Metabolic, Endocrine, and Appetite Responses to Carbohydrate Compared with Niacin Ingestion During Exercise in Healthy Females and Males

BACKGROUND: The substrates used during exercise may influence the postexercise response of systemic metabolites and hormones, along with the drive to refeed following exercise. However, to our knowledge, no study has assessed whether manipulating substrate metabolism during exercise independent of carbohydrate ingestion alters these responses. OBJECTIVES: The objective of this study was to assess the effects of niacin and carbohydrate ingestion during exercise on postexercise metabolite and hormone concentrations, and ad libitum energy intake. METHODS: Fifteen participants (5 females) performed 1 h of cycling at 95% lactate threshold 1 with carbohydrate ingestion [exercise with carbohydrate (CARB), insulin-induced lipolysis suppression], niacin ingestion [exercise with niacin (NIACIN), insulin-independent lipolysis suppression], and fasted exercise (FAST), in a crossover design with blood and breath samples. Two hours after exercise, energy intake was measured at an ad libitum meal. RESULTS: Compared with FAST, CARB, and NIACIN suppressed plasma concentrations of nonesterified fatty acids during exercise (time × condition: P < 0.001) and total fat oxidation (mean ± 95% confidence interval; CARB: ‒11 ± 4 g, P < 0.001; NIACIN: ‒9 ± 5 g, P = 0.001). CARB and NIACIN increased plasma glucagon-like peptide-1 (GLP-1)TOTAL concentrations compared with FAST (time × condition interaction: P < 0.001), with the highest concentrations in CARB (P < 0.001 compared with FAST and NIACIN). There was no evidence of differences in ad libitum energy intake between conditions (FAST 833 ± 206 kcal, CARB 786 ± 219 kcal, NIACIN 780 ± 158 kcal; P = 0.37). Ketone body and (large) very low density lipoprotein triglyceride (VLDL-TG) concentrations were lower with NIACIN compared with FAST (time × condition: both P < 0.001), whereas branched-chain amino acid concentrations were lower with CARB compared with FAST (time × condition: P < 0.001). CONCLUSIONS: These data demonstrate that niacin ingestion increases carbohydrate oxidation during exercise and postexercise GLP-1, ketone body, and VLDL-TG concentrations but does not seem to detectably influence postexercise energy intake. This suggests that manipulating fatty acid availability during exercise may have greater implications for metabolism than for energy balance behaviors. The study was registered on clinicaltrials.gov/study/NCT05417659.