But this week's research shows, for the first time, that they may also help us to maintain a healthy weight, as well as boosting our sex lives.
"We've known for a long time that a diet rich in fruit and vegetables has many health benefits," says Helen Bond, a dietitian and spokesperson for the British Dietetic Association. "Emerging evidence is suggesting that flavonoids may be responsible."
Researchers from Harvard School of Public Health and the University of East Anglia kept track of 124,000 people across the U.S. over 24 years. They were drawn from three groups with average ages of 36, 47 and 48.
Across the groups, men gained on average 2.2lb in weight every four years, and women 4.8lb.
But those who ate the highest number of flavonoid-rich foods tended to maintain a healthy weight, or even showed a modest weight loss.
The flavonoids with the most benefits were a group called anthocyanins - found in grapes, blueberries, strawberries, cherries, blackberries, radishes and blackcurrants.
Other types of flavonoids that had effects on weight maintenance and loss were flavonoid polymers - large antioxidant molecules found in tea, apples, plums and berries - and flavonols, chemicals similar to flavonoids, found in tea and foods such as onions and apples.
The results held true even after the researchers adjusted for factors such as how much exercise the participants did, and other aspects of their diet, including how much fat and fibre they ate.
"That suggests there is something specific about flavonoids," says author Aedin Cassidy, professor of nutrition at the University of East Anglia's Norwich Medical School.
"Rather than being the latest diet fad, this is about maintaining your weight. When people enter middle age, they tend to gain weight - it's just a fact of life.
"And we know that, at this age, weight gain raises the risk of heart disease and diabetes. This is about preventing that weight gain from happening, which is much better than putting it on then trying to lose it."
And better still, the flavonoids in berries, grapes and wine won't just keep you slim - they may also help your sex life.
Earlier this month, another study by Harvard and the University of East Anglia found middle-aged men who regularly ate flavonoid-rich foods were less likely to develop erectile dysfunction.
Anthocyanins, found in red and blue fruits such as berries, and flavanones and flavones, which are found in citrus fruit - were shown to have the greatest effect.
Researchers said a flavonoid-rich diet was as good for protecting against erectile dysfunction as briskly walking for five hours a week.
Even better, just a few portions of flavonoids a week were enough to have a positive effect on weight gain. Every tiny increase of daily intake - just 10mg of anthocyanins, or a tenth of a portion of blueberries - was associated with 70-100g less weight gained over four years.
"It's easy to look at the results and see that they're really tiny changes in weight," says lead author Monica Bertoia, of the Harvard School of Public Health. "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."
Professor Cassidy, who eats two portions of berries or grapes a day, says: "We should probably try to incorporate a few portions a week - ideally one a day."
She believes upping our flavonoid intake could be a valuable and easy way to prevent obesity.
"We're all told to eat five a day, but this new evidence suggests some fruit and vegetables might be better than others, particularly in terms of weight maintenance."
So, how might flavonoids help us to slim? Experts believe the compounds may help by reducing the amount of fat the body absorbs from food, reducing appetite, or raising our metabolism. "There have been studies where mice were fed a high-fat diet then fed flavonoids on top of that, and it seemed to prevent the increase in body weight that you'd expect," says Professor Cassidy.
Another area of focus is flavonoids' effect on gut bacteria - they may alter the type of microbes growing in our gut, meaning our food is metabolised differently and we gain less weight.
However encouraging the new research into flavonoids is, Professor Cassidy stresses that they aren't a magic bullet: no amount of blueberries will help you lose weight if you're also scoffing biscuits, cakes and cheese all day.
"We know fruit and veg are good for you and now it looks like you get a double benefit: you're lowering your risk of heart disease and helping to keep a healthy weight, too."
It takes three hours of sex to work off a slice of carrot cake
Step away from the biscuit barrel. It takes 24 minutes of running just to burn off three custard creams, or 27 minutes of tennis to work off two Hobnobs.
Although some forms of exercise are more appealing than others. Incredibly, it takes three hours and seven minutes of love-making to work off a slice of carrot cake.
This week, the Royal Society for Public Health called for "activity equivalent" calorie scores on food packaging to show how much exercise would be required to burn them off.
Although calorie burn varies according to gender, age, weight and metabolism, these figures are based on estimates that brisk walking burns 4.4 cals per minute, running burns 7.1, tennis 5 and sex 3.1 calories.
- Daily Mail
