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 plas ma doubled from 2.08 +/- 0.22 and 1.63 +/- 0.20 mumol.kg-1.min-1, resp ectively, after 12 h to 4.36 +/- 0.36 and 3.26 +/- 0.40 mumol.kg-1.min -1, respectively, after 72 h of fasting (P < 0.01). Of the total incre ase in lipid kinetics, 60% occurred between 12 and 24 h of fasting, th e greatest interval change occurred between 18 and 24 h of fasting. Gl ucose R. and plasma concentration decreased by approximately 25% betwe en 12 h (11.0 +/- 0.4 mumol.kg-1.min-1 and 5.58 +/- 0.08 mmol/l, respe ctively) and 72 h (8.3 +/- 0.3 mumol.kg-1.min-1 and 4.14 +/- 0.10 mmol /l, respectively) of fasting (P < 0.01), but no statistically signific ant changes occurred between 18 and 24 h of fasting. Plasma insulin de creased by approximately 50% between 12 h (64.6 +/- 12.9 pmol/l) and 7 2 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. The se results demonstrate that the mobilization of adipose tissue triglyc erides increases markedly between 18 and 24 h of fasting in young adul t men. The early alterations in lipid metabolism are associated with a decline in circulating insulin but do not seem to be regulated by cha nges in glucose kinetics or plasma glucose concentrations.