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Dietary Protein and Phosphorus: Effect on Calcium and Phosphorus Metabolism in Bone, Blood and Muscle of the Rat¹

U.S. Department of Agriculture, Agricultural Research Service, Protein Nutrition Laboratory, Beltsville Human Nutrition Research Center, Beltsville, MD 20705

Four- or 12-week-old rats were fed diets containing either 25 or 45% casein and either 0.35 or 0.8% phosphorus (P) for 7 weeks. Calcium (Ca) level of each diet was 0.9%. In the young, fast-growing rat (expt 1), plasma P was increased (P < 0.01) when 0.8%, rather than 0.35%, P was fed. In the more mature rats (expt 2), increased dietary P depressed Ca in plasma but elevated P; high protein intake elevated whole-blood Ca, but depressed plasma P. Dietary treatment had little effect on muscle of mature rats, but wet weight and total P content of muscle were higher in young rats fed 0.8%, rather than 0.35%, P. Femur dry weight and P concentration were greater when 0.8%, rather than 0.35%, P was fed to young rats; dietary P did not affect any other parameter of bone (expt 1, 2). Excessive protein intake elevated femur P concentration in young rats, but depressed length and strength of femurs in mature rats. Some parameters of bone, blood and muscle can be influenced by dietary levels of protein and P. The response, however, is dependent on the physical maturity of the rat when dietary treatment begins.

KEY WORDS: • high protein • bone Ca • bone P • blood Ca • blood P • bone strength • bone flexibility

¹ A preliminary report has been presented at the meeting of the Federation of American Societies for Experimental Biology, Dallas, TX. April 1979. Fed. Proc. 38:872 (abs.).

² Submitted in part to the Graduate School of the University of Maryland as partial fulfillment of the requirements for the M.S. degree.

³ Reprint requests to: Mrs. Juliette C. Howe, U.S. Department of Agriculture, Beltsville Human Nutrition Research Center, Room 202, Building 308, BARC-East, Beltsville, MD 20705.

Manuscript received 28 March 1983.