Fructose is increasingly being implicated in the development of insulin resistance and obesity in humans. Fructose can only be metabolised by the liver and fructose supplied in excess of the amounts needed for liver glycogen synthesis is converted to fatty acids and esterified to glycerol to form triglycerides. These triglycerides then form the very low density lipoprotein fraction of plasma phospholipids, as they are exported to peripheral tissues, where they may be involved in the development of insulin resistance. During intense exercise liver glycogen often becomes depleted and fructose can therefore be useful following exercise in order to replenish hepatic carbohydrate stores. However, there are limits to the amount of fructose that can be absorbed, and excess is fermented in the colon where it can contribute to abdominal pain and discomfort due to the production of hydrogen gas in the fermentation process.
For example, researchers1 have investigated the intestinal absorption of fructose in 10 healthy subjects. Subjects performed 30 minutes of exercise and then consumed a 50g carbohydrate drink containing either 100% fructose, 95% fructose and 5% glucose, 70% fructose and 30% glucose, 100% glucose or water, during a subsequent rest period. Subjects then performed another 30 min exercise during which and after, breath hydrogen was used as a measure of incomplete absorption of fructose, relating to fermentation in the colon. Following consumption of 100% and 95% fructose breath hydrogen increased significantly indicating malabsorption of fructose, but this did not occur after 100% glucose or water. In the 70% fructose, 5 of the 7 subjects displayed increased breath hydrogen. Exercise appeared to increase the hydrogen production in response to fructose ingestion, suggesting that absorption was decreased by physical activity.
These results suggest that the capacity to absorb fructose is saturated in amounts above 50g and that exercise has a detrimental effect on fructose absorption. The results of this is that the fructose becomes fermented by intestinal bacteria releasing hydrogen and leading to possible gastrointestinal discomfort. These results support other studies that have shown that ingestion of amounts of fructose over 50g results in malabsorption. Other researchers have reported that adding 4g of glucose to the 50g of fructose decreased the rate of malabsorption by half. Fructose is absorbed by passive diffusion and therefore as concentrations rise saturation of the absorptive process occurs. However, it is unclear why glucose may increase fructose absorption. Glucose is actively transported across the mucosa and this may increase solvent absorption by increasing osmotic pressure in the process of solvent drag. Fructose may then follow passively.
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