BioEd Online

It isn't news that eating fatty foods and smoking can shorten your life expectancy through heart attacks and cancer. But now a study shows that a lifetime of these unhealthy habits can directly 'age' DNA by years.

Strings of DNA are often capped by highly repetitive sequences known as telomeres. Like the plastic tips on the ends of shoelaces, telomeres help to protect genes against wear and tear. But each time a cell divides, the proteins involved in replicating our DNA fail to copy the telomeres completely. So these sections get shorter as the years pass.

Tim Spector, director of the twin research unit at St Thomas' Hospital in London, and his team have shown that telomeres shrink dramatically in patients who are obese or heavy smokers. "Shorter telomeres mean you will run out of steam sooner than people with longer ones," adds Edward Louis of the University of Nottingham, UK.

Obese women in the study were, according to their telomeres, nine years 'older' than slim women of the same age, explains Spector. Heavy smokers, who consumed a pack a day for 40 years, showed seven years of extra biological ageing.

Researchers already knew that smoking and obesity could cause a kind of stress in cells that produces reactive chemicals, which in turn are known to wear away telomeres. Spector's large study looks directly at this effect to quantify just how much a cigarette can age you.

Clocking up years

Spector's team collected information and blood samples from more than 1,100 women aged between 18 and 76 years. Within this group, 11% had a body mass index greater than 30, which classified them as clinically obese. About 18% were active smokers.

The researchers sequenced the DNA of the women's white blood cells to find the length of their telomeres. Overall, they found that a woman's telomeres shorten by about 27 base pairs a year; a base pair being a single letter of DNA in your genetic sequence.

But heavy smokers wore an additional 200 base pairs off their telomeres after 40 years of puffing. And obese participants' telomeres were, on average, 240 base pairs shorter than those of their lean counterparts.

According to the authors of the report, which appears in The Lancet¹, this ageing effect might help to explain why these women are at a greater risk of age-related health problems such as heart disease. But, Spector adds, it is important to note that the whole body ages faster under this kind of stress, not just the heart.

Spector says the immediate impact of such ageing could provide a different motivation to quit smoking or start a diet, alongside the fear of one day contracting cancer or heart disease. "People do distinguish between growing older and dying earlier," he says.

The research team now plans to examine the effects that exercise, diet and occupation have on telomere length.