Drinking sugary soft drinks may accelerate biological ageing as much as smoking, a study has found.
The findings, from an analysis of thousands of DNA samples, suggested that sweet fizzy drinks had worse effects on health than merely promoting obesity.
They may actually speed up the rate at which cells age - although scientists could not confirm that the effect was causal.
The study focused on telomeres, protective caps on the ends of the chromosomes that provide a measure of biological ageing.
Telomeres shorten with age, and short telomeres are associated with chronic problems of ageing such as heart disease, diabetes and some types of cancer.
The researchers found that people who regularly drank sugar-sweetened fizzy drinks - or sodas - had significantly shorter telomeres than those who did not.
Professor Elissa Epel, a member of the US team from the University of California at San Francisco (UCSF), said: "Regular consumption of sugar-sweetened sodas might influence disease development, not only by straining the body's metabolic control of sugars, but also through accelerated cellular ageing of tissues.
"This is the first demonstration that soda is associated with telomere shortness.
"This finding held regardless of age, race, income and education level.
"Telomere shortening starts long before disease onset.
"Further, although we only studied adults here, it is possible that soda consumption is associated with telomere shortening in children, as well."
The scientists measured telomeres in the white blood cells of 5,309 participants aged 20 to 65 with no history of diabetes or heart disease.
Consumption of 20 fluid ounces of soda a day - equivalent to about two cans of cola - was associated with 4.6 years of additional biological ageing, based on telomere shortening.
More than a fifth of the participants fell into this category.
The effect on telomere length was similar to that of smoking, the researchers said.
The findings appear in the American Journal of Public Health.