A High Mixed Protein Diet Reduces Body Fat without Altering the Mechanical Properties of Bone in Female Rats

Long-term consumption of high-protein (HP) diets at 35% of energy is postulated to negatively influence bone health. Previous studies have not comprehensively examined the biochemical, physical, and biomechanical properties of bone required to arrive at this conclusion. Our objective in this study was to examine the long-term effect of a HIP diet on bone metabolism, mass, and strength in rats. Adult female Sprague-Dawley rats (n = 80) were randomized to receive for 4, 8, 12, or 17 mo a normal-protein (NP) control diet (15% of energy) or a HP diet (35% of energy). Diets were balanced for calcium because the protein sources were rich in calcium. At each time point, measurements included weight, body composition, and bone mass using dual-energy X-ray absorptiometry, mechanical strength at the mid-diaphysis of femur and tibia, microarchitecture of femurs using microcomputerized tomography and serum osteocalcin, carboxy-terminal crosslinks of type 1 collagen (CTX), insulin-like growth factor-1 (IGF-1), leptin, and adiponectin. Effects of diet, time, and their interaction were tested using factorial ANOVA. The HIP diet resulted in lower body weight, total body, and abdominal fat and higher lean mass. Serum leptin and adiponectin were greater in HP-fed than in NP-fed rats, but IGF-1 did not differ between the groups. Whereas the HIP diet resulted in higher relative bone mineral content (g/kg) in the femur, tibia, and vertebrae, serum osteocalcin and CTX and bone internal architecture and biomechanical strength were unaffected. In conclusion, HIP diets at 35% of energy lower body fat content without hindering the mechanical and weight-bearing properties of bone. J. Nutr. 139:2099-2105, 2009.