Starvation entails a progressive selection of fat as body fuel. Soon after a meal glucose utilisation by muscle ceases and fatty acids are used instead. Ketoacid levels in blood become elevated over the first week, and the brain preferentially uses these instead of glucose. The net effect is to spare protein even further, as glucose utilisation by brain is diminished. Nevertheless, there is still net negative nitrogen balance, but this can be nullified by amino acid or protein supplementation. Insulin appears to be the principal regulatory hormone. Recent data suggest that decreased levels of active T3 may play a role by sparing otherwise obligated calories by decreasing metabolic needs.