For years people wanting to maintain a slim figure have been obsessed with calorie counting.
But new research, published today (WED), indicates that it is much more important to concentrate on eating healthy foods rather than fixating on how much one consumes.
Experts have found that the more good food in one's diet, the more weight one loses over the long term.
Their study of almost 120,000 people, five-sixths of whom were women, discovered that extra helpings of yoghurt, nuts, fruit, whole grains and vegetables were all linked to weight loss.
The team, from Harvard School of Public Health, quantified the effect that eating particular types of food daily had on weight gain or loss.
Perhaps surprisingly, eating more yoghurt and nuts every day had a bigger effect on losing weight than fruits and vegetables - probably because they keep people fuller for longer.
They found that people who ate an extra portion of yoghurt daily, compared to the study group as a whole, lost on average 0.82lbs (0.37kg) every four years, over a 20 year period.
For nuts the comparable figure was 0.57lbs (0.26kg), for fruits 0.49lbs (0.22kg), for whole grains 0.37lbs (0.17kg) and for vegetables 0.22lb (0.1kg).
The authors, led by Prof Dariush Mozaffarian, noted that this did not mean people could simply eat large amounts of these foods and lose weight.
"Obviously, such foods provide calories and cannot violate thermodynamic laws," they wrote in an article published in the New England Journal of Medicine.
However, people who ate them tended to eat less calorie-dense foods such as chips, meats, desserts and sugary drinks.
They wrote: "Their increased consumption would .. displace other, more highly processed foods in the diet, providing plausible biologic mechanisms whereby persons who eat more fruits, nuts, vegetables, and whole grains would gain less weight over time."
The worst foods to over-consume were chips - adding on extra 1.69lbs (0.77kg) per four years for every daily serving over the average - sugary drinks (1lb / 0.45kg) and meats (0.94lbs / 0.43kg).
Frank Hu, professor of nutrition and epidemiology at Harvard, who also worked on the paper, added: "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 study warned that people tended to put on weight so slowly that they never noticed.
The mean weight gain was less than a pound a year, but over 20 years that amounted to 16.8lbs (7.6kg).
At its root, this was because people consumed just 50 or 100 calories a day more than they expended.
Prof Mozaffarian commented: "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."
The study also found that sleeping between six and eight hours - no more no less - was ideal for minimising weight gain, while cutting down on television viewing was also important.
It was based on 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. All participants were initially free of chronic diseases and not obese.