Nutrition - Brain Drain


Feeling bushed at the end of a hard run is perfectly normal. In fact, it's inevitable. After all, we're not machines, and fatigue is your body's way of telling you enough is enough. But if fatigue has been hampering you lately, maybe there's more to it than physical exhaustion. Could be you're experiencing "brain fatigue," and certain foods can help you fight back.

Researchers have long pondered the origin of exercise fatigue, and they now agree that several factors contribute to it, including spent stores of muscle glycogen, overuse of specific muscle groups through repeated contractions, and lactic acid buildup. These are what exercise physiologists call peripheral fatigue mechanisms. That is, they originate in the muscles themselves.

However, recent research shows that exercise fatigue is also caused by mechanisms in the brain. Specifically, scientists are looking at neurotransmitters--brain chemicals responsible for (among other things) mood and alertness. According to the "brain theory," levels of the neurotransmitter serotonin rise during exercise and cause fatigue. Further research indicates that eating certain foods may inhibit this mechanism. But first, let's look more closely at brain fatigue. It's All In Your HeadThe brain theory, or "central fatigue hypothesis," of exercise exhaustion was developed by Oxford University biochemist Eric Newsholme, Ph.D. It begins like this: When you run for long periods, you drain your muscles of glycogen, which is the form that carbohydrates take when stored in muscle tissue. When glycogen stores run low, fatty acids (released from fat cells) become a primary energy source.

Now it gets tricky. Fatty acids require a special carrier to take them through the bloodstream. The problem is that there's another substance that rides this same carrier. That other substance is called tryptophan, an amino acid that the brain converts to serotonin. What happens is that during endurance exercise, increasing numbers of fatty acids bump tryptophan off its carrier. The free-floating tryptophan enters the brain (it has a biochemical "preference" to do this), where it converts to serotonin. The result? Serotonin levels increase, and you feel tired.

This fatty acid-tryptophan-serotonin chain of events isn't the only mechanism responsible for brain fatigue. There's a second biochemical interaction that brings on fatigue. It involves substances called branch chain amino acids (BCAAs), which compete with tryptophan for entry into the brain.

During a long bout of exercise such as a marathon, your muscles use BCAAs for fuel. This lowers the amount of circulating BCAAs, which means that more tryptophan can get into the brain, as it doesn't have to compete with as many BCAAs to do so. The result: more serotonin is produced, and fatigue sets in. Theoretically, if you could maintain higher levels of BCAAs in your blood during exercise, more BCAAs and less tryptophan would get into the brain, and this would help fight fatigue.

Food For Thought

The key to fighting brain fatigue is to prevent tryptophan from getting into the brain. Certain foods can help accomplish this--in particular, carbohydrates.

When you take in carbohydrates just before or during exercise, as with a sports drink, your muscles use the carbohydrates as fuel, and a signal is sent to fat cells to slow the release of free fatty acids. When fatty acid release slows, this decreases the amount of tryptophan going to the brain.

To examine this carbohydrate/brain connection, University of South Carolina exercise physiologist Mark Davis, Ph.D., put trained cyclists to the test. Two groups of cyclists were asked to pedal on a stationary bike at two-thirds of their maximum effort until exhaustion. While cycling, they drank a cup of fluid every 30 minutes. The first group drank a placebo beverage that contained no carbohydrates, while the second group drank a carbohydrate sports drink.

The cyclists who drank the placebo stopped pedaling after about 3 hours. The carbohydrate drinkers lasted a full 45 minutes longer. Furthermore, researchers discovered that the levels of free fatty acids and tryptophan climbed over 500 percent of normal in the placebo group and were highest at the time of exhaustion. Those who took in carbohydrates had far lower levels of both.

"The carbohydrate beverages clearly boosted performance in these cyclists," says Davis. "And while some of the improvement may be from increased fuel for the muscles, our results show that carbohydrates also play a role in delaying fatigue through central mechanisms." In other words, carbohydrates do this by holding down levels of fatty acids and tryptophan.

The Chain Gang

And what about the branch chain amino acids? Can we take a dose of BCAAs while running, so that less tryptophan reaches the brain for serotonin production? The theory sounds plausible, but studies have been inconclusive. According to Davis, the amount of BCAAs needed to lessen the amount of tryptophan entering the brain would be poorly tolerated and perhaps even dangerous. BCAA beverages taste awful, for one, and they slow fluid absorption, which increases risk of dehydration. Heavy doses could possibly even bring about toxic levels of ammonia in the body.

Yet, inevitably, sports drinks containing BCAAs have hit the market. While testimonials from endurance runners suggest that these beverages ward off fatigue, the amount of BCAAs in these products--less than 1 gram per serving--is small and would have negligible effect. Until we learn more about BCAAs and fatigue, you should avoid taking supplements containing these amino acids because of their potentially harmful side effects.

Brain Pleasers

To help fight brain fatigue, here are several tips for before and during your workouts and races:

References

Liz Applegate, Ph.D., is the Nutrition Editor for Runner's World. She is a member of The National Triathlon Training Camp Elite Team, and teaches at the University of California at Davis.