Journal Title (Medline/Pubmed accepted abbreviation): Am J Clin Nutr
Page numbers: 1071-1079
Summary of Background and Research Design
Background:During exercise, the maximum glucose (GLU) oxidation rate is 1 g/minute, regardless of exogenous glucose intake. Addition of fructose (FRU) to glucose (GLU; GLUFRU) during exercise, exogenous GLU oxidation rate can reach 1.75 g/minute. When GLUFRU is ingested during exercise, plasma lactate (LAC) significantly increases, suggesting an increase in systemic LAC fluxes. Recent research suggests that FRU may be converted into LAC during exercise, and may, therefore, be efficiently oxidized by working muscles.
Hypothesis/purpose of study:The objective of this study was to compare the metabolic effects of GLUFRU with those of GLU alone on LAC kinetics and oxidation rate as well as on gluconeogenesis from LAC (GNGL) or FRU (GNGF)
Subjects:Male cyclists (N = 7, age 20 to 40 yr, body mass index 18 to 25 kg/m2) with > 3 years of cycling endurance training participated in the study.
Experimental design: Single-blind, randomized, crossover design
Treatments and protocol:Subjects completed a preliminary exercise session during which maximal oxygen consumption (VO2max) and maximal aerobic power (MAP) output were assessed. In 3 separate exercise sessions, separated by 5 to 10 days, subjects performed 120 minutes of exercise at 60% VO2max while ingesting 1.2 g GLU/minute + 0.8 g of either GLU or FRU/minute (GLUFRU). In the first 2 trials, subjects received a glucose mixture (120 g every hour) or a GLU + FRU mixture (72 + 48 g every hour) during exercise, and an infusion of 6,6-2H2 glucose and 13C3-lactate to calculate GLU and LAC kinetics. In a third trial, volunteers received GLUFRU containing labeled 13C6-fructose to evaluate exogenous FRU metabolic disappearance and oxidation. Conversion of FRU to GLU and LAC was evaluated by monitoring for presence of plasma 13C- lactate and 13C- glucose. Total exogenous FRU oxidation was determined by exhaled 13CO2. Volumes of oxygen and carbon dioxide consumption were measured for 5 minutes every 20 minutes of exercise.
Summary of research findings:
- In GLUFRU, LAC appearance (120 ± 6 μmol/kg/min), LAC disappearance (121 ±7 μmol/kg/min), and oxidation (127 ±12 μmol/kg/min) rates increased significantly (P < .001) in comparison with GLU appearance, disappearance, and oxidation alone (94 ±16, 95 ±16, and 97 ±16 μmol/kg/min, respectively).
- Gluconeogenesis of LAC (GNGL) was negligible with ingestion of either GLU or GLUFRU.
- In contrast, GNGF and exogenous FRU oxidation increased with time and peaked at 18.8 ± 3.7 µmol/kg/minute and 38 ± 4 μmol/kg/minute, respectively, at 100 minutes with GLUFRU ingestion.
- Plasma GLU appearance rate was significantly higher with GLUFRU than with GLU alone (91 ± 6 µmol/kg/min vs 82 ± 9 μmol/kg/min; P < .01).
- Total carbohydrate oxidation rate was also higher (P < .05) in GLUFRU than with GLU alone.
Interpretation of findings/Key practice applications:
Co-ingestion of FRU with GLU increased the rate of GLU appearance slightly, but significantly. Ingestion of FRU led to an increase in total carbohydrate oxidation, LAC production and oxidation, and GNGF. Oxidation of FRU most likely occurred by a combination of FRU-derived LAC and GLU oxidation in muscles. This study is limited by the fact that it was performed exclusively during exercise and not during rest. Despite this limitation, these results show that co-ingestion of FRU with GLU not only allows total carbohydrate oxidation to increase beyond what may be obtained with increasing GLU doses, but also provides an additional oxidative fuel to active skeletal muscles during exercise. A likely explanation for these findings is that the absorptive capacity for GLU can be saturated, and feeding GLU at a high level (> 1 g/min) likely overloads the body’s ability to absorb GLU. However, FRU is absorbed from the intestine via a different transporter. Thus, the GLUFRU combination allows GLU to be absorbed at its maximal capacity and also contributes additional FRU that can be absorbed and metabolized as well.