The quality of the food you eat rather than how much you consume has the biggest impact on losing weight, a study has found.

Diet also has a greater impact than other factors such as physical activity or sleep duration.

But focusing on calories alone was not the best way to stay slim, according to the scientists. The secret was to focus on the quality of foods, especially carbohydrates.

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

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

One of the researcher 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."

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.

The team analyzed results from three large-scale studies of US health workers: the Nurses' health Study, the Nurses' health Study II, and the Health Professionals Follow-up Study. Together the studies monitored the health of more than 100,000 individuals for up to 20 years.

All were initially free of chronic diseases and not obese. Every four years, participants gained an average of 3.35lbs, putting on almost 17lbs over the full 20 year period.

Increased daily servings of certain foods were associated with the largest amount of weight gain per four year period. They included potato chips, which added 1.69lbs of extra weight, and sugar-sweetened drinks, which added one pound.

Some foods were actually associated with a lowering of weight gain when their consumption was stepped up. They included nuts (minus 0.57lbs per four years), vegetables (minus 0.22lbs), whole grains (minus 0.37 lbs) and fruits (minus 0.49lbs).

Physical activity also reduced weight gain.

The co-author said:

"An average adult gains about one pound per year.

"Because the weight gain is so gradual and occurs over many years, it has been difficult for scientists and for individuals themselves to understand the specific factors that may be responsible.

"Small dietary and other lifestyle changes can together make a big difference - for bad or good. This makes it easy to gain weight unintentionally, but also demonstrates the tremendous opportunity for prevention. A handful of the right lifestyle changes will go a long way."

Research Paper Details:

Mozaffarian D, Hao T, Rimm EB, et al. Changes in diet and Lifestyle and Long-Term weight gain in Women and Men. New England Journal of medicine, 2011; 364 (25): 2392.