Obesity and smoking accelerate aging, say researchers who saw the evidence at the cellular level.

The team measured shortening of telomeres, caps on the end of chromosomes, as an indicator of aging. Telomere length shrinks every time a cell divides, like a chromosomal clock that reflects the aging process.

"Our findings suggest that obesity and cigarette smoking accelerate human aging," the team wrote in the June 14 online issue of the medical journal The Lancet.

Telomeres help keep chromosomes from unravelling.

In the study, Dr. Tim Spector of St. Thomas Hospital in London and his team found women who were obese or smoked had shorter telomeres than leaner, non-smokers.

The team studied telomere length in 1,122 female twins aged 18 to 76. All participants completed a questionnaire about their smoking history.

Scientists suspect that as telomeres shrink, chromosomes are less stable and therefore more likely to mutate.

Observing aging

Spector's team used shrinking telomeres in white blood cells as a marker for the damage from oxidative stress and inflammation in cells – an indirect way of observing the pace of aging.

After taking lifestyle factors into account, the difference between being obese and lean corresponded to 8.8 years of extra aging for telomeres.

Telomeres in women who smoked or were ex-smokers showed an extra 4.6 years of aging, while those who smoked a pack a day for 40 years showed 7.4 more years of damage.

Since the researchers only looked at white blood cells, they can't say if the effect occurs in other tissues. And scientists haven't conclusively shown that shorter telomeres are linked to a shorter lifespan for organisms.

Nevertheless, the results offer more support for living a smoke-free lifestyle and maintaining a healthy weight.

"Our results emphasise the potential wide-ranging effects of the two most important preventable exposures in developed countries – cigarettes and obesity," the team concluded.

Nearly 120 of the women studied were clinically obese, 531 had never smoked, 203 were smokers and 369 had quit.

Blood samples were taken to measure telomere length in white blood cells. Researchers also looked at levels of a body fat regulator called leptin.