2 Comments »Posted on Saturday 18 April 2009 at 9:34 am by Sam Wong
In Biology

Call me cynical (many people do), but I’ve always been somewhat sceptical of energy drinks. On their website, Lucozade provide fact sheets on ‘The Science Behind Sports Drinks’. They have this to say about carbohydrate:

Carbohydrate is an important source of energy during moderate to high intensity activity. Unfortunately there is only a limited amount of carbohydrate (~ 2000 kcal) stored within the body which if depleted beyond a critical point decreases endurance performance by causing an individual to slow down. This is because they do not have sufficient energy, in the form of carbohydrate, to sustain their chosen exercise intensity for the remainder of their event.

It makes intuitive sense that giving your body a boost of readily metabolisable glucose during exercise would help you go on for longer. But studies have shown that even after an hour of intense exercise, there is still plenty of glycogen stored in the muscles, and it is this glycogen that is primarily being used as a source of energy, not blood glucose. Moreover, the amount of carbohydrate you absorb from an energy drink during an hour of exercise is relatively small, and doesn’t contribute much to your total energy supply.

Despite all this, most of the evidence suggests that energy drinks really can improve performance in events lasting around an hour, which is very puzzling to physiologists.

In 2004, researchers at the University of Birmingham found that intravenous infusion of glucose did not affect the time taken for cyclists to complete a time trial lasting roughly an hour, but performance on the same test was improved by merely rinsing the mouth with a maltodextrin solution (maltodextrin is a polysaccharide, consisting of chains of glucose). This suggests that taste can improve exercise performance, and is all the more surprising since maltodextrin isn’t sweet like glucose; in fact it doesn’t taste of anything very much.

In further work published in the Journal of Physiology this week, researchers from the same laboratory tested the effect on cycling performance of rinsing the mouth with solutions containing either glucose, maltodextrin, or a placebo, all of which were artificially sweetened. They also used functional magnetic resonance imaging (fMRI) to look at which brain areas were being stimulated by the different solutions.

Edward Chambers and his collaborators found that both glucose and maltodextrin resulted in faster times than the placebo, suggesting that we have as-yet unidentified taste receptors that respond to carbohydrates besides those that detect sweetness .

The fMRI scans showed that glucose and maltodextrin were activating areas of the brain associated with our neural reward system, including dopamine pathways in the striatum. These pathways are known to have a role in influencing motor activity. The authors hypothesise that the carbohydrates are detected by some putative non-sweet receptors in the mouth, which in turn stimulate parts of the brain that counteract the effects of fatigue.

The evidence for this is far from definitive. Among the problems with this study is the fact that the fMRI scans were done on resting athletes using much higher concentrations of carbohydrate than were used in the exercise tests. The stress of exercise might affect the way that the brain responds to the carbohydrate. But the results do show that carbohydrate solutions are sensed in the mouth, leading to an improvement in exercise performance, and suggest a possible mechanism through which this effect might operate.

The findings lend support to the ‘Central Governor‘ model: the idea that fatigue results from the brain limiting its own ability to activate muscle fibres so as to prevent damage to the heart caused by lack of oxygen. Chambers and colleagues propose that detecting carbohydrate in the mouth interferes with the central governor to reduce the perception of exertion, which fits with the finding that cyclists who tasted glucose or maltodextrin didn’t feel any more tired despite cycling faster. (Not everyone accepts the central governor hypothesis; here’s a critical review.)

Fascinating stuff. I might just give Lucozade a go the next time I play football. I probably will swallow it though – even if it’s no good as an energy source, it must help with the whole hydration business.

  1. 2 Comments

  2. It’s a very strange result, but I don’t think Lucozade will be running a “spit don’t swallow” advertising campaign any time soon..

    By Jacob Aron on Saturday 18 April, 2009 at 9:40 am

  3. I’ve always found glucose drinks to be very helpful during 1 hour runs, but it’s always hard to know how much of that is just psychological. Or just the benefit of drinking water.

    By Neuroskeptic on Saturday 18 April, 2009 at 3:02 pm

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