Quality of food is more important than quantity for preventing weight gain with the passing years, a US study has found.

Small lifestyle changes made the difference between staying slim and becoming overweight or obese, said researchers fromHarvard School of Public Health in Boston.

Diet had more of an impact than physical activity, watching TV or sleep duration. But focusing on calories alone was not the best way to stay slim.

The secret was to focus on the quality of foods, especially carbohydrates, according to the findings which were published in the New England Journal of Medicine.

Weight-watchers were advised to cut out sugar-sweetened drinks, potatoes, and refined grain foods such as white bread, white rice and low-fibre breakfast cereals.

At they same time they were urged to eat more “natural” foods, such as fruit and vegetables, whole grains, nuts and yoghurt, while avoiding anything processed.

Frank Hu, a nutrition expert at the Harvard School of Public Health, said: “These findings underscore the importance of making wise food choices in preventing weight gain and obesity. The idea that there are no ‘good’ or ‘bad’ foods is a myth that needs to be debunked.”

His team analysed results from three large-scale studies of US health workers. The findings also showed that getting the right amount of sleep helped people maintain a healthy weight.

People who slept six to eight hours a night gained less weight than those who slept less than six or more than eight hours. It was also found that physical activity reduced weight gain, while alcohol consumption added weight at the rate of 0.41 pounds per drink per day.

Co-author Dariush Mozaffarian, also from the Harvard School of Public Health, said: “Small dietary and other lifestyle changes can together make a big difference – for bad or good.