Comparable Exogenous Carbohydrate Oxidation from Lactose or Sucrose during Exercise

PURPOSE: Ingesting readily oxidized carbohydrates (CHO) such as sucrose during exercise can improve endurance performance. Whether lactose can be utilized as a fuel source during exercise is unknown. The purpose of this study was to investigate the metabolic response to lactose ingestion during exercise, compared with sucrose or water. METHODS: Eleven participants (age, 22 ± 4 yr; V̇O(2peak), 50.9 ± 4.7 mL·min(−1)·kg(−1)) cycled at 50% W(max) for 150 min on five occasions. Participants ingested CHO beverages (lactose or sucrose; 48 g·h(−1), 0.8 g·min(−1)) or water throughout exercise. Total substrate and exogenous CHO oxidation was estimated using indirect calorimetry and stable isotope techniques (naturally high (13)C-abundance CHO ingestion). Naturally low (13)C-abundance CHO trials were conducted to correct background shifts in breath (13)CO(2) production. Venous blood samples were taken to determine plasma glucose, lactate, and nonesterified fatty acid concentrations. RESULTS: Mean exogenous CHO oxidation rates were comparable with lactose (0.56 ± 0.19 g·min(−1)) and sucrose (0.61 ± 0.10 g·min(−1); P = 0.49) ingestion. Endogenous CHO oxidation contributed less to energy expenditure in lactose (38% ± 14%) versus water (50% ± 11%, P = 0.01) and sucrose (50% ± 7%, P ≤ 0.05). Fat oxidation was higher in lactose (42% ± 8%) than in sucrose (28% ± 6%; P ≤ 0.01); CHO conditions were lower than water (50% ± 11%; P ≤ 0.05). Plasma glucose was higher in lactose and sucrose than in water (P ≤ 0.01); plasma lactate was higher in sucrose than in water (P ≤ 0.01); plasma nonesterified fatty acids were higher in water than in sucrose (P ≤ 0.01). CONCLUSIONS: Lactose and sucrose exhibited similar exogenous CHO oxidation rates during exercise at moderate ingestion rates. Compared with sucrose ingestion, lactose resulted in higher fat and lower endogenous CHO oxidation.