Every other week, there's another revelation about the health-giving properties of a common foodstuff. If only we'd known sooner, we think, and shovel in another mouthful of broccoli, olive oil or dark chocolate. Is it already too late to start?

"Super" foods come in pretty disparate incarnations. Blueberries, green tea, chocolate and fatty fish have all been touted as amazing sources of this, that or the other beneficial goodie.

The magic is usually identified as a single component that fights symptoms of ageing and the causes of degenerative diseases, such as heart disease and cancer. In recent years there has been publicity over:

• quercetin contained in red onions, apples, red wine and black tea
• flavonoids in chocolate
• antioxidants in blueberries and grapes
• lycopene in tomatoes and watermelon
• isothiocyanates in broccoli and brussels sprouts.

Concentrated supplements

The media reports that follow these scientific discoveries often declare the particular food has the highest levels of the new magic ingredient ever found. In the blink of an eye, that ingredient appears concentrated in a supplement with advertising claims promising all but immortality. The reality is rather less exciting.

Lab tests can identify each component in food and even get them reacting with human cells in a test tube, but the problem is linking those results to real life.

Most of the evidence so far has been gathered from observations of groups of people linking diet and lifestyle with health. Absolute proof of the links is difficult to establish. Scientists can't spend years feeding potentially harmful diets to people, just to see how it affects them.

And there are always complicating factors: how do you decide on the relevance of other lifestyle and environmental factors, as well as genetic influences?