Comparable Exogenous Carbohydrate Oxidation With Lactose Or Sucrose Feeding During Endurance Exercise

Abstract

PURPOSE: Endogenous carbohydrate (CHO) availability can limit endurance exercise performance. CHO ingested during exercise such as glucose, glucose polymers and sucrose are readily oxidized and can improve endurance performance. The extent to which lactose can be utilized as a fuel source during exercise is unknown. The purpose is to elucidate the metabolic response to lactose ingestion during endurance exercise, compared to sucrose or water. METHODS: 11 participants (mean ± SD: 22 ± 4 years, 50.9 ± 4.7 ml.min-1.kg-1) cycled for 150 min at 50% of Wmax on 3 occasions. During exercise participants ingested, in a randomized order, water (WAT), lactose (LAC) or sucrose (SUC) containing beverages; a CHO dose of 0.8 g.min-1 (48 g.h-1) was used. Indirect calorimetry was used to calculate fat and total CHO oxidation and stable isotope tracer methodology (natural high 13C abundance CHO ingestion, 13C:12C ratio determination in expired breath by isotope-ratio mass spectrometry) was used to quantify exogenous CHO oxidation. Venous blood samples were taken throughout exercise and analyzed for glucose, lactate and non-esterified fatty acids (NEFA). RESULTS: Mean exogenous CHO oxidation rates in LAC (0.55 ± 0.19 g.min-1) and SUC (0.58 ± 0.10 g.min-1) were similar (P=0.68). Endogenous CHO oxidation contributed less to energy expenditure (EE) in LAC (39 ± 14 %) than in WAT (50 ± 11 %, P=0.01) and SUC (50 ± 8 %, P=0.04). Fat oxidation contributed most to EE in WAT (50 ± 11 %), which was significantly greater than in LAC (42 ± 8 %, P=0.04) and SUC (28 ± 6 %, P<0.01), with fat oxidation higher in LAC than SUC (P<0.01). Plasma glucose was significantly higher in LAC (5.2 ± 0.4 mmol.L-1) and SUC (5.3 ± 0.5 mmol.L-1) than WAT (4.6 ± 0.5 mmol.L-1, P≤0.01). Plasma lactate was significantly higher in SUC (1.7 ± 0.5 mmol.L-1) than WAT (1.5 ± 0.4 mmol.L-1, P<0.01). Plasma NEFA were significantly higher in WAT (1.1 ± 0.4 mmol.L-1) than SUC (0.5 ± 0.3 mmol.L-1, P<0.01), and tended to be higher in WAT than LAC (0.7 ± 0.4 mmol.L-1, P=0.08). CONCLUSIONS: Exogenous CHO oxidation during exercise was comparable from ingested lactose and sucrose. However, lactose ingestion promoted higher fat and lower endogenous CHO oxidation during exercise than sucrose. This suggests that lactose may be at least as efficacious as commonly used CHO sources for improving exercise performance.