Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation

Despite extensive experimental studies on total starvation, many of the fin dings relating to protein, fat (plus ketone body), and carbohydrate metabol ism remain confusing, although they become more consistent when considered in relation to the degree of initial obesity. During prolonged starvation, protein loss and percent energy derived from protein oxidation are 2- to 3- fold less in the obese than in the lean; percent urine N excreted as urea i s 2-fold less in the obese; and the contribution of protein to net glucose production is only about half in the obese compared to lean subjects. Durin g short-term starvation (first few days) the following differences are repo rted: hyperketonaemia is typically 2-fold greater in lean subjects, but ass ociated with a 2-fold lower uptake of ketone bodies by forearm muscle; gluc ose tolerance becomes impaired more in lean subjects; and both protein turn over and leucine oxidation increase in the lean, but may show no significan t change in the obese. It is no longer acceptable to describe the metabolic response to starvation as a single typical response. The differences betwe en lean and obese subjects have important physiological implications, some of which are of obvious relevance to survival.