Fructose and glucose co-ingestion during prolonged exercise increases lactate and glucose fluxes and oxidation compared with an equimolar intake of glucose.

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.