Fructose has gained renewed interest for its suspected role in cardiometabolic diseases and as a performance enhancer in sports nutrition. Both are related to fructose metabolism in the liver resulting in no plasma glucose or insulin peaks, but causing postprandial hyperlactatemia and hypertriglyceridemia. The general aim of this work was to further investigate fructose metabolism and to find potential new applications of fructose that may improve performance.
In a first experimental study, we evaluated how fructose metabolism is modulated by an exercise session performed before or after fructose ingestion. Although confirming that fructose is extensively oxidized during exercise, our results also showed that fructose metabolism was largely unaltered when ingested during recovery. In a second study, we used fructose in mixed-meals provided for 24 h postexercise to specifically favor muscle energy storage and subsequent exercise performance. Compared to an isocaloric control, fructose did however not further improve muscle recovery, and fructose impaired whole-body glycogen storage and subsequent exercise performance. In a third study, we investigated the effects of glucose-fructose ingestion during training sessions on lactate metabolism, at rest and during exercise, pre-training and post-training. Interestingly, training with glucose-fructose increased lactate production, consumption and oxidation at rest but not during exercise, and an important part of lactate was directed to non-oxidative fates in all conditions.
Altogether, the present results indicate an ambivalent role of fructose, efficient to fuel muscle work, but inefficient during postexercise recovery. This may be largely due to fructose conversion into lactate, furnishing an alternative fuel during exercise that however needs to be stored for an extra energy cost in resting times. In turn, this “reverse Cori cycle” may represent an important mechanism by which fructose effects are matched to physical activity.