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Human Utilization of Urea Nitrogen in Low Calorie Diets¹

Metabolism and Nutrition Unit, Instituto de Investigaciones Médicas, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires, República Argentina

To evaluate the role of calorie intake on the ability of nonprotein nitrogen to substitute for part of a high biological value protein, urea was added to the low caloric milk diets of five obese women during periods of negative nitrogen balance. In five studies, negative nitrogen balance was induced by decreasing milk nitrogen intake to 3.7 g/day and in two others by decreasing carbohydrate intake to 50 g daily. Nitrogen retention was increased when urea was added to the low calorie milk diet resulting in nitrogen equilibrium (four out of five studies) but not a positive nitrogen balance as was observed when milk nitrogen was increased. Addition of methionine (limiting essential amino acid in milk), lysine or both did not improve nitrogen retention when urea was added to the low milk protein diet. Increased nitrogen retention was not observed when urea was added to the low carbohydrate diets. These results indicate that in low calorie diets nonprotein nitrogen may replace part of the high biological value protein only when the diet provides a sufficient amount of glucose for nonessential amino acid synthesis.

Manuscript received 15 January 1971.