Although many people think raw vegetables offer the best protection against heart disease and cancer, this simply is not true, they said.

Now a Europe-wide study has shown that the body can absorb more of an important substance from cooked vegetables than from raw ones.

The research suggests that cooking can improve the performance of carrots, broccoli and spinach when it comes to protecting health.

However, nutrition specialists say there is no point in people focussing on how to eat vegetables until they are eating enough of them in the first place.

Cancer prevention

The active ingredients studied in this research were carotenoids. They are one of a group of chemicals known as antioxidants that are thought to protect against cancer.

Dr Sue Southon, from the Institute of Food Research in Norwich, led the research, which is reported in New Scientist magazine.

She said that while the gut could absorb between three and four per cent of the carotenoids in raw carrots, that could increase by up to five times if the carrots were cooked and mashed.

"One of the problems with getting carotenoids into your body is the structure of the food, particularly the tough-walled cells like those in carrots. Cooking helps to release them," Dr Southon told the magazine.

The team also found that it was easier for the body to absorb vitamins from vegetables than from supplements.

The scientists - from the UK, the Netherlands Spain, Ireland and France - conducted their study using an artificial gut.

Producing guidelines

Interim results from the study were presented to the European Commission last month.

The researchers hope in future to work out precise amounts of carotenoids absorbed from food prepared in different ways so they get set recommended daily amounts.

But Sarah Schenker, a nutrition scientist at the British Nutrition Foundation, said people did not yet eat the recommended five portions of fruit and vegetables per day, and so there was little point getting too concerned about how food was prepared.

She told BBC News Online: "It's a common misconception that raw vegetables are better for you, and perhaps this study will help rectify that, but it is far more important that people eat a variety of fruit and vegetables.