Urinary Creatinine as an Index of Body Composition

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Abstract
The relationships between the amount of creatinine excreted in the urine and the quantities of the major chemical components of the ingesta-free body (determined by direct analysis) were investigated in 65 sheep. The effects of age of animal, kind of diet and level of feed intake on the rate of creatinine excretion were also studied. These studies revealed that the protein and water content and the fat-free mass of the ingesta-free body are highly correlated with the amount of creatinine excreted in the urine. Prediction equations for these relationships were derived which permit an accurate estimate of the quantity of protein, water, and the fat-free mass of the empty body of living sheep. Body weight was an accurate predictor of protein content in sheep containing less than 28% of fat and weighing less than 55 kg. As an index of protein content in sheep containing 28 to 47% of fat, creatinine output was superior to body weight. Thus, the rate of urinary creatinine excretion is an index of much promise for use in nutritional and genetic investigations with ruminants. Although no direct means of estimating body fat from the creatinine output was found, body fat may be estimated indirectly from an estimate of the fat-free mass or of the water content. The relationships observed between urinary creatinine output and the weights of protein, water and fat-free mass in the empty body were independent of the age of sheep between 4 and 27 months, the kind of diet ingested, the level of feed intake, the size of animal, and the degree of body fatness. The animals used in these studies ranged in body weight from 17.1 to 75.8 kg and in fat content from 4.9 to 46.6%. One milligram of creatinine was excreted per 24 hours for each 5.13 ± 0.43 gm of protein contained in the ingesta-free body.