OBJECTIVE: We wanted to establish and understand how the fractional losses of fat, fat-free tissues, and selected nutrients compare with that of body mass during a 44-d voluntary starvation (water only) and measurements of nutrient status. METHODS: We used anthropometry, sequential measurements of urinary substances during the fast, and blood analytes at the end of the fast. RESULTS: At the start of the fast, body weight was 96.0 kg (20% fat) and body mass index was 28.36 kg/m(2). The changes in body mass and arm anthropometry and in the pattern of urinary excretion of creatinine, ammonia, sodium, and ketone bodies during the study were consistent with starvation. At the end of the fast, body mass had decreased by 25.5%, of which a quarter to a third was due to loss of fat and the remainder to fat-free mass, predominantly muscle. There was an estimated loss of 20% of total body protein, 20-25% of fat-free mass, and a greater fractional loss of fat. Total energy expenditure was estimated to be 1638-2155 kcal/d of which 13.0-17.1% was from protein oxidation. Differential losses of minerals in urine ranged from 1.2% of estimated initial body content for manganese to 17.3% for selenium and 40.5% for zinc. At the end of the study, plasma concentrations of zinc and vitamin B12 were increased, those of copper, selenium, and manganese were normal, and there was biochemical evidence of deficiency in thiamine, riboflavin, and vitamin K (prothrombin time). CONCLUSION: The data confirm and extend the available information on prolonged fasting in lean individuals and have relevance to the understanding of the physiologic responses to starvation and the associated homeostatic mechanisms.