ANY RUNNER who has gone 26.2 miles or beyond knows the feeling. I’m not talking about the leaden, tenderized feeling you get in your legs or the sapped, hollowed-out feeling that spreads throughout your body in the latter miles of a long run or race. I’m talking about the misery and suffering that overtake your brain at about the same time—the brooding, the negativity, the fantasies of quitting. Yeah, that feeling.

Scientists have actually given a name to this most unpleasant state of mind so familiar to all long-distance runners: central fatigue. What’s more, they have even gathered evidence to suggest that central fatigue may have as important a limiting effect on performance as muscle fatigue, which usually gets all the credit, or criticism.

“We don’t know exactly what central fatigue is,” says J. Mark Davis, a professor of exercise physiology at the University of South Carolina and one of the world’s leading researchers in the study of the phenomenon. “It appears to have two components. One is a group of psychological factors, including loss of motivation and will, negative mood state, and lethargy. The other is an automatic, inhibitory mechanism in the brain that actually prevents the transmission of electrical impulses to the muscles and may exist to protect us from going too far into exhaustion.”

The causes of central fatigue are only partially understood. In general terms, it appears to result from the accumulation of certain neurotransmitters associated with negative mood states in specific parts of the brain during extensive exercise. The primary culprit, or at least the one we know the most about at this time, is serotonin, which is familiar to many as a culprit in relation to various mood disorders.

The factors that lead to these changes in neurotransmitter concentrations are closely related to the factors that lead to muscle fatigue, which is why they tend to happen at about the same time. “Blood glucose is a primary muscle fuel during exercise of moderate to high intensity and is the only fuel for brain activity,” Davis explains. “The longer an effort is maintained, the less glucose remains to fuel both neural and muscular activity.”

WHAT TURKEYS DO TO US

But that’s not the whole story. As blood glucose is depleted, hormones trigger the release of other, less preferred fuels into the bloodstream. These include branched-chain amino acids and free fatty acids. For reasons that are a bit too technical for this discussion, the increased use of branched-chain amino acids and free fatty acids as fuel causes a corresponding increase in the concentration of an amino acid called free tryptophan in the blood. Tryptophan is best known as the ingredient in turkey that makes you so darn sleepy after Thanksgiving dinner. It is also capable of crossing into the brain, where it is converted into—you guessed it—serotonin.

Exactly when this process begins to occur depends on the intensity of exercise and one’s level of fitness, but it always unfolds in virtual lock step with muscle fatigue. Its first and primary symptom is an increase in perceived effort. The more fatigued the brain becomes, the harder it feels to continue running at the same pace. Motivation and mood head due south, and even the ability to concentrate and make decisions begins to suffer.

Delaying central fatigue as long as possible is therefore clearly in the best interest of every distance runner. How is it done? At this time, the prescription for extending brain endurance is identical to the recipe for prolonging muscular endurance: train well and consume plenty of fluids and carbohydrates during races and longer workouts. By training properly and consuming a well-formulated sports drink when you need it, you can maintain blood glucose levels much longer than you could otherwise and thereby postpone the muscular and neural consequences of glucose depletion.

In one study led by Davis, cyclists exercised to exhaustion at 70 percent VO₂max while drinking either water or a sports drink containing carbohydrates and electrolytes. On average, the cyclists who consumed the sports drink lasted a full hour longer than those who drank only water, and they had 80 percent less free tryptophan buildup in the blood. While the better performance of the carbohydrate group must have been due in large part to delayed muscle fatigue, Davis notes that the degree of the performance gap “could not be explained by typical markers of peripheral muscle fatigue involving cardiovascular, thermoregulatory, and metabolic functions.”

Scientists like Davis are currently searching for other nutritional factors that might address central fatigue specifically. One candidate is caffeine. “It’s far too early to begin recommending caffeine intake during exercise,” says Davis, “but we do know that caffeine leads to increased levels of dopamine, which is basically the opposite of serotonin in terms of its effect on psychological states.”

PROTEIN REPLACEMENT

Might we one day find ourselves using sports drinks that contain such things as caffeine? “It’s possible,” says Davis. In the meantime, some sports drink makers have already begun fiddling with the water-electrolytes-carbohydrate formula that has been the norm for many years now. For example, a small amount of protein is being included in some of the newer sports drinks because of research into its effect on the hormone insulin.

“Insulin plays an important role in delivering glucose through the bloodstream to the working muscles and the brain,” explains Edmund Burke, Ph.D., author of Optimal Muscle Recovery. “Both carbohydrate and protein stimulate insulin release. Research shows that when protein is consumed with carbohydrate in the proper ratio (about one gram per four grams), insulin release is maximized and therefore so is glucose fuel. This delays both muscular and central fatigue.”

In a study performed at the University of Texas, one of the new sports drinks containing carbohydrate and protein in a 4-to-1 ratio (Accelerade) was compared to a conventional sports drink containing no protein. Subjects who used the former lasted 24 percent longer than those who used the latter. However, to what degree the difference was owing to delayed central fatigue is not known.

Runners are notoriously more casual about midexercise nutrition than some other types of endurance athletes, such as cyclists and triathletes. In light of what we have recently learned about central fatigue, this oversight now seems doubly detrimental. If you want to finish strong in body and mind in your longest runs, you need to make proper use of a quality sports drink. And keep an eye out for the newest (and future) formulations that take the latest knowledge into account.

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