Evidence of beneficial effects of combining ingestion of fructose and glucose based carbohydrates during endurance exercise is very strong. However, less clear is whether such an approach could be recommended during the recovery period as well.
Fructose and glucose share the molecular formula – 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms, however the structure is different. As a result of the structural difference, glucose and fructose differ in the metabolism. After ingestion all carbohydrates are first broken down to monosaccharides, so fructose and glucose. Fructose and glucose are transported from the small intestine into the bloodstream via different transporters. SLGT1 is the primary pathway for the transport of glucose, whereas fructose uses a different transport, GLUT 5.
It is believed that during exercise SLGT1 and GLUT2 transporters become saturated when more than 60 g of glucose based carbohydrates (i.e., maltodextrin and dextrose) are ingested, whereas addition of fructose can enhance the absorption and thus increases the carbohydrate availability.
What happens after the absorption?
As you are probably aware of glucose can be utilised by all cells in the body and maintaining stable concentration of glucose in the bloodstream is essential for normal operation of the body. If it happens that blood glucose concentration that is usually around 5 mmol/L drops, the condition called hypoglycaemia occurs which can cause numerous problems. But let’s not delve into this issue now.
In contrast with glucose, fructose can only be up-taken and utilised by the liver. So, liver takes up the fructose and converts it to glycogen within the liver or glucose that can subsequently be used by other cells.
This is during exercise. But what about the recovery period?
The fundamentals of metabolism are the same. Glucose is converted to glycogen in all body cells (muscle and liver), whereas fructose mostly affects liver glycogen.
The evidence is pretty clear – ingesting a combination of glucose and fructose based carbohydrates results in optimal replenishment of liver and muscle glycogen stores.
A team of researchers from the University of Birmingham that included our Dr Tim Podlogar were the first to investigate whether this also translates into improved recovery of muscle function.
They first asked trained runners to run until exhaustion at a moderate intensity. By doing this runners depleted the carbohydrate stores within the body as would happen during a race as well. Then, a four hour long period of feeding and rest was initiated. Study participants received either glucose based carbohydrates or a combination of glucose and fructose.
Then, participants again run until exhaustion. When participants received a combination of fructose and glucose, the time to exhaustion was improved by 32.4%!
Subsequently a similar study was published by researchers from the University of Bath confirming the improved recovery in cyclist as well.
Our Dr Tim Podlogar did another study that yielded no positive effects of fructose-glucose combination over glucose only on the recovery of exercise capacity.
Instead of measuring exercise capacity (i.e., how much time people can sustain certain intensity), exercise performance was measured (i.e., how quickly can people cover certain distance). This approach is more representative of the real world scenarios.
In this study no benefits of ingesting a combination of fructose and glucose were found on exercise performance
However, oxidation rates of during recovery ingested carbohydrates were higher in fructose-glucose condition indicating higher storage of carbohydrates. Unfortunately this hasn’t translated into improved performance.
It is speculated that had the carbohydrates were ingested during the subsequent exercise bout as well, as would have happened during a normal race, this would then unlock the potential of co-ingestion of fructose and glucose in the recovery period.
To sum up – ingesting glucose-fructose based carbohydrates in the recovery period very likely translates into improved recovery of exercise capacity and performance provided that during the subsequent exercise bout guidelines for ingestion of carbohydrates are followed as well.