Background: When fructose is ingested together with glucose (GLUFRU) during exercise, plasma lactate and exogenous carbohydrate oxidation rates are higher than with glucose alone.
Objective: The objective was to investigate to what extent GLUFRU increased lactate kinetics and oxidation rate and gluconeogenesis from lactate (GNG(L)) and from fructose (GNG(F)).
Design: Seven endurance-trained men performed 120 min of exercise at ≈60% VO₂max (maximal oxygen consumption) while ingesting 1.2 g glucose/min + 0.8 g of either glucose or fructose/min (GLUFRU). In 2 trials, the effects of glucose and GLUFRU on lactate and glucose kinetics were investigated with glucose and lactate tracers. In a third trial, labeled fructose was added to GLUFRU to assess fructose disposal.
Results: In GLUFRU, lactate appearance (120 ± 6 μmol · kg⁻¹ · min⁻¹), lactate disappearance (121 ± 7 μmol · kg⁻¹ · min⁻¹), and oxidation (127 ± 12 μmol · kg⁻¹ · min⁻¹) rates increased significantly (P < 0.001) in comparison with glucose alone (94 ± 16, 95 ± 16, and 97 ± 16 μmol · kg⁻¹ · min⁻¹, respectively). GNG(L) was negligible in both conditions. In GLUFRU, GNG(F) and exogenous fructose oxidation increased with time and leveled off at 18.8 ± 3.7 and 38 ± 4 μmol · kg⁻¹ · min⁻¹, respectively, at 100 min. Plasma glucose appearance rate was significantly higher (P < 0.01) in GLUFRU (91 ± 6 μmol · kg⁻¹ · min⁻¹) than in glucose alone (82 ± 9 μmol · kg⁻¹ · min⁻¹). Carbohydrate oxidation rate was higher (P < 0.05) in GLUFRU.
Conclusions: Fructose increased total carbohydrate oxidation, lactate production and oxidation, and GNG(F). Fructose oxidation was explained equally by fructose-derived lactate and glucose oxidation, most likely in skeletal and cardiac muscle. This trial was registered at clinicaltrials.gov as NCT01128647.