Exercise is the opiate of the people, and jogging is good for snogging, but does it shrink your brain?

"Everything gives you cancer," or at least according to Joe Jackson. I'm not sure on what evidence he based his assumption but it's a damn fine song, if a little on the bleak side: "There's no cure, there's no an-swer." That bit is plainly wrong - well over half of all cancers can be cured - but I guess it didn't scan so well. I was reminded of Mr Jackson's pathological outpourings last week when Dr Kenneth Cooper, "the father of jogging", announced to the world that too much of it can cause you- know-what. Ken cites "undeniable" evidence that "cancer-causing free radicals" can be unleashed on your body if you do too much heavy breathing. A harsh punishment for a few nuisance calls, you might think, but Ken's new book, The Anti-Oxidant Revolution, cites legions of marathon runners who've succumbed to malignancy.

This is a tad at odds with the official line that exercise can reduce the risk of colon cancer and the antithesis of Ken's 1968 best-seller Aerobics, which convinced millions of Americans that you couldn't get enough heavy breathing. Still, if you want to follow Ken's new line, it's as good an excuse as any to keep those jogging pants in wrapping paper. I suspect marathon runners are more likely to die of cancer because their hearts are so healthy - and if you don't have a cardiac arrest there's only one other normal way to die. My excuse for sitting down a lot is even flimsier, but it was passed to me at St Thomas's by a physician to the Royal Family. "All vigorous exercise does is divert blood towards the muscles and away from the brain. It's bound to make it shrink." Any family with a history of in-breeding and uninspiring marriages needs all the help it can to hold onto its neurones, but I've yet to uncover any proof of the theory. Not that you want to take any chances. I mean, when did you last see the Queen in Reeboks?

But enough nihilism. Are there any benefits of working up a sweat? Research seems to suggest that you need to do at least 30 minutes of "moderate intensity activity" on at least five days a week to significantly cut down on your risk of dying prematurely; 20 minutes of it three or more times a week will reduce your chance of death a bit and maximise your aerobic fitness (ie ability to snog for long periods without panting). Whether this improves the quality of your life is a different matter. The tortured face of your average jogger is enough to put me off. Still, leaving the cancer debate aside, regular exercise can reduce your blood pressure and risk of developing osteoporosis, obesity, diabetes, strokes and heart attacks. It can also raise your self-esteem and improve your mood, though these benefits obviously have to be balanced against the risk of the activity itself. Your mood may well plummet if you break something.

The good news for those of you who don't move like gazelles is that a brisk walk in sensible shoes qualifies as "moderate intensity activity". And if you're too self-conscious even for that, I find hiding behind a damp Labrador does the trick. You then get the added benefit to your heart of owning a pet and a riveting social life as you encounter other pet owners. Also, carrying a baby on your back builds up your upper body and advertises your fertility. Great for your heart and self-esteem, although you do have to watch out for low branches.

So is it possible to become hooked on exercise? Not personally, no, but as with any habit you can become psychologically addicted ("I can't go to sleep until I've run to Wimbledon and back") and perhaps even pharmacologically addicted (exercise releases endorphins, the body's natural opiates, which are blamed for just about everything these days). However, these fears tend to be overblown for the sake of documentaries. The point is that exercise doesn't have to be synonymous with sweaty bores. Digging the garden, hand-washing your underpants and making chutney are more than a match for step aerobics, and pets are as good as press-ups. Press-ups with a pet on top of you is best of all, and still legal in most parts of the UK.

But if you still want to construct an argument to get you out of exercise, economics is your answer. If you're under 44, the average annual medical care costs due to sports injuries is pounds 30 per person, but the annual disease prevention benefits are only 68p. Hence you could say you were saving the nation pounds 29.32 a year by being a sloth. Unfortunately, if you're intending to live beyond 44, the figures reverse - disease risk increases with age as the desire to bunjee jump fades. Indeed, the older you get the more beneficial exercise becomes. Until you're mugged, of course