Oxidation of independent and combined ingested galactose and glucose during exercise

Coingestion of glucose and galactose has been shown to enhance splanchnic extraction and metabolism of ingested galactose at rest; effects during exercise are unknown. This study examined whether combined ingestion of galactose and glucose during exercise enhances exogenous galactose oxidation. Fourteen endurance-trained male and female participants [age, 27 (5) yr; V̇o_2peak, 58.1 (7.0) mL·kg^-1·min^-1] performed cycle ergometry for 150 min at 50% peak power on four occasions, in a randomized counterbalanced manner. During exercise, they ingested beverages providing carbohydrates at rates of 0.4 g.min^-1 galactose (GAL), 0.8 g.min^-1 glucose (GLU), and on two occasions 0.8 g.min^-1 total galactose-glucose (GAL + GLU; 1:1 ratio). Single-monosaccharide ¹³C-labeling (*) was used to calculate independent (GAL, GLU, GAL* + GLU, and GAL + GLU*) and combined (GAL* + GLU*, COMBINE) exogenous-monosaccharide oxidation between exercise. Plasma galactose concentrations with GAL + GLU [0.4 mmol.L; 95% confidence limits (CL): 0.1, 0.6] were lower (contrast: 0.5 mmol.L; 95% CL: 0.2, 0.8; P < 0.0001) than when GAL alone (0.9 mmol.L; 95% CL: 0.7, 1.2) was ingested. Exogenous carbohydrate oxidation with GAL alone (0.31 g·min^-1; 95% CL: 0.28, 0.35) was marginally reduced (contrast: 0.05 g·min^-1; 95% CL: -0.09, 0.00007; P = 0.01) when combined with glucose (GAL* + GLU 0.27 g·min^-1; 0.24, 0.30). Total combined exogenous-carbohydrate oxidation (COMBINE: 0.57 g·min^-1; 95% CL: 0.49, 0.64) was similar (contrast: 0.02 g·min^-1; 95% CL: -0.05, 0.09; P = 0.63) when compared with isoenergetic GLU (0.55 g·min^-1; 95% CL: 0.52, 0.58). In conclusion, coingestion of glucose and galactose did not enhance exogenous galactose oxidation during exercise. When combined, isoenergetic galactose-glucose ingestion elicited similar exogenous-carbohydrate oxidation to glucose suggesting galactose-glucose blends are a valid alternative for glucose as an exogenous-carbohydrate source during exercise.NEW & NOTEWORTHY Glucose and galactose coingestion blunted the galactosemia seen with galactose-only ingestion during exercise. Glucose and galactose coingestion did not enhance the oxidation of ingested galactose during exercise. Combined galactose-glucose (1:1 ratio) ingestion was oxidized to a similar extent as isoenergetic glucose-only ingestion during exercise. Galactose-glucose blends are a viable exogenous carbohydrate energy source for ingestion during exercise.