Energy restriction may require additional calcium to prevent bone loss

Nutrition Research Newsletter, August, 2004

Many studies have indicated that weight loss of >5% is associated with decrease in bone mass. Additionally, fracture risk is increased in women who lose weight. During weight reduction in obese postmenopausal women, bone turnover is increased along with changes in serum hormone profiles including elevation in parathyroid hormone (PTH) and reductions in the concentrations of sex steroids.

A recent study set out to determine whether intestinal calcium absorption is changed by short-term moderate energy restriction at two different amounts of calcium intake and to better understand the regulation of calcium metabolism, PTH, and bone turnover during weight loss in overweight postmenopausal women.

Seventy-three weight-stable, healthy, overweight (BMI 25-29.9 kg/[m.sup.2]), postmenopausal women were recruited into either a weight-loss (WL) or weight-maintenance (WM) program. The subjects underwent a one-month stabilization period, during which they were instructed to consume a total of 1.0 g of calcium per day and asked to maintain body weight. The subjects took a standardized multivitamin and mineral supplement throughout the course of the study. Following the stabilization period, baseline measurements were performed. The Subjects in the WL group then started on a standard nutrition education and behavior modification weight-reduction program, including weekly instruction. WL was achieved through a reduced energy intake while maintaining habitual exercise levels. At baseline and after six weeks of weight reduction or maintenance, calcium absorption and body weight were measured, and three-day dietary intake records and fasting blood and second-morning-void urine samples were collected. Women were randomly assigned in a double-blinded fashion to receive an additional daily supplement of calcium citrate containing either 0.2 g or 1.0 g of calcium.

The 57 women who completed the study had a baseline true fractional calcium absorption (TFCA) of 24.9 [ or -] 7.4%. Energy restriction significantly decreased the total calcium absorbed (P < 0.05) in the WL group compared with the WM group. Regression analysis showed that a greater rate of weight loss suppressed TFCA and the total calcium absorbed (P < 0.05) in the high calcium group. The women in the normal calcium WL group absorbed inadequate amounts of calcium, whereas the women in the high calcium WL group absorbed adequate amounts. PTH explained 22% of the variance in calcium absorbed in the normal calcium group only.

It is suggested that WL is associated with elevated calcium requirements that, if not met, could activate the calcium-parathyroid hormone axis to absorb more calcium. Normal amounts of calcium during energy restriction result in inadequate total calcium absorption and could lead to comprised bone mass.

M Cifuentes, Cl Riedt, R. Brolin, et al. Weight loss and calcium intakes influence calcium absorption in overweight postmenopausal women. Am J Clin Nutr;80:123-130 (July, 2004). [Correspondence: SA Shapses, Department of Nutritional Sciences, Rutgers University, 96 Lipman Drive, New Brunswick, NJ 08901-8525. E-mail: shapses@aesop.rutgers.edu].