1. We report here studies of the interrelationship of bone and muscle growth in the rat and the regulatory role of dietary protein. Two experiments were undertaken. In experiment 1, growth inhibition was induced by ad libitum feeding of low protein diets containing 7%, 3.5% or 0.5% protein, with a control group fed a 20% protein diet. Measurements were made at 1, 3 and 7 days. In experiment 2, complete growth inhibition was induced by ad libitum feeding of a 0.5% protein diet with measurements at 7, 14 and 21 days followed by refeeding diets of 3%, 6%, 9%, 12% and 20% protein, with measurements after 3, 7, 10 and 14 days of refeeding (experimental days 24, 28, 31 and 35). Controls fed a 20% protein diet were studied at 0, 14, 21, 24, 28, 31 and 35 days.

2. Body weight growth stopped immediately in all reduced protein groups, with subsequent weight maintenance on the 7% protein diet, slight loss on the 3.5% protein diet or marked weight loss on the 0.5% protein diet, although food intake was maintained for 3 days, falling in all groups after this time. Inhibition of muscle growth was delayed in the 7% and 3.5% protein fed groups, with 12–15% increases in muscle weight after 7 days, but prompt growth inhibition occurred with the 0.5% protein diet with subsequent weight loss. In animals fed the control 20% protein diet, muscle weight (W) reflected tibial length (L) as W = L3.85/102.93 (r = 0.98, n = 98). Calculation of the muscle weight/bone length ratio (μg/mm3.85) indicated that a significant muscle deficit was apparent on day 3 and subsequently in the 0.5% protein fed rats, but not until day 7 in the 3.5% and 7% protein fed animals.

3. Total tibial length, epiphysis length and epiphyseal cartilage width were measured radiographically. In all groups there was no significant reduction in bone length growth during the first 3 days. After 3 days there were graded reductions on reduced protein intakes with complete inhibition on the 0.5% protein diet. Epiphyseal cartilage width responded sensitively, with a reduction within 24 h of the 0.5% and the 3.5% protein diets, and within 3 days of the 7% protein diet. The epiphysis length was only minimally affected.

4. In experiment 2, food intake increased immediately on refeeding in all except the 3% protein fed group. Accelerated body weight growth occurred in the 20%, 12% and 9% protein fed groups, slower growth in the 6% protein fed and little growth in the 3% protein fed group. Muscle growth commenced immediately in all groups, continuing at an accelerated rate in the 20%, 12% and 9% protein fed groups, at a slower but substantial rate in the 6% fed group and with little further growth in the 3% fed group. This allowed muscle repletion in relation to tibial length (i.e. μg/mm3.85) by day 7 in 9%, 12% and 20% fed protein groups.

5. Bone growth recovered slowly on refeeding, in a graded manner with the protein intakes. Significant increases in tibial length were only observed after 7 days of refeeding with 7–10 days required to fully restore growth in the 20% protein fed group and 10–14 days for the 12% and 9% protein fed groups. Only 50% of the age control rate was achieved in the 6% protein fed group, with little growth in the 3% protein fed group. Although gradual restoration of the epiphyseal cartilage width occurred in a graded manner with increasing protein intakes, complete restoration did not occur in any group. The small reduction in epiphysis length was partially, although not entirely, reversed by refeeding.

6. These studies demonstrate an anabolic drive of dietary protein on bone growth which responds in a graded manner to protein intake at levels in excess of those necessary for maximal rates of muscle growth. Muscle growth appears to be dependent in part on bone length growth, possibly through the anabolic influence of passive muscle stretch.

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