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Lactose is a disaccharide of glucose and galactose, found exclusively in milk. Carbohydrates represent an important fuel for endurance and prolonged exercise. Recommendations for athletes include high carbohydrate diets to maximise performance, especially before, during and after exercise. However, lactose does not feature in guidelines for carbohydrate intake for athletes, despite athletes likely consuming nutritionally relevant amounts. This review will explore possible applications for lactose in a sports nutrition context. These include lactose as a fuel source, for before and during exercise, where maximizing availability of readily oxidisable carbohydrate can optimise performance. Lactose could play a role in a post-exercise recovery setting, as a vehicle for the delivery of glucose and galactose, for the optimisation of muscle and liver glycogen. Lactose may also act as a prebiotic, possibly promoting beneficial changes to gut microbiota. A discussion of the possible risks associated with lactose overconsumption and intolerance will also be considered.

Co-ingestion 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. 14 endurance-trained male and female participants (age, 27 (5) years; V̇O2peak, 58.1 (7.0) ml·kg-1·min-1) performed cycle ergometry for 150 min at 50% peak power on 4 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-labelling (*) was used to calculate independent (GAL, GLU, GAL*+GLU, GAL+GLU*) and combined (GAL*+GLU*, COMBINE) exogenous-monosaccharide oxidation between during exercise. Plasma galactose concentrations with GAL+GLU (0.4 mmol.L; 95%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; 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; 0.52, 0.58). In conclusion, co-ingestion 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.

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