DIETARY RESTRICTION OF ENERGY AND CALCIUM ALTERS BONE TURNOVER AND DENSITY IN YOUNGER AND OLDER FEMALE RATS

To determine the influence of weight loss with or without adequate cal cium intake on bone turnover and density, we examined the influence of dietary restriction of calcium or energy on body weight (BW), bone mi neral density (BMD) and bone turnover in both younger (3 mo) and older (10 mo) female rats (n = 66), Diets were designed to allow feeding at two levels of calcium intake (normal = 78 mg/d and low = 15 mg/d) and two levels of energy intake (normal and 40% restriction) while keepin g the intake of protein, fat, fiber, vitamins and other minerals equal between groups. Thus rats received either a control diet (CNTL), a di et restricted in calcium, energy or both for 9 wk, Energy restriction reduced BW 5-21% (P < 0.01) and elevated bone formation 10-20% (P < 0. 05) in both age groups. Bone resorption was 20-40% above CNTL values ( P < 0.05), in rats fed all three restricted diets. In younger rats, BM D increased over time in all groups (P < 0.05), but final BMD was lowe r in calcium restricted groups compared with CNTL (P < 0.01). In older rats, CNTL had a significantly greater final BMD (P < 0.05) than diet -restricted groups. These data indicate that, in both younger and olde r rats, dietary restriction of calcium or energy results in an elevate d rate of bone turnover. BMD is compromised by calcium restriction in both younger and older rats, whereas only older rats were negatively i nfluenced by dietary energy restriction. Thus the present study indica tes a detrimental effect of low-energy diets, as well as inadequate ca lcium intake, on bone density in mature rats.