Eating berries, grapes and other fruits daily can convert excess white fat into calorie-burning "beige" fat to prevent weight gain, scientists have found.

In the study, Washington State University scientists fed mice a high fat diet. Those receiving resveratrol in amounts equivalent to 12 ounces of fruit per day for humans - about two or three servings daily - gained 40 per cent less weight than control mice.

Resveratrol is a polyphenol, a type of antioxidant found in most fruits.

Previous studies have suggested that resveratrol can help prevent obesity but how it did that was unclear. Most of the research, including highly publicised studies on wine, also used very large concentrations of resveratrol, much more than a human could consume in a normal diet.

Professor of animal sciences Min Du and visiting scientist Songbo Wang demonstrated that mice fed a diet containing 0.1 per cent resveratrol were able to change their excess white fat into the active, energy-burning beige fat.

"Polyphenols in fruit, including resveratrol, increase gene expression that enhances the oxidation of dietary fats so the body won't be overloaded," said Du.

"They convert white fat into beige fat which burns lipids off as heat - helping to keep the body in balance and prevent obesity and metabolic dysfunction," Du said.

The researchers also showed that an enzyme called AMPK, which regulates the body's energy metabolism, stimulates this transition of white fat into beige fat.

Du said resveratrol is only one of the polyphenolic compounds found in fruit that provides beneficial health effects.

Du said those compounds are high in all fruits but especially rich in blueberries, strawberries, raspberries, grapes and apples.

Wines like merlot or cabernet sauvignon, in contrast, contain only a fraction of resveratrol and other phenolic compounds found in grapes, he said.

"Many of the beneficial polyphenols are insoluble and get filtered out during the wine production process," he said.

The study was published in the International Journal of Obesity.