The forgotten cause of muscle fatigue...
A chemical trigger that tells the brain to feel tired after tough
exercise has been identified by scientists, raising the prospect
both of new treatments for chronic fatigue and new doping methods
in sport.
Research in South Africa has revealed that a signalling molecule
called interleukin-6 (IL-6) produced during physical exertion seriously
impairs the performance of athletes, adding to feelings of fatigue
that build during a race.
The findings, from a team at the University of Cape Town, suggest
that blocking the brain’s ability to respond to IL-6 could
be a promising approach to treating chronic fatigue syndrome.
Drugs that work in this way, however, also have the potential for
abuse by athletes, allowing them to train for longer and to stave
off pain in important races. But drug cheats who took it as a performance-enhancer
would probably risk serious damage to the immune system, experts
said.
IL-6 is a protein that helps to regulate the immune system, determining
when the body’s defences are switched on and off in response
to illness and injury. It is produced in large quantities at injured
or infected sites that become inflamed, or in response to stress
or changes in energy metabolism.
Paula Robson-Ansley, who led the research, said that it appeared
to be a defence mechanism, prompting fatigue to force the body to
conserve energy. "I see it as a distress flare that says the
body’s in trouble, so slow down," she said.
Previous research has shown that blood concentrations of IL-6 rise
to between 60 and 100 times the normal levels after prolonged exercise.
This probably reflects small injuries to the muscles, low levels
of energy available to the muscles and psychological stress. Injecting
healthy people with the chemical also makes them feel tired.
Dr Robson-Ansley, now at the University of Portsmouth, has shown
that IL-6 also has a measurable detrimental effect on athletic performance.
She recruited seven club-standard runners, and injected them either
with IL-6 or an inert placebo before they took part in a 10km race.
A week later, the experiment was reversed, so every athlete had
done the run on both IL-6 and placebo.
She found that athletes ran almost a minute faster after receiving
the placebo than they did after being injected with IL-6. The difference
was statistically significant, given average finishing times of
41 minutes.
"The participants did noticeably worse after taking IL-6,
and it looks very much as if fatigue was the reason,” she said.
It is hoped that a drug which blocks the IL-6 receptor in the brain,
currently in clinical trials, could help to relieve chronic fatigue
syndrome, which is often difficult to treat.
Another possibility is that unscrupulous athletes might take IL-6
blockers as performance- enhancing drugs, allowing them to train
for longer and avoid “hitting the wall” in big events.
References
Robson, P. (2003). Elucidating the unexplained underperformance
syndrome in endurance athletes : the interleukin-6 hypothesis. Sports
Medicine, 33, 771-781
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