'Free radicals' form when oxygen (a highly reactive element) combines with other molecules. This produces high-energy chemical substances that attack cell membranes, proteins and even DNA, as they seek out something to combine with causing irreversible damage.

Many degenerative diseases that are common in later life are likely to be exacerbated or even caused by free radical damage, including cancer; vascular diseases; degenerative eye diseases; and possibly neuro-degenerative diseases like Parkinson's.

There are other sources of 'oxidative stress', including tobacco smoke and other pollutants, certain drugs and excess alcohol.

Antioxidants

The crucial antioxidant vitamins (A, C, E) and minerals (zinc, copper, manganese, selenium) in fruit and vegetables can stop free radicals in their tracks. Carotenoid vitamins are particularly powerful antioxidants. These include beta-carotene (found in carrots), lycopene, lutein and zeaxanthine.

We have other antioxidant defences, such as natural enzymes, but scientists have established that we need to consume antioxidants in large, regular doses. That's why the Department of Health and nutritionists advise us to eat five portions of fruit and vegetables every day.

Smokers use antioxidants faster and therefore need an even higher intake. Different fruits and vegetables offer different protection. For example, green leafy vegetables and vitamin C rich fruits and vegetables offer the greatest protection against heart disease.

Eating foods rich in lutein - such as sweetcorn, spinach, courgettes, green peppers, cucumbers, red grapes and kiwis - may protect against cataract and macular degeneration, as may zeaxanthine, which comes from everything orange -peppers, corn, oranges and mangoes.

Lycopene appears to protect against heart disease and prostate cancer, and a particularly rich source is tomatoes - if they are cooked. So you really can tuck into tomato ketchup and tomato sauces knowing they are health foods!

Fruit and vegetables have many other important jobs to do, including lowering our blood pressure, because the potassium they contain increases sodium (salt) excretion. Potassium is found in many fruits and vegetables including bananas, citrus fruits, raisins and other dried fruits, potatoes and avocados.