There are a wide range of phytochemicals or plant compounds in fruit and vegetables that may have beneficial effects on health

British Nutrition Foundation spokesman

In a separate experiment, researchers looked at lengths of artery removed during surgery, and found that artery walls pre-treated with lutein were less likely to experience a chemical reaction which is thought to contribute to hardening.

The higher the dose of lutein, the smaller the chemical reaction was.

The researchers suggested that lutein may form part of the reason why people with a high consumption of fruit and vegetables are less likely to develop heart disease.

Good sources of the pigment include broccoli, lettuce, cucumber, peas, asparagus, avocado, beans, Brussel sprouts, cabbage, celery, spinach and watercress.

However, the study failed to find any beneficial effects of beta-carotene, another nutrient which has been suggested as preventing or slowing the progress of artery hardening.

A spokesman for the British Nutrition Foundation said that the link discovered by the research team was interesting.

Egg yolks are a source of lutein

She said: "Most evidence for health effects of plant compounds such as lutein has been obtained from laboratory techniques or using experimental animal models.

"There are a wide range of phytochemicals or plant compounds in fruit and vegetables that may have beneficial effects on health and is likely that the complex mixture of these compounds, rather than any one nutrient, contributes to their protection against heart disease and cancer."

"This emphasises the need for a healthy balanced diet and supports the recommendation of five or more portions of fruit and vegetables every day."