Assessment Of Postprandial Substrate Oxidation Following A Glucose Tolerance Test In Young Adults

PURPOSE: To compare the postprandial metabolic response to an oral glucose tolerance test in young adults of varying body compositions. METHODS: Young adults (ages 18-40 y; n = 23, 11 females) without overt pathology completed a glucose tolerance test with indirect calorimetry. Indirect calorimetry was conducted prior-to (fasting) and following (30, 60, 90, 120 min) consumption of a 100 g glucose beverage. Serum and plasma were collected at corresponding timepoints and analyzed for blood glucose, insulin, non-esterified fatty acids, and inflammatory markers. Linear mixed models were used to test for the effect of body composition on postprandial metabolism over time, while controlling for sex, fasting metabolism, and the repeated measures design. For sensitivity analyses, we compared results using relative and absolute measures of substrate oxidation (respiratory exchange ratio [RER] and carbohydrate oxidation [CHO]), as well as body composition (percent body mass and fat mass). An α = 0.05 was determined a priori. RESULTS: Results are presented as mean ± SD, range. Participants were heterogeneous in their fat mass (23.2 ± 9.3 kg, 9.0 – 41.5), lean mass (52.2 ± 10.5 kg, 39.3 – 76.0), percent body fat (29.2 ± 8.7%, 11.9 – 42.8), visceral adipose tissue mass (0.44 ± 0.41 kg, 0 – 1.46), and relative VO2PEAK (34.9 ± 9.3 mL/kg/min, 17.4 – 56.3). Postprandial RER increased over time (p < 0.001) and was significantly influenced by fasting RER (p < 0.001), without significant time-by-fat mass (p = 0.974) or time-by-percent body fat interactions (p = 0.308). Fat mass (p = 0.426), percent body fat (p = 0.921), and sex (p = 0.436) did not significantly influence postprandial RER. Similar findings were observed for postprandial CHO. Across all participants, RER increased from 0.79 ± 0.06 at fasting to 0.81 ± 0.05 at 30 min, 0.84 ± 0.05 at 60 min, 0.84 ± 0.05 at 90 min, and 0.85 ± 0.05 at 120 min. CONCLUSION: Overweight and obesity in young adulthood is a significant risk factor for chronic disease across the lifespan. Contrary to findings in older adults, our data indicate postprandial substrate oxidation is not significantly impaired in young adults with higher fat mass or percent body fat. An understanding of the metabolic derangements underlying obesity in young adults is important to improving identification and treatment of chronic disease. GWU MISPH Research Innovation Award