Can exercise age you?

She may be one of the world's most glamorous celebrities but even Madonna, it seems, isn't immune to the tell-tale signs of ageing. Scenes from her flop film Swept Away - and recent snaps of her out shopping - show just how wrinkled the 45-year-old's face has become - despite all the make-up and favourable camera angles.

Years of dancing, weight-training, running and astanga yoga - a very physical and punishing form - have given her the body of a much younger woman, but the same cannot be said for her face.

Madonna is now one of the most advanced astanga yogis in the country - she's on the Third Series, which is the yogic equivalent of being an Olympic standard athlete. She is a keen runner (she covers around 20 miles a week, according to a previous trainer), dances and used to do weight training every day.

"Doing astanga on this level is like running a marathon on a weekly basis," says one teacher. "It tends to wither people and, like anything done to excess, it will age you."

After prolonged exercise, such as a 25-mile run, the body changes its metabolism, with a resulting increase in free radicals, atoms that can cause permanent damage to your cells and can also speed up the ageing process.

Free radicals, which are also created by external factors such as pollutants, ultraviolet light or cigarette smoke, are highly reactive and attack nearby healthy molecules, including your collagen and DNA. Collagen damage will make your skin look older.

Free radicals attack healthy molecules until they are mopped up by a radical scavenger or an antioxidant. It has been calculated that under normal circumstances, each of our 100 trillion cells gets hit by 10,000 free radicals per day.

Antioxidants convert the free radicals back into a good molecules. To keep up your intake of antioxidants - sources include vitamins C and E - eat plenty of fruit and vegetables or take supplements.

Undoubtedly, we all need to keep fit, but how does too much exercise adversely affect your body?

The face

Long-distance runners traditionally look gaunt, and a number of plastic surgeons believe that running is the biggest cause of premature wrinkles after smoking and sunbathing. "Running is not good for the facial tissues," says plastic surgeon Laurence Kirwan. "All that pounding up and down causes shearing forces of the skin against bone and loosens everything, so you get the slack neck and jowls that Madonna has now. I see a lot of New Yorkers who run five to six days a week and I tell them to use bikes instead, because otherwise they won't just end up with saggy breasts, they'll get a slack face."

Personal trainer Matt Roberts says you'd need to run at least 25-30 miles a week to cross the fine line between running making you look younger (improved muscle tone and circulation) and looking older. Kirwan advises casual exercisers to limit sessions to two runs a week.

The bones

Weight-bearing exercise is linked to increased bone strength, but do too much and it can have the opposite effect - intensive aerobic training has been linked to a fall in bone density in women, as a result of low oestrogen levels. This is a particular problem for child gymnasts and ballet dancers, where intensive training and low body fat in childhood delay the onset of periods, or in athletes who train to the extent that their periods stop. At milder levels the reduction in oestrogen that occurs as a result of exercise is generally beneficial - leading to less PMS, and lower breast cancer risk.

The knees

We know that running can help prevent arthritis, but there's a limit. A British Medical Journal study of 2,049 Finnish athletes found that endurance and sports athletes had a higher incidence of osteoarthritis, as a result of increased wear and tear on their joints. The BMJ's advice? Prevent premature osteoarthritis by treating injuries. So if you have knee or joint pain, see a physio.

Sports scientists have found "overtraining syndrome" is a problem not just in Olympic athletes, but in domestic sports people - in the US, it is said that 10 per cent of college swimmers "burn out" every year.

"Overtraining syndrome" is a form of chronic fatigue, and is linked to intensive work and inadequate rest periods ( sprinters are less likely to suffer from it because they train with large amounts of rest).

Symptoms include depression, loss of appetite, increased anxiety and irritability, fatigue, sleep problems (in more than 90 per cent of cases) and frequent minor infections.

Overtraining raises cortisol levels and suppresses the immune system.

The advice to anyone suffering-from the above symptoms is to take a break: the only known cure is rest, but athletes who fail to do that generally find they take six to 12 weeks to recover from chronic fatigue.

The heart

Regular running can cut your risk of heart disease by 50 per cent, but do too much, too soon and it can be deadly. An American study found that heart attacks and strokes triggered by tough workouts have trebled since 1980.

The key is to increase cardiovascular demands gradually - blood pressure can more than double in intense exercise, so you need to allow your body to adapt to these changes. Otherwise, the arteries are placed under immense novel pressure, which can cause a heart attack or stroke.