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Danny Kim for TIME

It may not feel like news when scientists uncover more evidence that fruits and vegetables are healthy. But Americans seem to need every shred of science to convince them to eat them. Despite reams of research backing up a plant-heavy diet for overall health and for a healthy weight, fewer than one in five adults eat enough of them, according to recent federal data.

In a study published today in The BMJ, researchers found that one little-understood group of compounds—flavonoids—might be partly responsible for the weight-loss power of produce. The researchers looked at up to 24 years’ worth of data from 124,000 people in the U.S. The people in the data set, who were ages 27 to 65, had reported their weight every two years and their diet every four years between 1986 and 2011.

The results revealed a relationship between diets high in flavonoids and less weight gain. The ones that had a biggest impact were anthocyanins, found in dark red foods like blueberries, cherries, grapes and strawberries, and flavonoid polymers, found in tea and apples.

Previously, the researchers had found that some foods, including blueberries, apples, pears, prunes, strawberries, grapes, peppers and celery, were linked to lower weight gain. “We wanted to better understand why those particular fruits and vegetables stuck out,” says Monica Bertoia, research associate at Harvard T.H Chan School of Public Health and lead author on the paper.

Fiber and glycemic load—how much a food raises blood sugar when consumed—didn’t explain the association, so they focused on flavonoids, natural compounds found in fruits and vegetables that seem to have a protective effect on their host plants. Past animal studies have suggested that flavonoids in whole foods might decrease the absorption of fat and may increase calorie expenditure.

Every extra daily standard deviation—a unit that varied by produce type—of flavonoids was associated with 0.16 to 0.23 pounds less weight gained over four years. That might sound small, but in the study, one serving of a fruit often provided more than one standard deviation of a class of flavonoids, Bertoia says. For example, just half cup of blueberries gives you about 12 standard deviations of anthocyanins; half a cup of blackberries gives you about seven. In other words, you don’t have to eat a bucket of blueberries to get the impact.

“It’s easy to look at the results and see that they’re really tiny changes in weight,” Bertoia says. “But weight maintenance is really important. Just maintaining weight from adulthood onward would have a significant public health impact, because most people are gaining weight.”

The authors hope the findings will help refine dietary recommendations for obesity prevention by pointing eaters toward produce with the most flavonoids, like apples, berries and pears—which may also have an enhanced weight loss effect.

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