Research from Brigham Young University (BMU), published in medical journal Preventive Medicine, found people who consistently did high levels of activity had significantly longer telomeres - a protein that protects chromosomes from deterioration - than those who were sedentary and even of those who were moderately active.

The older a person gets, the shorter telomeres become.

But exercise science professor Larry Tucker says a person's chronological age does not always reflect their biological age.

"We all know people that seem younger than their actual age. The more physically active we are, the less biological ageing takes place in our bodies," he said.

Researchers at BMU, led by Professor Tucker, analysed data from nearly 6000 adults who took part in the National Health and Nutrition Examination Survey.

The survey included telomere length values for study subjects.

The analysis showed the shortest telomeres belonged to those who led a sedentary lifestyle.

There was also no significant difference in telomere length between those with low or moderate physical activity and the sedentary people.

Adults who did at least 30 minutes of jogging a day, five days a week, had the longest telomeres, giving them a biological ageing advantage of nine years, the research estimated.

"If you want to see a real difference in slowing your biological ageing, it appears that a little exercise won't cut it," Prof Tucker said.

Although the exact mechanism for how exercise preserves telomeres is unknown, it is thought it might be tied to inflammation and oxidative stress - the body's inability to counteract the production of damaging free radical molecules.

AAP