Previous studies have suggested different effects of high or low protein diets on urinary loss of calcium and bone health. In a study published in the American Journal of Clinical Nutrition, Dawson-Hughes and Harris explored the associations between dietary protein intake and change in bone mineral density (BMD) and fracture risk in a group of elderly subjects.
One hundred sixty-one men and 181 women aged 65 years participated in a 3-year study of the affects on BMD of supplementation a diet supplemented with calcium citrate and vitamin D in a placebo-controlled study. BMD was measured using dual-energy X-ray absorptiometry every six months. Dietary protein was assessed by a food frequency questionnaire midway through the study. The mean protein intake of the participants was 79 g/day, and calcium intake was 871 mg/day in the placebo group and 1346 mg/day in the supplemented group. In general, the men consumed less protein than the women. Those with the greatest intake of protein had significantly less total body BMD loss in the calcium-supplemented subjects, whereas there was no association between protein intake and BMD in the placebo group.
The supplemented subjects also had higher rates of calcium absorption than the placebo group, which suggests that the extra calcium may have offset protein-related deficits in calcium balance due to urinary calcium excretion.
An accompanying editorial by Heaney discusses the homeostatic feedback system controlling the body's calcium economy. He suggests that if both protein and calcium are present in the body in adequate quantities, they can act synergistically to promote bone health, whereas excess protein in the diet may result in bone loss when calcium intake is low. The finding that BMD may be improved by increasing protein intake in conjunction with calcium supplementation is important, given that bone fracture is a major cause of mortality among the elderly.