Washington: A study has found that potatoes can make people pile on the pounds while fruits, vegetables, grains, nuts and yoghurt can help them stay slim.
The Harvard study for the first time details how much weight individual foods make people put on or keep off.
"The conventional wisdom is simply, `Eat everything in moderation and just reduce total calories` without paying attention to what those calories are made of," the Washington Post quoted Dariush Mozaffarian of the Harvard School of Public Health, who led the study, as saying.
"All foods are not equal, and just eating in moderation is not enough," Mozaffarian explained.
From all the foods studied, potatoes stood out. It was found that every additional serving of potatoes people added to their regular diet each day made them gain about a pound over four years.
It was no surprise that french fries and potato chips are especially fattening.
But the study found that even mashed, baked or boiled potatoes were unexpectedly plumping, perhaps because of their effect on the hormone insulin.
Similarly, while it was no shock that every added serving of fruits and vegetables prevented between a quarter- and a half-pound gain, other foods were strikingly good at helping people stay slim.
Every extra serving of nuts, for example, prevented more than a half-pound of weight gain. And perhaps the biggest surprise was yoghurt, every serving of which kept off nearly a pound over four years.
The findings could have significant political, economic and policy implications, supporting, for example, growing pressure to levy taxes and take other steps to discourage certain menu options, such as sugary soda for kids.
For the study, Mozaffarian and his colleagues analysed data collected from a total of 120,877 healthy American men and women.
The study has been published in the New England Journal of Medicine.