Eleven obese patients were placed on a liquid formula diet containing 320 kcal (1.34 MJ), 31 g protein, 44 g oligosaccharides, 1.5 g fat, vitamins, and essential minerals for a period of 4 weeks under metabolic ward conditions. The diet was well tolerated, and mean weight loss was 2.49 kg/week during the 4-week period. Nitrogen excretion diminished in all patients during the period of treatment, but nitrogen balance remained slightly negative in most patients, mean daily deficit being 1.3 g N/day at the end of the study. The rate at which nitrogen was loss declined in a biphasic fashion, a slower second phase after an initial rapid period of adjustment to the diet. During the study a mean of 4.1 +/- 1.4% SD of calculated total body nitrogen was lost. Potassium excretion studies did not reveal significant potassium losses. The amount of nitrogen loss was correlated with the urinary creatinine excretion, suggesting that nitrogen loss during reduced dietary intake of protein is largely dependent on the size of the lean body mass.