Frances Sutton, of Karori, asks :-

Nutritionists often stress the importance of including green and leafy vegetables in your diet. Do broccoli, green bean pods and green peppers have the same nutritional value? If not, how do they differ?

Murray Skeaff, a human nutritionalist at the University of Otago, responded.

What nutritionists stress most is to eat plenty of fruit and vegetables. Our conviction that this is an important message is not based on the type or amounts of nutrients in these foods, however important this may be, but on large surveys of the population which show that people who eat plenty of fruit and vegetables – all things equal – are generally healthy and live longer than those who don't eat their fruit and veg.

Of course, we are interested to know if some types of fruit or vegetables are healthier than others, and we suspect that green and leafy vegetables are fairly good, but the healthiest people eat a wide range of different types of fruit and vegetables not just the same two or three types. So the main message for vegetables is to eat plenty of them, including different types and colours.

But what is it in vegetables that makes us healthy? Nutritionists have traditionally considered that it was the vitamins and minerals in vegetables, and ranked the nutritional value of different vegetables according to the amounts of vitamins and minerals in them. Had I answered your question 20 years ago, this is how I would have compared the nutritional value of broccoli, green bean pods, and green peppers. However, we now know that vegetables, as well as fruit, contain an incredible number of different molecules – called phytochemicals – that affect metabolic processes in the body in a way that helps prevent cancer and heart disease. At this stage our knowledge of how all this works is not advanced, but everything points to a complex balance of phytochemicals, vitamins and minerals.

Human curiosity, being what it is, drives the conviction that there must be "superfoods" or a few powerful phytochemicals that we can package into a "magic" pill. However, the last twenty years of learning about nutrition and the health of people, has convinced us that the benefits of vegetables comes from eating them, and a wide variety of them.