Effect of energy compared with carbohydrate restriction on the lipolytic response to epinephrine

Fasting enhances the lipolytic sensitivity of adipose tissue to β- adrenergic stimulation. The importance of carbohydrate compared with total energy restriction in regulating the lipolytic response to epinephrine was evaluated. Five normal volunteers participated in two study protocols in a randomized crossover design separated by a 3-wk period. In one study (total energy restriction) the subjects fasted for 84 h, whereas in the other (carbohydrate restriction) a lipid emulsion was infused for 12-15 h each day to meet resting energy requirements during an 84 h oral fast. Glycerol flux, an index of whole-body lipolysis, was measured by infusing [²G₅]glycerol. Each subject was studied in the basal state and during a 1-h infusion of epinephrine (0.015 μg · kg^-1 · min^-1) after 84 h of total energy restriction and after 84 h of carbohydrate restriction (12 h after the final lipid infusion). The lipolytic response to epinephrine, defined as the total area between the glycerol flux curve and baseline during 1 h of epinephrine infusion, was similar after total energy restriction (241 ± 141 μmol/kg) and carbohydrate restriction (294 ± 58 μmol/kg). We conclude that carbohydrate restriction, not total energy restriction, is responsible for the increase in lipolytic sensitivity observed during fasting.