Obese teenagers who eat a lot of salty food may be getting old before their time, a study suggests.
Scientists found evidence the teenagers' cells were ageing more quickly than those of overweight teens who consumed less salt, or slimmer individuals.
"Lowering sodium intake, especially if you are overweight or obese, may slow down the cellular ageing process that plays an important role in the development of heart disease," said lead scientist Dr Haidong Zhu, from Georgia Regents University in Augusta, US.
The findings are based on a study of telomeres, protective caps on the ends of chromosomes that shorten as we age.
Telomeres have been compared with the plastic tips that stop shoelaces fraying. When they get too short, cells become inactive or die.
Research has shown that the speed of telomere shortening can vary between individuals, corresponding to different rates of biological ageing.
High levels of body fat were already known to hasten the shortening of telomeres.
The new study found that sodium in salt seemed to work hand-in-hand with obesity to speed the effect up further.
Zhu's team divided 766 teenagers into different groups according to their reported sodium intake.
Low-intake teens consumed an average of just over two grams of sodium a day, compared with more than four grams for the high-intake group.
Both groups consumed far more than the daily maximum 1.5 grams of sodium recommended by the American Heart Association - equivalent to about two-thirds of a teaspoon of table salt.
The scientists found that among overweight and obese teenagers, those with a high sodium intake had significantly shorter telomeres than those in the low intake category. This was after adjusting for a number of factors that influence telomere length.
"Lowering sodium intake may be an easier first step than losing weight for overweight young people who want to lower their risk of heart disease," Zhu said.
"The majority of sodium in the diet comes from processed foods, so parents can help by cooking fresh meals more often and by offering fresh fruit rather than potato chips for a snack."