They tell us to eat it, they tell us to stop. As fibre becomes the latest foodstuff to lose its halo, Jeremy Laurance chews over the `facts'
The F-Plan Diet is one of the best-selling diet books of all time. Audrey Eyton's simple message that has moved 4 million copies off bookseller's shelves since it was published in 1982 was that eating high-fibre foods could keep you healthy and slim. But critics were never sure about the slimming effects of fibre and now, it seems, we cannot be sure of the health-giving effects, either.

Fibre has been a totem of the health establishment for almost three decades but a report published last week rocked that belief to its foundations. One of the world's largest studies, of 88,000 American women, found that the raison d'etre of most breakfast cereal did not protect against bowel cancer. Instead, it suggested, it is the presence of sugar in the diet, rather than the absence of fibre, that could be the culprit.

It was a grim blow for medical and dietary organisations, which have been urging consumers to eat more lentils and wholemeal bread as protection against the chronic diseases of modern living. Only last year the Cancer Research Campaign, one of Britain's largest cancer charities joined forces with Kelloggs, makers of All-Bran, to promote high-fibre breakfast cereals in a deal worth pounds 1m over three years.

But this is only the latest setback for the diet and health industry. A glance at the history of the last few decades shows that on fat, alcohol, sugar, vitamins, fruit and vegetables and now fibre, advice has changed or research has challenged the prevailing wisdom. This leaves consumers bemused, confused and increasingly sceptical of all health advice. Time perhaps to make up your own mind...

FIBRE

The indigestible, cellular material of most plants has been a key element of the healthy diet since Dr Denis Burkitt observed in 1972 that bowel cancer was almost unknown in Africa, where vegetables and grains are the staple diet. Fibre acts like a colonic broom, sweeping the food through the gut more quickly and diluting toxic chemicals that build up there.

The US researchers at Harvard Medical School, who found 787 women with bowel cancer but no link with their consumption of fibre during the 16 years of their study, offer two possible explanations. Either sugar could be the cause - other studies have consistently suggested a link - or different types of fibre, such as that from vegetables or from grains, may have different effects.

Don't, however, throw out the muesli and the All-Bran just yet. This is just one study, albeit a major one, among many others which suggest fibre does help prevent bowel cancer. Fibre also offers protection against heart disease, high blood pressure and diabetes - according to existing research - as well as helping to banish constipation.

FAT

"Cut fat to reduce heart disease" has been repeated like a mantra by doctors for even longer than they have been telling us to increase our fibre intake. But the advice took a nasty knock last year with the publication of the world's largest and longest study of heart disease, which found no link between the two.

The World Health Organistion's "Monica" project compared the health records of 38 populations in 21 countries over the last two decades and examined the records of 150,000 heart attacks. What it found is that heart disease is declining everywhere, but at very different rates in different countries. This cannot be explained by changes in the traditional risk factors of smoking, blood pressure and cholesterol.

We may eat too much fat but statistics fail to prove it. Although hundreds of studies have shown that cutting fat saves lives in people with existing heart disease or raised cholesterol, there is no hard evidence - from a controlled trial - that reducing intake benefits the rest of us who are averagely healthy. The areas with the highest heart deaths - Scotland and the North - do not eat a fattier diet than the rest of us. The rich eat more fat, and have higher cholesterol levels, but suffer less heart disease than the poor.

SUGAR

In the 1970s it was regarded as the fount of all dietary evil. An influential book, Pure, White and Deadly by John Yudkin, the former professor of nutrition at London University, cast it as the cause of heart disease, diabetes and breast cancer. It was later accused of providing "empty calories" - energy without true nutritional value.

Its rehabilitation began in the late 1980s when the Committee on Aspects of Food Policy, the Government's nutritional advisory committee, drew a distinction between intrinsic sugars found in fruit and vegetables, which it said were healthier than extrinsic sugars taken by the spoonful from the bowl.

The process was completed last year when an expert international committee assembled by the World Health Organisation and the Food and Agriculture Organisation of the United Nations concluded that sugar was good because it provides a rapid source of energy which, when combined with a high carbohydrate diet and regular exercise, stops people eating too much other food. Even on tooth decay it said fluoridation and regular brushing were more important than cutting sugar consumption. Its rehabilitation could be short-lived, however, if the latest suggested link with bowel cancer gains research support.

ALCOHOL

As one of the most popular drugs the world has known, alcohol was always regarded as too pleasurable to be healthy. In the 1980s red wine fell under the spotlight as the possible explanation of the French paradox - the fact that Mediterranean dwellers ate a diet rich in fats and oils but had low rates of heart disease. Perhaps, researchers argued, it was a constituent of red wine that was protective.

Several major studies have now demonstrated that the protective constituent of red wine is the same as that in white wine, beer and spirits - it is the alcohol itself. Two or three alcoholic drinks a day cuts the risk of heart attack in middle-aged men by at least 30 per cent. Moderate drinkers live longer and suffer less ill health than teetotallers.

VITAMINS

They are regarded as a modern panacea, a defence against the debilitating demands of modern life and the toxic effects of the environment. Vitamins were necessary when diets were deficient - of fruit and vegetables, for example - but today most people eating a balanced diet do not need them. Now research has shown that high doses can themselves be toxic.

Major studies of beta-carotene, an antioxidant found in carrots, green vegetables and fruit, and vitamin A have shown that far from protecting men against cancer, as had been thought, they increased the incidence of the disease. An American trial, called CARET was stopped early after the group taking the pills were found to have 28 per cent more cancers and a 17 per cent higher death rate.

Even Vitamin C, mega doses of which are taken by thousands of people to ward off colds, has come under suspicion since a small study by researchers at Leicester University published in Nature last year suggested it could increase oxidative damage increasing the risk of heart disease. The implication is that vitamins are not a magic bullet and cannot provide the health benefits of the food from which they are extracted.

THE HEALTHY DIET

It is in the nature of scientific research that from time to time contradictory findings will be thrown up. If all research consistently reached the same conclusion there would be no need to do the studies. But it is alarming when a major study contradicts a hallowed principle of healthy living - such as that fibre protects against bowel cancer.

One one matter, however, advice over the last two decades has been consistent and growing - the need to eat more fruit and vegetables. An apple a day may not be enough to keep the doctor at bay - but on current advice five portions of fruit and veg will.

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