It's because they lose some of their flavonoids - which gives them some of their taste - during storage.
For example, onions will lose about one-quarter to one-third of their original flavonoid content over six months, with most of the loss occurring in the first two weeks.
So, wherever possible, choose fresh, local, seasonal produce to get the most health benefits.
But cooking could kill them off
But it's not just storage that may have an impact on flavonoid content. They can also be can be lost through contact with water - in some cases, up to 80 per cent of some flavonoids can be lost into cooking water during the boiling of foods.
Loss of colour-rich flavonoids during boiling may often be seen in a dulling of the food's colours.
Avoid this by steaming instead, and serving vegetables al dente.
Flavonoids also can be damaged by heat, so your grilled veg will have fewer health-boosting benefits than steamed or raw.
Eat fruit and vegetables whole
Avoid cutting, slicing or peeling fruit and veg wherever possible - flavonoids are often concentrated in the skin and outer portions of fruits and vegetables.
Pick red grapes over white
Because flavonoids provide the colour in many foods, to get the best dose you should seek out those fruit and vegetables with the brightest, deepest colours - which means choosing red grapes over white.
Juices don't count - but frozen berries do
"There are much higher levels of flavonoids in the whole fruit than there are in the juice. That's because most of them are found in the skin or the pith, which are extracted when juiced," says Professor Cassidy.
"That's why you get more in a blueberry than a strawberry, for example - they're tiny little berries, so there's more skin for each one you eat."
So it's much better to eat it whole, rather than in a juice.
Frozen fruits are just as good a source as fresh, as the freezing process traps nutrients and prevents flavonoid loss.
And best of all . . . wine is fine!
Red wine is a good source of anthocyanins - the type of flavonols shown to have the greatest effect in protecting against weight gain.
Indeed, it's these compounds that give it its rich red colour.
A medium glass of red wine - 140ml - provides 28mg of anthocyanins, about the same as you'd get from half a cup of raspberries.
But any more than a glass a day and the effects are outweighed by the harmful effects of alcohol, says Professor Cassidy. "Red wine does contain good amounts of flavonoids, but moderate intake is best."
- Daily Mail
