Exercise may hold the key to youth, according to a study published on Monday which showed people who keep fit are up to nine years biologically younger than those who do not.

The findings are the first to show in humans how keeping fit affects the ageing process.

The study of 2,401 twins found that a sedentary lifestyle raises the risk of a range of problems from heart disease to cancer and appears to play a key role in the ageing process.

It all appears to boil down to the length of structures called telomeres -- which protect the DNA on the chromosomes, the researchers from King's College London wrote in the Archives of Internal Medicine.

Many studies have shown telomeres get shorter over time, suggesting the cells are ageing or dying. The study, which extracted a DNA sample from their volunteers, found people who exercised more each week had longer telomeres.

Exercise lowers the risk of a range of problems such as heart disease, diabetes and cancer, the researchers said.

"It is not just walking around the block. It is really working up a sweat," said Tim Spector, a genetic epidemiologist who led the study, in a telephone interview.

The study found people who exercised vigorously 3 hours each week had longer telomeres and they were 9 years biologically younger than people who did under 15 minutes.

Spector's team, who also adjusted for body weight, smoking, economic status and physical activity at work, also said moderate exercise for 1-1/2 hours each week provided a four-year advantage.

Studying twins also provided a unique opportunity to gauge the effects of exercise on people with the same or similar genetic make-up, the researchers said.

The reason why exercise has this effect is not clear but the researchers said they believe physical activity somehow defends against the natural process called oxidative stress, which damages and kills cells.