Anyway, crashed out yesterday reading the Guardian magazine in one of those New Year sort your life out articles, these paragraphs caught my eye:
Exercise only in moderation
The two key terms for energy researchers in 2010 have been BDNF (brain-derived neurotrophic factor) and VEGF (vascular endothelial growth factor): BDNF stimulates the formation of new connections between brain cells; and VEGF produces new blood vessel-lining cells, potentially keeping the arteries free of flaws that are the potential sites of clots, and therefore preventing heart attacks and strokes.
Regular exercise increases levels of both, so it should be good for you – but there's a snag: too much exercise lowers BDNF levels. Does that have a damaging effect on brain cells? We don't yet know, but anecdotal evidence of the breakdown in health of athletes and enthusiasts who train to near-exhaustion every day tends to suggest that it does.
The main message, then, is to give your body time to recover after exercise. The current advice is to exercise to breathlessness (it doesn't matter what you do – anything you continue to enjoy) for around 30 minutes, and avoid exercising more than three or four times a week.
Do something....but not too much.