Fructose can constitute a substantial energy source in the human diet. It is a dispensable (non-essential) nutrient, and no adverse effects of a fructose-deprived diet have been reported. However, specific fructose-metabolising enzymes are expressed in most mammals, including humans, indicating that this source of energy most likely conferred some metabolic advantage at some point during evolution. In athletes, fructose is often used in addition to glucose or maltodextrins as a way to deliver more carbohydrate and improve endurance performance. This blog describes the possible role of fructose during exercise, and discusses the implications of fructose for overall health.
While glucose and fatty acids remain the primary energy substrates for contracting skeletal muscle, fructose presents some potential advantages for individuals involved in strenuous physical exercise when ingested in combination with glucose. Fructose is absorbed differently to glucose. Glucose uses the SGLT1 transporter and fructose a GLUT5 transporter. Because the SGLT1 transporter becomes saturated, using fructose and thus a second transporter allows more carbohydrate to be absorbed. Fructose ingested on its own in larger quantities causes GI-problems (diarrhea) in many individuals, suggesting that the GLUT5 transporter can also be saturated.
How is fructose used in the body?
Fructose is absorbed slower than glucose and it is metabolised in a two-step process. Initially fructose is converted to lactate in the liver. This lactate is then transported to the muscle and used as a fuel. Fructose cannot be used in the muscle cell directly.
Fructose (and glucose) after exercise
After exercise we need to replenish both liver and muscle glycogen and it has been demonstrated that glucose is used more to replenish muscle glycogen and fructose is better at restoring liver glycogen. A combination could be the most effective way to restore both stores.
What are the negative effects of fructose for health?
Excess fructose ingestion has been linked with various health problems including liver disease, insulin resistance, inflammation and certain forms of cancer. It has been suggested that liver problems may occur because excess fructose is converted to liver fat which in turn can cause fatty liver disease and contribute to insulin resistance. The work in animals is more convincing than the human studies, but there is evidence of increased lipogenesis with excess fructose intake. This means that fructose can be turned into fat in the liver and there is also evidence that prolonged overfeeding with fructose can alter insulin sensitivity of the liver. The links with cancer do not have consistent evidence with some studies showing such relationships and other studies not showing any links. The interest in fructose originated from the association between fructose intake (in particular high fructose corn syrup and obesity). It appeared that as the intake of high fructose corn syrup increased over the years, so did obesity. However, in the last 20 years or so, we have seen a marked drop in the intake of high fructose corn syrup whilst obesity continues to rise.
However, in the last 20 years or so, we have seen a marked drop in the intake of high fructose corn syrup whilst obesity continues to rise.
But such effects of fructose occur only with overfeeding
Studies in animals and humans have demonstrated such negative health effects of fructose only in situations of overfeeding. Overfeeding is a term that is usually not well-defined but it refers to two situations: more calories are ingested than expended (positive energy balance), and/or more carbohydrate is ingested than is used (positive carbohydrate balance). The negative effects are reported when this positive energy and carbohydrate balance is sustained for longer periods of time. The use of fructose by athletes who use relatively large amounts of carbohydrate, are in energy balance and would consume fructose only during their longer workouts, the situation is very different.
The current knowledge of fructose physiological effects in relation to exercise can be summarised as follows:
Fructose on its own, ingested in larger quantities can cause gastro-intestinal problems. When glucose (or maltodextrins) is added much larger amounts seem to be tolerated well without side effects.
Fructose on its own is not used to a major extent as a fuel, and not recommended.
Immediately before and during exercise, fructose can be conveniently ingested in easily consumed, energy- rich, solid or liquid foods in combination with a glucose source like glucose, maltodextrins or soluble starches. The ratios depend on the amounts ingested, but if carbohydrate is ingested at rates of around 90 g/h, we would suggest 2:1 (glucose:fructose).
In the recovery period, fructose administered together with glucose may be a convenient, well tolerated way of increasing total energy intake. Furthermore, fructose and glucose exert mutually synergic effects to enhance intestinal fructose absorption, hepatic energy uptake and glycogen synthesis.
There is no evidence that fructose has negative health effects if energy expenditure and carbohydrate use are relatively high.
