The effect of ketone bodies and dietary carbohydrate intake on protein metabolism

To evaluate the effect ketone bodies on protein metabolism beta-hydroxybutyrate was infused into healthy nonobese and obese subjects and insulin dependent diabetics in the postabsorptive state and into obese subjects after 3 days and 3-10 wks of starvation. In association with blood ketone increments of 1-2 mM, plasma alanine fell by 25-35% in all treatment groups. Furthermore, the hypoalaninemic effect of beta-hydroxybutyrate was equally demonstrable in fasted subjects, in whom alanine was already reduced. In association with repeated 12 hr infusions of beta-hydroxybutyrate in subjects fasted 5-10 wks, urinary nitrogen fell by 30%, returning to baseline after cessation of the infusions and paralleling the changes in plasma alanine. When endogenous ketonemia was produced by isocaloric carbohydrate restriction (less than 25 gm/day), protein ingestion was associated with a 40-50% greater increase in plasma branched chain amino acids as well as a reduced rise in plasma insulin. The enhanced rise in branched chain amino acids was attributable to decreased net utilization since intravenous leucine also produced a 40% greater elevation in plasma leucine after carbohydrate restriction. When nitrogen balance was compared during hypocaloric (400 Kcal) feeding of a pure protein diet and a mixed diet containing 50% protein and 50% carbohydrate, no significant differences were observed. Isocaloric replacement with carbohydrate failed to accentuate nitrogen wasting, despite a marked lowering of blood and urinary ketones. Our findings support the possibility that ketone bodies contribute to the reduction in proteolysis and decrease in muscle alanine release which characterizes prolonged starvation. However, when endogenous hyperketonemia is induced by carbohydrate restriction, plasma insulin declines and the disposal of ingested protein is impaired. Furthermore, the addition of carbohydrate during hypocaloric feeding reduces hyperketonemia, but does not enhance negative nitrogen balance. These observations suggest that dietary carbohydrate and insulin also promote nitrogen retention and that ketogenic, high protein diets do not confer a unique protein sparing advantage.