The Urinary Excretion of Protein, Non-Protein Nitrogen, Creatinine and Uric Acid in Ageing Male Rats

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Abstract
The urinary excretion of protein in the male rat increased sharply with age, showing a tenfold increase from youth to senility. The excretion of creatinine decreased progressively with age, commencing in middle age and reaching the lowest level just before death. The excretion of both uric acid and non-protein nitrogen, expressed as mg per gram of food consumed, increased in middle age when growth ceased. A second rise in the excretion of these constituents occurred in the last 200 days of life, as the rat approached death. The age changes in the excretion of non-protein nitrogen, creatinine and uric acid were not affected by the presence of renal disease associated with a high protein excretion.