We are constantly exhorted to take up a particular diet, like the New Diet Revolution in the story above, to make us healthier. But how many supplements and diets have actually been proved to do what they are supposed to?

Some illnesses, of course, need special attention to food. Like diabetes, in which it's best to avoid sugar and fats and make the most of starches such as bread and potatoes. And there are rarer conditions, detected soon after birth, like phenylketonuria, in which foods containing a particular type of protein must be avoided to protect the brain from deteriorating in early childhood. A special diet is then a life-saver.

But for most normally healthy people, eating a wide variety of foods is the best option. Restricting what you eat won't make you healthier, and supplementing what you eat with extra vitamins, minerals or particular products won't protect you against illness. The logic is this: no normally healthy person eating a normal range of foods ever becomes vitamin or mineral deficient, so why take extra? Then you are taking them like a medicine, rather than a food. You don't need a medicine if you are already healthy.

What I have written is probably already annoying some readers. So to be fair, I've looked into the claims and the scientific proofs for the best-known supplements and diets.

As for supplements, many people take high doses of vitamin C or zinc tablets for colds. They can help cold sufferers feel better and may even shorten the time you have the cold. But we need not take them all winter to prevent colds. People taking vitamin C or zinc have as many colds as people taking placebo (dummy) tablets.

The vogue for giving high dose vitamin C to people with cancer has no scientific support. Trials have shown that cancer patients get no benefit from it. There's a fashion, too, for giving people with learning difficulties, including Down's syndrome, high doses of multivitamins. Sadly, trials have shown they do not improve their abilities. Nor do multivitamins help people with schizophrenia, shattering another popular belief.

On the other hand, evening primrose oil and oils from fish such as herring, mackerel, sardines, pilchards, trout and salmon may offer benefits. Good trials have shown that they reduce blood fat levels (and therefore help prevent heart attacks and strokes) and that they help some people with rheumatoid arthritis or inflammatory bowel diseases, such as Crohn's disease or ulcerative colitis. However, it's probably as effective to eat the fish itself two or three times a week, rather than take the oil in capsules or spoonfuls of medicine. They are a good alternative, though, if you can't stand fish.

Vitamin B6 is promoted for premenstrual syndrome (PMS). I have doubts about the only trial that showed it helped in PMS, but if women want to try it I wish them luck.

As for diets, I find it horrifying that anyone, let alone a top American actress, would follow a low-carb high-fat diet. Eating fat to lose weight is crazy because even if you do lose it, which is doubtful, you may still be driving fat into your blood vessel walls so that you may be the thinnest person with a coronary or a stroke. The idea is to keep healthy, not to be thin and dead.

John Kellogg had the right idea a century ago. When doctors said red meats were the best food, he challenged them by promoting cereals, fruit and fibre. He was right. Breakfast cereals are now accepted as healthy foods, provided they're not covered in sugar.

The newer 20th century 'health' diets don't carry as much conviction. The Hay diet recommends eating proteins (such as meats) and starches (such as bread and potatoes) at different times, because it is claimed we need different conditions in the stomach to digest them. Expert nutritionists doubt this. They know of no evidence that separating foods like this changes the way they are digested, and that's what matters. There are no good follow-up studies to show that people on the Hay diet are better physically or mentally than the rest of us.

At least Hay gives people all the foods that they need, and if they feel well on it, that's fine. But other diets, such as 'raw foods only', the Stone Age diet and 'macrobiotics', can cause harm.

Most raw vegetables are indigestible. We don't have the digestive systems to break down raw vegetable material: we are humans, not rabbits or cows. So eating only raw foods is nonsense. Eating raw eggs, meat and fish, which need to be cooked to kill any possible contamination from food-poisoning germs, can even be dangerous.

Stone Age dieters believe that the human digestive system has not yet adapted to the agricultural changes we brought in since our ancestors started farming - so we should not eat cultivated vegetables like grains, peas and beans. In complete contrast are macrobiotic fans, who eat mainly cereals and vegetables and drink very little water, and base their eating on the ancient oriental concepts yin and yang. A Chinese doctor friend, steeped in his country's philosophy, doesn't see the connection.

Vegans avoid all animal-based foods, including dairy products and eggs. I respect their beliefs. But they and macrobiotic adherents should not impose their decision on their children, who may become anaemic and have weaker bones than normal because of the diet.

Finally, because something is labelled 'vitamin' or 'mineral' doesn't mean it is safe. High doses of vitamin C can give diarrhoea, and high doses of vitamin B1 can cause flushing. High doses of vitamin A can cause birth defects and bone and liver damage. High doses of vitamin D cause stones in the kidneys and bladder. Too much zinc and selenium can depress the immune system. Even the much praised evening primrose oil can make a type of epilepsy (temporal lobe epilepsy) worse.

So if you are contemplating changing your diet or taking supplements, ask for the evidence that it will improve your health. If it isn't forthcoming, think again.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.