Starvation in the rat. II. Effect of age and obesity on protein sparing and fuel metabolism.

Sixteen-week-old control and obese rats survive longer than 8-wk-old control rats. In addition, unlike the 8-wk-old group, they conserve tissue RNA and protein. To evaluate the basis for this, the effects of starvation on circulating fuels and hormones and the urinary excretion of nitrogen and 3-methylhistidine (3MH) were compared in the three groups. Urinary nitrogen and 3MH diminished during prolonged starvation in 16-wk-old obese and control rats, suggesting that both groups are able to conserve protein and curtail muscle proteolysis. In contrast, urine nitrogen and 3MH did not decrease in 8-wk-old control rats. Protein conservation in the older rats was associated with diminished blood levels of alanine and increased levels of lipid fuels, ketone bodies, and free fatty acids. Although ketone bodies and free fatty acids were also increased during the first few days of starvation in 8-wk-old rats, there was no evidence of protein sparing. In all groups, as fat stores became exhausted terminally, blood lipid levels decreased and protein catabolism increased. Starvation caused insulin to decrease to comparable levels in all rats; however, minimal levels were reached later in the older groups. Thyroxine and triiodothyronine (T3) decreased during the fast in both control groups; however, T3 did not decrease in the obese rats. These findings support the contention that the conservation of protein during prolonged starvation requires the continued availability of lipid fuels. The role of insulin and thyroid hormone in modulating these adaptations is unclear.