Influence of high protein diets on cartilage and bone formation in rats

The influence of diets containing combinations of high protein and low calcium on discrete stages of bone formation was investigated in 28-day-old rats. A bone matrix-induced bone forming system was utilized to determine the stages of endochondral ossification that were being affected. Mesenchymal cell proliferation as assessed by [3H]-thymidine incorporation and ornithine decarboxylase activity were unchanged in animals fed a high protein (80% casein)/normal calcium (0.61% Ca; 0.40% P) diet. However, osteogenesis was reduced by 78% in the rats fed high protein/normal calcium as measured by 45Ca incorporation. Alkaline and acid phosphatase activities in bone were increased 2.5 and 2.3 times, respectively, reflecting increased matrix turnover induced by the high protein availability. Bone that did form was not remodeled nor was there evidence of marrow formation. The animals were normocalcemic and normophosphatemic and showed no evidence of acidosis. A combination diet of high protein and low calcium resulted in a 62% reduction of cell proliferation and chondrogenesis and a 98% inhibition of bone formation. High dietary protein-induced osteoporosis in animals is due to a failure of osteogenesis of the stage of ossification possibly a result of restricted availability of calcium at the site of mineralization.