Ask anyone about the secret to a healthy diet and the odds are they'll tell you to eat lots of fruits and vegetables. It's one nutritional message that seems to have been generally accepted. In the 1994-5 Asian Urban Adult Study, respondents claimed that the main change they had made to their diet over the last two years was that they were eating more fruits and vegetables. And with good reason. Fruits and vegetables are packed with vitamins and minerals and provide valuable levels of dietary fibre.

But these nutritional benefits are just the tip of the iceberg. Fruits and vegetables also contain a host of other plant nutrients (phytonutrients), that are thought to offer even more health benefits.

Understanding the role these phytonutrients play in plants gives us a clue as to how they may help humans. Phytonutrients help to protect plants in different ways such as protecting against pests or solar radiation. They add flavour, fragrance and colour to foods. The red colour of a watermelon and the citrus smell of a lemon are due to phytonutrients. Compounds in garlic and onions that give their characteristic smell are present to help protect from bacterial and viral infections.

Plants have developed literally hundreds of thousands of phytonutrients that help them fight everything from fungal and bacterial infestation to the stresses and strains of the environment. The theory is that as we consume these plant foods, we may assume some of these protective benefits secondhand.

Many of the phytonutrients that scientists have discovered have antioxidant properties. By helping to fight free radicals (chemicals that are believed to trigger changes in cells) antioxidants are thought to help in the prevention of heart disease, cancer, cataracts and aging.

Researchers were first alerted to the possible effects of phytonutrients; by examining disease patterns around the world. Such work led them to believe that certain diets may contain something that helped to explain differences in disease patterns. For example, Japanese women have lower rates of breast cancer than women in other parts of the world. One of the major differences between the Japanese diet and many Western diets is the presence of high levels of soy products in the typical Japanese diet. Soy beans have been found to contain high levels of phytoestrogens, oestrogen-like compounds that are believed to block the action of human oestrogen in the body, thereby lowering the risk of breast cancer.

The good news is that there are many foods enjoyed all over Asia that are packed with protective phytonutrients, giving many good reasons to enjoy the benefits of fruits and vegetables.