Effect of Age, Caloric Restriction, and Dietary Protein on Bone Composition of Male Rats

Thirty-day old male Wistar rats were fed 1 of 8 purified diets. Three groups were fed diets containing either 12, 20, or 28% casein (protein) ad libitum. Three other groups were fed diets containing 18, 30, or 42% casein with a feed intake restricted to two-thirds of that consumed by the ad libitum groups. In addition, two groups were initially fed diets containing 20% casein, then the casein level was either gradually (0.8%/month) decreased to 12% or increased to 28% by 1 year of age. At either 1 or 2 years of age, the rats were sacrificed, and their femurs were analyzed for length, wet weight, dry weight, fat-free dry weight, fat and ash. The femurs of the ad libitum groups were longer (p < 0.0001), heavier (wet bone wt, p < 0.0001), and contained greater amounts (p < 0.0001) of absolute ash than those of restricted rats. Increased age of the rats in both the ad libitum and restricted groups was also associated with increases in the previously listed responses, but they were of lesser magnitude. Restricted-feeding (p < 0.0001) and aging (p < 0.006) were associated with increased percentages of water. There was a significant diet by age interaction (p < 0.002) for the percentage fat in the femurs. Ash expressed as a percentage of fat-free dry bone was neither significantly affected by caloric or protein restriction nor by age when the ad libitum group was compared to the restricted group. Length and wet weight of the femurs were remarkably similar regardless of whether the protein was constant or changing. Ash expressed in an absolute amount was greater (p < 0.01) in the changing rather than in the constant protein ad libitum group but only approached significance when expressed as a percent of fat-free dry femur weight. Within the changing group, the 20 → 12% protein group had femurs which were shorter (p < 0.001) and weighed less (wet bone wt, p < 0.001) than the 20 → 28% group, but percent water was not significantly different.