Prolonged fasting significantly changes nutrient oxidation and glucose tolerance after a normal mixed meal

The aim of this study wasto establish the experimental paradigm of fasting, followed byrefeeding, to investigate individual differences in nutrientpartitioning. Eight nonobese men were fed a normal meal (25% ofdaily energy requirements) on two occasions, after an overnight (13-h)fast and after a prolonged (72-h) fast. During the entire fastingperiod, subjects were resident in a whole room indirect calorimeter,and blood samples were drawn periodically. Because no other food wasconsumed over the 12 h after either meal, negative energy balancewas observed after the overnight and prolonged fast. Postprandialcarbohydrate oxidation was significantly reduced after the 72- vs. 13-hfast (P < 0.0001), whereas fat oxidation wassignificantly increased (P < 0.0001). Interestingly,carbohydrate balance was positive after the prolonged fast but negativeafter the overnight fast (24 ± 17 vs. 57 ± 16 g/12 h,respectively; P < 0.001), whereas fat balance wasnegative under both conditions (78 ± 7 vs. 47 ± 8 g/12h, respectively; P < 0.002). With 72 h offasting, the glucose and insulin excursions in response to the mixedmeal were significantly greater compared with the 13-h fast(P < 0.001). In conclusion, prolonged fasting resulted in a significant decrease in carbohydrate oxidation and anincrease in fat oxidation, after a normal mixed meal, in healthy men.This was associated with a significant decrease in glucose tolerance.Because circulating free fatty acids were greatly elevated at all timesafter the prolonged fast, these may be mediating some of the changes inpostprandial metabolism.