Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men

Stable isotope tracers and indirect calorimetry were used to evaluate the progressive alterations in lipid and glucose metabolism after 12, 18, 24, 30, 42, 54, and 72 h of fasting in six healthy male volunteers. The rates of appearance (R(a)) of glycerol and palmitic acid in plasma doubled from 2.08 ± 0.22 and 1.63 ± 0.20 μmol · kg^-1 · min^-1, respectively, after 12 h to 4.36 ± 0.36 and 3.26 ± 0.40 μmol · kg^-1 · min^-1, respectively, after 72 h of fasting (P < 0.01). Of the total increase in lipid kinetics, 60% occurred between 12 and 24 h of fasting; the greatest interval change occurred between 18 and 24 h of fasting. Glucose R(a) and plasma concentration decreased by ~25% between 12 h (11.0 ± 0.4 μmol · kg^-1 · min^-1 and 5.58 ± 0.08 mmol/l, respectively) and 72 h (8.3 ± 0.3 μmol · kg^-1 · min^-1 and 4.14 ± 0.10 mmol/l, respectively) of fasting (P < 0.01), but no statistically significant changes occurred between 18 and 24 h of fasting. Plasma insulin decreased by ~50% between 12 h (64.6 ± 12.9 pmol/l) and 72 h (30.1 ± 7.9 pmol/l) of fasting (P < 0.001). Of the total decline in plasma insulin, 70% occurred within the first 24 h of fasting. These results demonstrate that the mobilization of adipose tissue triglycerides increases markedly between 18 and 24 h of fasting in young adult men. The early alterations in lipid metabolism are associated with a decline in circulating insulin but do not seem to be regulated by changes in glucose kinetics or plasma glucose concentrations.