Vitamin A is one of nature's most valuable anti-oxidants, and is also vital for preventing and fighting infection. Well known animal sources are foods such as egg yolks, milk and liver, along with the traditionally reviled fish liver oils.

Plant sources, however, will be more palatable for many, and may also have the advantage of providing additional anti-oxidant activity.

"Carotenoids" is the generic term used to describe a large range of more than 600 pigments synthesized by plants. In modern Western diets those most frequently encountered are alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin, and lycopene.

Much attention has been focussed on alpha-carotene and beta-carotene as these can be synthesised by the body to form vitamin A, whereas this is not true of lutein, zeaxanthin, and lycopene.

Alpha and beta-carotene from plant foods are readily synthesised into vitamin A by the body. But only about 20% of dietary alpha-carotene will be used in this way, the remainder will function as a highly effective anti-oxidant in its own right, particularly within the fatty membranes of the body's cells. The same is true of the better known beta-carotene, except that this is much more active in terms of vitamin A provision, around 40% of dietary intake being converted into vitamin A. Surplus beta-carotene functions as an anti-oxidant in its own right, and also provides a significant boost for the immune system.

Both alpha and beta-carotene are highly fat-soluble anti-oxidants and therefore particularly effective in protecting against free radical damage to the vital fatty structures of the body's cells, such as the membranes.

Research also suggests that high levels of carotenoids in the blood may help prevent the oxidation of Low Density Lipids (LDLs), the so-called "bad cholesterol", which is now heavily implicated in the development of the atherosclerosis, or hardening of the arteries which is a common precursor of cardio-vascular disease such as heart attack and stroke. One study of 5,000 middle aged and elderly adults showed a reduced heart attack risk of between 45-55% for people following a high beta-carotene diet, the higher figure being in respect of smokers within the group.

Numerous studies, too, have demonstrated the potential of beta-carotene as a weapon against cancer. This is probably because of its general anti-oxidant function, but also because of its proven role in keeping open the pathways between cells, which are vital to allow the immune system to kill off cancer cells in the early stages of the disease.

Conventional medical wisdom tends to accept the value of a diet rich in carotenoids but, confusingly, argues that the positive effects may be due to factors associated with such diets other than the carotenoids themselves. But alternative practitioners, of course, admit to no such doubts and are convinced of the benefits - particularly of the highly vitamin A active beta-carotene.

Whilst affirming the benefits of a diet naturally rich in carotenoids - ie in the kinds of fruits and vegetables detailed below, they insist that the maximum benefits of alpha and beta-carotene, the latter particularly, can only be secured through high dosage supplementation. Practitioners argue that studies discounting the effects of such supplementation are flawed because they have looked only at dosages too low to be effective.

And certainly the dosages they recommend appear massive - between 70,000 and 90,000 IU being proposed as the optimum to assist in the fight against both heart disease and cancer. To get this in perspective, these figures are equivalent to around 42,000 and 54,000 mcg of beta-carotene respectively, that's to say approximately 3 to 4 cups of cooked carrots (around 13,000 mcg each).

Now, clearly an awful lot of fruit and vegetables need to be consumed to achieve the kind of intake of beta-carotene that the alternative practitioners recommend. And it also needs to be noted that optimum absorption of carotenes depends on the presence of a reasonable amount of dietary fat. So chopping, pureeing, and cooking carotenoid rich vegetables in oil is the ideal way to maximise the bio-availability of these nutrients. But it's neither convenient nor appetising. And whilst an 8 fl oz glass of carrot juice provides around 22,000 mcg (37,000 IU) of beta-carotene, would you consider giving up your morning coffee in its favour? Me neither!

But apart from the convenience, another point in favour of supplementation is that it appears not to be harmful in any likely dosage. In fact the only side effect to have been noted even from doses as high as 250,000 IU has been a benign, if unflattering, "fake tan" skin pigmentation, which very quickly vanishes upon the reduction of the dose. So it appears that supplementation may do some good, is extremely unlikely to do any harm, but for best results should be taken as part of a normal daily diet already well supplied with carotenoid rich foods.