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Original Paper: Coyle, E. F., A. R. Coggan, M. K. Hemmert and J. L. Ivy. 1986. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrates. Journal of Applied Physiology 61: 165–172.

An athlete trained for endurance can sustain repeated muscle contractions far longer than someone without such training. But even trained athletes have a limit on how long they can exercise before their muscles become fatigued. One factor necessary to sustain muscle activity is the type and amount of metabolic fuel used. In this study, researchers investigated whether administering carbohydrates to athletes as they exercised would affect their time to fatigue.

Seven male, endurance-trained cyclists performed two strenuous cycling trials until they fatigued. At 20-minute intervals throughout one trial, subjects ingested a placebo (flavored water). On another day, the athletes performed the same trial, this time ingesting a glucose solution at 20-minute intervals. The researchers measured time to fatigue, muscle glycogen content in the vastus lateralis (thigh muscle), blood glucose (Figure A), carbohydrate oxidation rate (Figure B), and blood free fatty acid (FFA) concentration (Figure C). Results are shown in the table and graphs. Glycogen levels were measured in millimoles (mM) of glucosyl units per kilogram of muscle tissue. Values are means ± SE, and means are compared with a t-test. In the figures, significant differences are indicated with an asterisk.

Questions

Question 1

In both groups muscle glycogen fell to about 25 percent of its resting value by the point of fatigue. There were no significant differences (P > 0.05) in muscle glycogen at any time point in the two groups. Thus over 3 hours both groups depleted their muscle glycogen at about the same rate. Blood glucose in the placebo group fell steadily after the first hour, but blood glucose did not fall in the glucose-fed group, and blood glucose levels were significantly different in the two groups after the first hour. Both groups had similar levels of carbohydrate oxidation for the first 2 hours, after which the placebo group had declining carbohydrate oxidation levels that were significantly different from those of the glucose-fed group.

Question 2

The difference between the two groups in time to fatigue was significant, and therefore the data show that carbohydrate feeding delayed muscle fatigue. Carbohydrate feeding also maintained blood glucose levels and the rate of carbohydrate oxidation. In addition, the data show that up until hour 3, this effect does did not involve a slowing of muscle glycogen depletion.

Question 3

For the glucose-fed group, there was no significant decline in muscle glycogen between hour 3 and 4, indicating that other sources of fuel were being metabolized once muscle glycogen was depleted. A possible explanation for these effects of carbohydrate feeding is that it enabled the subjects to support more of their metabolic work with carbohydrates. We can hypothesize that the placebo group had to compensate sooner with fuel sources other than carbohydrates.

Question 4

The body uses a mixture of carbohydrate and fats as fuel for ATP production via oxidative metabolism. Fats are slower to be mobilized than carbohydrates. The blood FFA data indicate that the placebo group began to depend more on FFA metabolism after 1 hour of exercise, but the glucose-fed group did not increase utilization of FFAs until between 3 and 4 hours of exercise. At that time the subject depended less on muscle glycogen. Overall, the carbohydrate feeding extended the time that the subject could exercise without increasing utilization of FFAs.

Question 5

Suggest eating small snacks composed of simple sugars periodically during runs to prolong the time to fatigue.