Fructose and galactose enhance postexercise human liver glycogen synthesis


Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc.
Year: 2011
Volume: 43
Issue: 10
Page Numbers: 1964-1971

Summary of background and research design:

Background: During exercise, the liver synthesizes glucose to maintain blood glucose levels and replenish glycogen stores. Fructose is taken up primarily by the liver and made into glucose and it is thought that glucose made from fructose is more often converted glycogen than secreted as opposed to being secreted into the blood as free glucose. This phenomenon has been observed for galactose once in rats but not in 2 other studies (neither of them human studies).  

In this study, maltodextrin (MD) is used as a glucose source. MD is a long chain of glucose molecules.

Hypothesis: The combination of maltodextrin with galactose or fructose will increase the rate of glycogen synthesis after exercise.

Subjects: Ten endurance-trained males, age 29 ± 1 y.

Experimental design:randomized, double-blind, cross-over design
 

Treatments: Immediately before the exercise session, 450 g of a powdered treatment (MD:test sugar = 2:1) was dissolved in 3 L of water. The test sugar was glucose, fructose, or galactose. A few drops of lemon juice were added for taste.

Protocol:The participants reported to the laboratory 4 times. First, they were assessed for VO2max and Wmax on a stationary bicycle and familiarized with the study protocol. On the days of the trials, they reported to the laboratory after an overnight fast. They then, after a warm up, cycled alternating between 90% and 50% Wmax until they could not maintain the workload. The upper intensity was reduced as needed until complete exhaustion. Afterwards, magnetic resonance imaging (MRI) was used to quantify liver glycogen concentrations. Approximately 1 h and 15 min after cessation of cycling, the athletes began drinking one of the treatment beverages. Beverages were provided at 8 time points throughout recovery (6.75 h). Also, blood samples and MRI images were acquired and questionnaires were completed at various times.


Summary of research findings:
  • After the glycogen-depleting exercise, liver glycogen concentrations were 99 mM (about 29.2 g total) and were not different between treatments.
  • The rate of liver glycogen replenishment was significantly higher with fructose (24 ± 2 mM/hr) and galactose (28 ± 3 mM/hr) compared to glucose (13 ± 2 mM/hr). Statistically, fructose and galactose were not different from one another and were both higher than glucose.
  • Total glycogen content increased in all treatments with fructose = galactose > glucose.
  • Liver volume increased for the fructose and galactose trials but not glucose.
  • Plasma glucose and insulin concentrations were not statistically significant at most time points.
  • The participants experienced worse gastrointestinal discomfort with the galactose beverage than with the others, specifically stomach problems, heartburn, and stomach cramps, but the severity of these symptoms was low.

Key practice applications:

A combination of different sugars has been shown to increase fluid and energy delivery to tissue because different transporters for sugar uptake are involved. Now, these authors have shown that combinations of sugars can help increase the rate of recovery, too, through increased rate of liver glycogen synthesis. It is unknown what effects this result could have on performance, but it could be particularly beneficial for athletes who require fast recovery (for 2 matches in one day, for example).


Limitations:

The authors did not look at muscle glycogen, which is also important in endurance events. It is possible that muscle glycogen levels increased at a faster rate, too, with the fructose and galactose, but that hypothesis would have to be confirmed.

 

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