Biochemical Form of Carbohydrate Craig (1993) reviewed many of the exercise stud

Question

Biochemical Form of Carbohydrate

Craig (1993) reviewed many of the exercise studies using fructose feedings and noted that fructose consumption prior to exercise does not cause an increase in either blood glucose or insulin. Although one study demonstrated a reduction in muscle glycogen use when fructose had been ingested prior to a 30 min exercise bout compared to feedings of glucose or water (Levine et al 1983), the glycogen-sparing effect of fructose ingestion was not confirmed in later studies using longer exercise bouts when glycogen could become more limiting (Hargreaves, 1987; Koivisto, 1985). This absence of effect on muscle glycogen occurred in spite of different glycemic and insulinemic patterns for the glucose versus fructose trials in the various studies. In addition, no performance benefit, as measured by time to exhaustion, was noted in most experiments for fructose compared to glucose. However, there may be some benefit to fructose ingestion prior to exercise when it is used in combination with other carbohydrate sources. For example, muscle glycogen sparing and performance enhancement were observed in athletes consuming a mixture of maltodextrin and fructose when compared to maltodextrin alone (Brouns et al 1989).

To summarize the studies that have used fructose feedings prior to exercise, blood glucose was maintained at higher levels during the initial period of exercise relative to high GI carbohydrates like glucose, but there were few reports of an ergogenic effect or a change in rate of muscle glycogen use. Because virtually all authors who fed fructose commented on the high incidence of gastrointestinal distress (due to slow fructose digestion), fructose alone is probably not practical in the concentrations required to provide sufficient energy; it may be useful as part of a mixture with other carbohydrates.

17. Explain using your vast knowledge of biochemistry why the phenomena bolded above is observed.

Explanation / Answer

Fructose and glucose have different metabolites when ingested.Fructose when ingested is diverted towards tryglyceride synthesis and replenishing liver glycogen.While glucose stimulates secretion of hormone insulin, fructose doesnt trigger insulin secretion that regulates blood glucose levels.Glucose on the other hand is directed to muscles where it generates energy by its breakdown to CO2,water and ATP.Fructose is a good substrate for replenishment of liver glycogen, post replenishing liver glycogen to certain level, fructose is directed towards tryglyceride synthesis.

Thus, feeding only fructose before exercise ,would not elevate muscle glycogen level, and can cause hypertriglyceridimia. After fructose ingestion , exercising increases total energy expenditure,there is increased fructose oxidation, and net decrease in glycogen storage.

When fructose is ingested with other carbohydrates, it is better since there is post prandial excursion of tricglycerides via impairment of extrahepatic tryglyceride clearance.

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