Increased postprandial metabolic flexibility is associated with higher body fat percentages in healthy young adults

Because higher adiposity is associated with cardiometabolic disease, we assessed the relationship between body composition (body fat percentage; BF%) and postprandial metabolic flexibility [change in respiratory exchange ratio (RER) from fasting]. Young adults [<i>n</i> = 27, <i>n</i> = 15 females, body mass index (BMI) = 27.1 ± 4.5; BF% = 30.4 ± 8.7; mean ± SD] without overt pathology completed a 100 g oral glucose tolerance test (OGTT). Indirect calorimetry before (fasting) and following (30, 60, 90, and 120 min) consumption was used to calculate respiratory exchange ratio (RER) and oxidation of carbohydrates (CHOX) and fats (FOX). Serum and plasma were collected at corresponding time points and analyzed for glucose, insulin, and nonesterified fatty acids (NEFAs). Two-way repeated measures ANOVA was used to compare data between normal weight and overweight/obesity by BMI. The effect of BF% on postprandial metabolic flexibility was tested via linear mixed models while adjusting for potential confounders. During the OGTT, blood glucose, serum insulin, plasma lactate, RER, and CHOX all significantly increased, whereas plasma NEFAs and whole body FOX decreased (all <i>P</i> < 0.05). BF% modified the relationship between postprandial RER and time (<i>P</i> = 0.019); individuals with higher BF% increase their RER faster and to a greater extent (i.e., greater metabolic flexibility) than those with lower BF%. Body fat percentage is associated with greater postprandial metabolic flexibility during an OGTT in young adults. Despite increased adiposity, metabolic flexibility may be preserved, representing a compensatory adaptation to decreased glucose storage in the postprandial period.<b>NEW & NOTEWORTHY</b> Previous studies have suggested that obesity blunts postprandial metabolic flexibility. By directly analyzing the effect of body composition on postprandial metabolic flexibility, we show that healthy young adults with higher body fat percentages have increased postprandial metabolic flexibility in response to concurrent hyperglycemia. This suggests a preserved, metabolically flexible phenotype in young adults with higher body fat percentages.