Liver and muscle glycogen recovery

Ample carbohydrate availability is essential to optimize prolonged moderate- to high-intensity exercise performance. These carbohydrates can be stored within the human body (in the form of glycogen) in the muscle and liver. After prolonged (>2.5 h) and moderate- to high-intensity endurance type exercise these glycogen stores will be depleted, thereby reducing the capacity to optimally perform during subsequent exercise. Therefore, it is of great importance to replenish these carbohydrate stores as quickly as possible when optimal performance is required within short periods of (recovery) time. This is mainly of importance for athletes that have to perform maximally within 24 hours after a previous exercise bout, such as athletes during the Tour de France. Previous research has shown that, with large carbohydrate ingestion after exercise, muscle glycogen stores can be replenished within 24 hours. With regards to liver glycogen, it is suggested that this may also be replenished within 24 hours. However, this has never been directly assessed. Recently we have demonstrated that when well-trained athletes ingest sucrose (instead of glucose), that their liver glycogen repletion is doubled. This would suggest that after intense endurance type exercise, liver glycogen stores can be fully replenished within 12 hours. We hypothesize that high carbohydrate intake with sucrose will fully replenish liver and/or muscle glycogen repletion within 12 hours after exhaustive exercise in healthy well-trained endurance athletes.

We hypothesize that high carbohydrate intake with sucrose will fully replenish liver and/or muscle glycogen repletion within 12 hours after exhaustive exercise in healthy well-trained endurance athletes

MRI and biopsy measurements will be performed before and after exercise, and at 6 and 12 h in the post-exercise recovery period. Blood samples will be collected frequently during the test day.

All subjects will participate in a cross-over design and will come to the laboratory on 3 different occasions, of which 2 post-exercise recovery trials and 1 nutrition trial (to increase liver glycogen content in the morning). In all 3 trials, after arrival in a fasted state, an MRI scan will be performed to assess muscle and liver glycogen concentrations and muscle and liver volume (to assess total glycogen storage). Afterwards, during the 2 post-exercise recovery trials, participants will perform a glycogen depletion (exercise) protocol to lower their glycogen concentrations followed by another MRI scan. Subsequently, participants will remain in the laboratory for an additional 12 h to assess recovery. On one occasion they will remain fasted (water can be ingested ad libitum), and on the other occasion they will receive 1.2 g/kg/h of sucrose for the first 6 h and afterwards 2 carbohydrate-rich carbohydrate meals in the following 6 hours. On both test days, at 6 and 12 h another MRI scan will be performed to assess glycogen concentrations in the muscle and liver. Muscle biopsies will be collected after every MRI scan to assess and compare muscle glycogen concentrations with the MRI results. Blood samples will be collected to assess plasma glucose, lactate, insulin, free fatty acids, and glycerol concentrations. During the nutritional trial, participants will receive a carbohydrate-rich breakfast (3 g/kg body mass of carbohydrate) after the MRI scan session in the morning. Subsequently, participants will rest for 3 hours during which blood samples will be collected to assess plasma glucose, lactate, insulin, free fatty acids, and glycerol concentrations. After 3 hours of rest another MRI scan session will be performed to assess muscle and liver glycogen concentrations and muscle and liver volume (to assess total glycogen storage).