The effects of a two-year randomized, controlled trial of whey protein supplementation on bone structure, IGF-1, and urinary calcium excretion in older postmenopausal women

The effects of dietary protein on bone structure and metabolism have been controversial, with evidence for and against beneficial effects. Because no long-term randomized, controlled studies have been performed, a two-year study of protein supplementation in 219 healthy ambulant women aged 70 to 80 years was undertaken. Participants were randomized to either a high-protein drink containing 30g of whey protein (n=109) or a placebo drink identical in energy content, appearance, and taste containing 2.1g of protein (n=110). Both drinks provided 600mg of calcium. Dual-energy X-ray absorptiometric (DXA) hip areal bone mineral density (aBMD), 24-hour urinary calcium excretion, and serum insulin-like growth factor 1 (IGF-1) were measured at baseline and at 1 and 2 years. Quantitative computed tomographic (QCT) hip volumetric bone mineral density (vBMD) and a femoral neck engineering strength analysis were undertaken at baseline and at 2 years. Baseline average protein intake was 1.1g/kg of body weight per day. There was a significant decrease in hip DXA aBMD and QCT vBMD over 2 years with no between-group differences. Femoral neck strength was unchanged in either group over time. The 24-hour urinary calcium excretion increased significantly from baseline in both groups at 1 year but returned to baseline in the placebo group at 2 years, at which time the protein group had a marginally higher value. Compared with the placebo group, the protein group had significantly higher serum IGF-1 level at 1 and 2 years (7.3% to 8.0%, p<.05). Our study showed that in protein-replete healthy ambulant women, 30g of extra protein increased IGF-1 but did not have beneficial or deleterious effects on bone mass or strength. The effect of protein supplementation in populations with low dietary protein intake requires urgent attention.