The purpose of this work was to clarify the essentiality of glucose production from amino acids in obese subjects undergoing prolonged starvation and to provide an explanation for death after the depletion of lean body mass when some body fat is still available to meet body energy requirements. Five obese subjects fasted for 21 d. Nitrogen balance studies were combined with measurements of blood metabolite and hormone concentrations, indirect calorimetry, determination of body-composition changes, and catheterization techniques. Phenylacetate was administered from day 19 to day 21 to remove glutamine from the body and to assess this perturbation on energy requirements, ammoniagenesis, ureagenesis, gluconeogenesis, and ketogenesis. The obese subjects lost body fat and fat-free mass in parallel and resting metabolic energy requirements per mass remained constant during starvation. Urinary nitrogen excretion reflected continuous demands for amino acid oxidation. Phenylacetate administration decreased blood glutamine concentrations, increased plasma epinephrine concentrations, and increased urinary nitrogen loss through phenylacetylglutamine excretion; urinary excretion rates of urea, ammonium, urate, creatinine, and ketone bodies remained unchanged. The essentiality of amino acid oxidation was therefore shown. Late in prolonged starvation, aminogenic oxidation amounted to 7% and fat provided the remaining energy requirements. Hepatic and renal gluconeogenesis were not curtailed. Blood glutamate served as a vehicle for carbon and nitrogen transport; the contribution of glycerol to gluconeogenesis equaled that of all amino acids combined. The minimal quantities of amino acid (0.27 +/- 0.08 and 0.52 +/- 0.10 g) and fat (1.53 +/- 0.21 and 2.98 +/- 0.15 g) oxidized per kg body wt or fat-free mass/d, respectively, were determined. Included within amino acid and fat oxidation were the minimal amounts of precursors needed for synthesizing the essential quantity of glucose (0.34 +/- 0.14 and 0.66 +/- 0.20 g) oxidized per kg body wt or fat-free mass, respectively.