Slight changes in eating habits, such as eating an extra serving of potato chips or fries each day, can add plenty to a person's weight over the years, US researchers said on Wednesday.

Three studies that spanned 20 years and more than 120,000 people showed that the notion of eating less and exercising more for good health may be too simplistic.

However, overall food choice - picking fruits and whole grains instead of starches and meats - appears to have the strongest link to how much a person gains, or doesn't, in the long term, said the research led by the Harvard School of Public Health.

The average adult gains about one pound (0.45 kilos) per year. To find out what drives weight gain, researchers examined data from three large studies of nurses and health professionals, said the report in the New England Journal of Medicine.

The subjects' lifestyle changes and weight gain were tracked every four years for two decades. Participants gained an average of 3.35 pounds (1.5 kilos) over each four-year period, for a 16.8 pound (7.6 kilo) gain over 20 years.

The food linked to the greatest amount of weight gain were French fries, one extra daily serving of which could add 3.35 pounds every four years. An extra handful of potato chips each day could add 1.69 pounds in the same time period.

Similar results were seen among people who consumed extra sugary drinks (one pound) and meats (0.95 pounds for unprocessed, 0.92 pounds for processed).

However there was less weight gain as time went on among people who ate more of certain foods, such as yogurt, vegetables, fruits and whole grains.

People who ate an extra serving of vegetables per day gained 0.22 fewer pounds over a four year period than people who did not. More yogurt meant a 0.82 pound dip, more fruit equaled 0.46 fewer pounds and more nuts meant 0.57 fewer pounds.

Researchers said people who ate more low-fat, high-fiber foods may have effectively "displaced other, more highly processed foods in the diet," which could explain why they gained less than their counterparts over time.

"These findings underscore the importance of making wise food choices in preventing weight gain and obesity," said senior author Frank Hu, professor of nutrition and epidemiology at Harvard.

"The idea that there are no 'good' or 'bad' foods is a myth that needs to be debunked."

Some lifestyle changes such as less exercise and more television watching were also linked to weight gain.

A good night's rest, but not too much rest, appeared beneficial too. People who slept between six and eight hours per night gained less weight over time than people who slept fewer than six hours or more than eight.

"Small dietary and other lifestyle changes can together make a big difference - for bad or good," said lead author Dariush Mozaffarian of the Harvard School of Public Health.

"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."