People looking to lose weight should eat more fruit and veg, scientists have suggested.
Fruit and vegetables that have high levels of flavonoids - such as apples, pears, blueberries and radishes - could prevent weight gain.
Flavanoids are plant compounds which are thought to have antioxidant and anti-inflammatory health benefits.
Scientists believe that consuming just 80g of fruit or veg per day could help to maintain a healthy weight and could even lead to "moderate" weight loss.
Researchers from Harvard University and the University of East Anglia (UEA) examined data from 124,086 men and women in the US over a period of 24 years.
The study focused on three groups of people - women with an average age of 36, women aged 48, and men with an average age of 47.
They found that those who ate a few portions of flavonoid-rich fruits and vegetables every week maintained a healthy weight, and even lost a little.
Foods rich in flavanoids
Apples
Oranges
Blueberries
Bananas
Grapefruit
Strawberries
Lemons
Peaches
Cherries
Pears
Watermelon
Plums
Cranberries
Raspberries
Onions
Lettuce
Tomatoes
Almonds
Parsley
Bell Peppers
Celery
Cabbage
Chili peppers
Sweet potatoes
Quinoa
Professor Aedin Cassidy, from UEA's Norwich Medical School, said: "This is the first large study to examine the associations between consumption of all flavonoids and weight gain in middle-aged and older adults.
"Most adults gain weight as they age and even small increases in weight can have a substantial impact on risk of high blood pressure, developing heart disease, cancer or diabetes - so strategies to help individuals maintain a healthy weight in middle-age are needed.
"We found that an increased consumption of most flavonoids was associated with weight maintenance, and even a modest weight loss. The results were found to be consistent across men and women, and different ages."
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"However losing even small amounts of weight, or preventing weight gain, can improve health and these modest effects were seen with a small, readily achievable increase in intake of many of these fruits.
"Just a single portion of some of these fruits per day would have an important impact on health at a population level."
He added that the most effective foods for maintaining a healthy weight were those containing anthocyanins. These include blueberries, strawberries, cherries, blackberries, grapes, radishes and blackcurrants.
"We hope that the results will help refine previous dietary recommendations and provide guidance on which specific fruits and vegetables to choose for preventing obesity and its potential consequences," said Professor Cassidy.
"Losing or preventing even small amounts of weight can reduce the risk of diabetes, cancer, hypertension and cardiovascular disease."
The study was published in the British Medical Journal.