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Posted: October 1, 2004
Multisport: Mental Fatigue - When your brain tells you to stop going, here's how to tell it to shut up
What actually causes you to become mentally fatigued when you're out for a long bicycle ride or run or involved in a extended game of squash? Theories about mental fatugue abound, but one popular hypothesis is that most of the lethargy is caused by a chemical called tryptophan.
Tryptophan is an amino acid which normally rides through your bloodstream attached to a very important blood plasma protein, albumin. However, when blood fat levels rise, as they do during prolonged exerclse, the fats 'kick' tryptophan loose from its albumin moorings. This 'free' tryptophan then enters the brain in large quantities, where - converted to another chemical called serotonin - it may induce fatigue and produce a drop-off in performance (serotonin is noted for its calming and even sleep-producing effects on brain cells).
This biochemical scenario has a protective effect: before you exercise so long that you tear your muscles to shreds, your brain fills up with tryptophan and you go to sleep - or at least you lose the willpower to force your muscles to keep going.
However, if you're an athlete, you would like to keep exercising at a high level until you reach the finish line or the end of the match, so how can you prevent your brain cells from taking a tryptophan bath? Exercise physiologists have had two answers: (1) you could try to limit the natural rise in blood fats, the ascendancy of which frees tryptophan to do its wicked work on the brain, or (2) you could pop some branchedchain amino acid (BCAA) tablets as you carry out your extended exercise.
Since everyone, even the skinniest Kenyan runner, has fat cells which insist on releasing fat into the blood during exercise, the latter strategy has gained a certain appeal. BCM-popping is an intriguing idea because BCMs 'compete' with tryptophan to enter the brain. As blood-BCAA levels increase, less tryptophan enters the neuron palace. Voila! If you're an athlete with high blood-BCM levels, you should be able to stay fatigue-free for a longer period of time as you exercise.
However, as BCAA-expert J. Mark Davis of the University of South Carolina points out, there are some problems with the BCAA-ingestion strategy. If you take a moderate dose of BCMs as you exercise, blood levels of BCAAs probably don't increase enough to thwart tryptophan. Large servings of BCAAs might work, but the problem is that hefty helpings reduce the absorption of water in the gut, potentially producing dehydration. High BCAA intakes might also increase your blood concentrations of ammonia, a toxic chemical which has been linked with fatigue. Not surprisingly, researchers in a number of studies have had a very hard time linking BCMs with improved exercise performances.
So what should you do? Fortunately, a triedand-true strategy - taking in carbohydrate during exercise - may have a profound effect on mental fatigue. Scientists have known for years that ingested carbohydrate keeps muscles perking along at a high level during triathlons, extended bike rides, marathons and multi-game squash or tennis matches, but they're just beginning to realise that carbohydrate may also have a profound effect on brain cells. It happens this way: carbohydrate feedings during exercise reduce the amount of fat circulating in the blood. As a result, less tryptophan is freed from its tenuous marriage with blood albumin, and so a lower amount of tryptophan enters the brain. Your nerve cells can't chill out on serotonin.
According to Davis, the realisation that carbohydrate delays central (mental) fatigue leads automatically to the following practical recommendations:
1. Don't fast before workouts or competitions. Fasting raises blood-fat levels, an effect which lets tryptophan do its dirty brainwork.
2. Avoid fatty foods during the 12-hour period before training sessions or competitions (fatladen foods increase your blood-fat concentrai tions and launch a cascade of tryptophan towards your brain).
3. In order to reduce blood-fat levels, eat a carbohydrate meal two to four hours before workouts or competitions.
4. Take in three to four swallows or more of a sports drink every 10 minutes or so during extended exercise to delay the onset of mental fatigue - and improve your performance.
('Carbohydrates, Branched-Chain Amino Acids and Endurance: The Central Fatigue Hypothesis,' Gatorade Sports Science Institute Nutritional Ergogenic Aids Conference, Chicago, November 1994)
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