So one of the major reasons for the common advice for everyone to eat a diet rich in fresh fruit and vegetables is that such diets will also of course be rich in these phytochemicals, the plant anti-oxidants that appear to have a tremendously positive effect on health. Numerous studies have reported the benefits of such a diet in fighting cardiovascular disease, cancer, cognitive degeneration and diabetes ? the characteristic diseases, in fact, of Western affluence. But the science remains in its infancy, so although thousands of phytochemicals are already known about, this is probably just the beginning.

For example, the class of phytochemicals known as carotenoids alone numbers more than 600. These are the highly coloured molecules formed naturally by plants, which lend them their characteristic yellow, orange or red pigments.

Only a few of these carotenoids have so far been the subject of serious research, but it's known that in plants the carotenoids function as anti-oxidants in neutralising the so-called "singlet oxygen" molecule, which forms during photosynthesis. It's less clear, however, whether this particular kind of reaction is significant for animal or human health.

What is clear, however, is that certain of the carotenoids ? notably alpha and beta-carotene and beta-cryptoxanthin are significant as anti-oxidants because they are readily synthesised by the body into vitamin A. The most important of these "pro-vitamin A" carotenoids are beta and alpha-carotene.

Certain types of food such as pumpkins, raw, cooked or canned carrots and winter squash are rich in both alpha and beta-carotene (as well as the other carotenoids), so eat these and you will be getting both.

The principal differences are that beta-carotene is present in greater quantities and that the body converts about twice as much of the beta-carotene it absorbs into vitamin A when compared with alpha-carotene, the relevant figures being 40% and 20% respectively. A given quantity of any of these foods will typically contain only between a quarter and two-thirds as much alpha as beta-carotene. For these reasons only the figures for beta-carotene are given here.

So an 8 fl oz can of carrots will typically contain around 21,955 mcg of beta-carotene, canned pumpkin 17,000, a cup of cooked carrots around 13,000. Beta-carotene is also present in a wider variety of foods such as turnip green 10,593 mcg and kale 11, 470 mcg per cup respectively.

In both cases the anti-oxidant effects of the vitamin A produced are enhanced by the unconverted alpha and beta-carotene which continue to act as anti-oxidants in their own right, and being highly fat soluble are particularly valuable in protecting the delicate fatty structures of the body's cells such as the membranes.

Along with other fat soluble anti-oxidants, alpha and beta-carotene also appear to assist in preventing the oxidation of the low density blood lipids (LDL), the so-called "bad" cholesterol which is widely believed to be a major precursor of cardiovascular disease.

Perhaps surprisingly, the effective absorption of beta-carotene particularly depends on the presence of this "bad" cholesterol. But with the obvious exception of artificially introduced toxins, if a substance is found in the body it's there for a good reason. So in fact this is a very good example of the body's holistic and balanced functioning, because although you need LDL to absorb and carry beta-carotene around the body, that beta-carotene will itself help to raise levels of High Density Lipids (HDL), or so-called "good" cholesterol.

In addition to this role in combatting cardiovascular problems, many research studies, have highlighted the potential of beta-carotene as a weapon against cancer, probably because of its general anti-oxidant function, but also because of the part it plays in the biochemical communication between cells. This is vital if the immune system is to be able to kill off cancer cells in the early stages of their development and before the disease takes a hold.

Conventional medicine recognises the benefits of a diet rich in carotenoids, but tends to be sceptical that the very high supplementary doses advocated by many alternative practitioners will provide any significant advantage. As so often, it is a question of balancing the risks.

But where a substance appears to be almost entirely benign in any realistically conceivable quantity of intake, and may offer very substantial health benefits at little expense, the question must be: why wouldn't you take it? .