Prolonged fasting as conditioned by prior protein depletion: effect on urinary nitrogen excretion and whole-body protein turnover.

To study the influence of previous dietary protein depletion on nitrogen (N) loss and protein turnover during a total fast, we measured plasma leucine kinetics and urinary N and 3-methylhistidine (3MH) excretion in obese and normal subjects. In one study, 10 moderately obese women fasted for 2 weeks after adaptation either to a normal maintenance intake of 80 g protein and 150% of estimated resting energy expenditure (control group), or to 10 days of a 950-kcal, 200-g carbohydrate, 4-g protein diet (depletion group), with measurement of postabsorptive (or fasting) plasma leucine turnover on the maintenance diet and after 3 and 10 days of fasting. As measured after 10 days of fasting, body N loss was blunted by 17% when preceded by the protein-deficient diet. Plasma leucine flux and oxidation of the control group increased in early fasting and decreased by 10 days, in accordance with previous reports. Results for the depletion group were similar in absolute magnitude, despite the preceding protein-deficient diet. In a second study of five normal men, leucine kinetics were measured on a maintenance diet, after 10 days of a protein-free diet, and after 3 days of fasting. After protein depletion, leucine flux decreased by 19% (P less than .05). After 3 days fasting, leucine flux was 16% higher than on the maintenance diet (P less than .05), but 44% higher than the value on the protein-free diet 3 days earlier (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)