For many years it has been widely accepted that a diet rich in fruit and vegetables is healthy and can protect against illnesses like heart disease and reduce the risk of cancer.

As a result of this scientists have worked hard to look for chemicals in plants which might be the reason for these benefits. So far several thousand different plant chemicals (or phytochemicals) have been discovered although only a hundred or so of these have been studied in any detail.

Phytochemicals which might have an effect in reducing the chances of cancer developing fall into several groups. These include:

n antioxidants: these work by removing harmful chemicals, called free radicals, from the body. Free radicals can damage the DNA in the cell's nucleus and this might lead to cancer formation. Antioxidants are found in vegetables like broccoli, cauliflower, brussels sprouts and cabbage. Lycopene is an antioxidant found in tomatoes, red peppers and red grapefruit and has been claimed to protect against prostate and lung cancer in particular.

• carotenoids: these cause the orange colour in a number of fruits and vegetables, like carrots and apricots.
  
  
• anthocyanins: found in grapes, aubergines and red cabbage.
  
  
• sulfides: found in garlic and onions.
  
  
• lutein & zeaxanthin: found in spinach
  
  
• flavonoids: these are actually a subgroup of a larger group of phytochemicals called polyphenols. They are found in a wide range of fruits, cereals and vegetables. Some flavonoids (found in soy beans, chickpeas and tea) have a similar action to the female hormone oestrogen and are called phytoestrogens.

Phytochemicals are present in almost all the fruit and vegetables we normally eat. Most of these will contain a mixture of compounds, for example it has been estimated that carrots contain over 100 different phytochemicals.

This means that eating a balanced diet rich in a range of fruit and vegetables should provide all the phytochemicals needed to promote health and minimise the risk of cancer. Although many phytochemicals are available as supplements in various forms there is no evidence that these have any additional benefit over eating the natural fruit and vegetables from which they come, indeed they are probably less effective.