1. Differing patterns of protein metabolism are seen in wasting due to undernutrition and wasting due to chronic infection. 2. We investigated whole body energy and protein metabolism in nine subjects with pulmonary tuberculosis, six undernourished subjects (body mass index < 18.5 kg/m2) and seven control subjects from an Indian population. Fasting subjects were infused with L-[1-13C]leucine (2.3 mumol.h-1.kg-1) for 8 h, 4 h fasted then 4 h fed. Leucine kinetics were derived from 13C-enrichment of leucine and alpha-ketoisocaproic acid in plasma and CO2 in breath. 3. Undernourished subjects, but not tuberculosis subjects, had higher rates of whole body protein turnover per unit lean body mass than controls [163.1 +/- 9.4 and 148.6 +/- 14.6 mumol compared with 142.8 +/- 14.7 mumol leucine/h per kg, based on alpha-ketoisocaproic acid enrichment (P = 0.039)]. 4. In response to feeding, protein oxidation increased in all groups. Tuberculosis subjects had the highest fed rates of oxidation (47.0 +/- 10.5 compared with 37.1 +/- 5.4 mumol.h-1.kg-1 in controls), resulting in a less positive net protein balance in the fed phase (controls, 39.7 +/- 6.2; undernourished subjects, 29.2 +/- 10.6; tuberculosis subjects, 24.5 +/- 9.3; P = 0.010). Thus fed-phase tuberculosis subjects oxidized a greater proportion of leucine flux (33.2%) than either of the other groups (controls, 24.0%; undernourished subjects, 24.0%; P = 0.017). 5. Tuberculosis did not increase fasting whole body protein turnover but impaired the anabolic response to feeding compared with control and undernourished subjects. Such 'anabolic block' may contribute to wasting in tuberculosis and may represent the mechanism by which some inflammatory states remain refractory to nutrition support.