ENERGY CONTRIBUTION OF PROTEINS AND LIPIDS DURING PROLONGED FASTING IN THE RAT

It has previously been shown that there are many similarities between the metabolic responses during fasting in rats with large initial lipid reserves and man: in particular, they both withstand long-term fasting through an efficient sparing of body proteins. Three metabolic phases were characterized. However, to further elucidate the regulation of protein and lipid utilization, data were needed on the change in the contribution of these body fuels. We found that in rats, after the first two days (phase I), a steady state in protein utilization is reached (phase II). Proteins then account for only 17 % of energy expenditure whereas lipids provide the most important part, 83 %. There is thereafter a progressive rise in protein utilization (phase III). Importantly, the occurence of this phase is not a consequence of the total exhaustion of lipid reserves, since 25 % still remain at the transition between phases II and III. This study also indicates that the rise in protein utilization (phase III) is a reversible phenomenon. This rise is associated with an important increase in corticosterone level, suggesting that glucocorticoids could be responsible for the changes in protein turnover during phase III.