.

FIGURE 4

Significant correlations between total dietary protein and urinary N-telopeptide crosslinks of collagen normalized for creatinine (NTX) at 12 mo (P = 0.023, r = −0.23) and bone-specific alkaline phosphatase (BAP) (P = 0.005, r = −0.28) expressed as a percentage change from baseline in the 97 women who completed the study.