Nearly one in five dishes served at US restaurants has at least 100 more calories than advertised, a difference that could pack on up to 15 kilograms per year, a study said.

While most of the 269 foods measured at random in the three-state survey were close to the calorie count listed on the menus, some varied wildly, such as one order of chips and salsa totalling 1,000 more calories than expected.

That finding, while “not typical... is an amount that is nearly half the total daily energy requirement for most individuals,” said the study led by Lorien Urban of Tufts University and published in the Journal of the American Medical Association.

Salads, as well as high-carbohydrate dishes containing rice, beans, potatoes or bread, tended to have “significantly more variability,” while sandwiches and meat dishes were more likely to remain near or below the advertised calories, said the study.

In a country where obesity has risen to 34 per cent of the population and up to 35 per cent of the average person’s calorie intake comes from food eaten out instead of at home, the findings point to a potential source of a nationwide epidemic.