Health

Being a couch potato in your free time could make you a decade older biologically than someone who is physically active, according to a major study of people's "chromosomal clocks".

Tim Spector at St Thomas' hospital in London, UK, and colleagues measured the lengths of telomeres - the DNA that bookends our chromosomes - in the white blood cells of 2401 twins.

Telomeres shorten each time a cell divides, and when they become too short a cell can no longer divide, so telomeres act as a kind of timer counting down our biological age.

The researchers found that people who did not exercise in their spare time had shorter telomeres than very active people.

Psychological stress

On average, the least active (getting just 16 minutes exercise a week) had telomeres 200 base pairs shorter than the most active (exercising 3 hours a week), which translates to them being about 10 years biologically older.

The team previously showed that smoking and obesity can shorten telomere length to the equivalent of 10 years. But they found that exercise and telomere length were linked independently of whether people smoked or were overweight.

Accounting for whether or not participants had a chronic disease (possibly making them sedentary) did not change the findings either, suggesting that other factors linked with an inactive lifestyle affect biological age.

One of those factors seems to be psychological stress, which could translate into oxidative stress, a source of free radicals that can cause DNA mutations and shorten telomere length.

Although exercise can alleviate such stress, it only seems to help with cellular ageing if done in people's leisure time. Slogging away in manual jobs, on the other hand, tends to have the opposite effect and can shorten telomeres because, Spector says, of "the psychological stress of being in the lower social pecking order".

Age-related diseases

Telomere length can be inherited, but even after analysing a subgroup of twin pairs, who share both genetics and upbringing, he says, "the twin who was doing more exercise had longer telomeres - on average, an 88 base pair difference".

Whether having shorter telomeres means you die younger is unproven, but you are "more likely to have age-related diseases", such as Alzheimer's, says Spector. Nevertheless, although telomere length cannot be restored, it may be possible to slow down the process by starting to exercise, "even if you'd been a slob".

David Gems, who researches ageing, at University College London, notes: "These days older folks spend more and more money on various dietary supplements in the hope of ageing more slowly, but really they would be better off keeping the money and just walking the dog more often".

Journal reference: Archives of Internal Medicine 2008 (vol 168[2], p 154)

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