An additive that promises to transform products from high- to low-fat sounds like the Holy Grail of food technology. But is fat itself really so sinful? Jerome Burne reports
It is a dieter's favourite fantasy. Puddings and fry-up that aren't either drenched in calorie-packed fats or taste like aerated cardboard; a way to get the rich creamy taste of fats without the associated health and waistline penalties. It's a dream with a price tag of $200m - that's what the food and household products giant Procter and Gamble has spent on trying to transform it into a reality and this month it looks as though their American division might be about to succeed.

However there are those who believe that if they do, far from ushering in dieter's heaven, this could be the beginning of a nightmare. Not only that, but our current fat-phobia has led to widespread and unhealthy confusion about what it is and is not safe to eat. Far from promoting health there is plenty of evidence that low-fat foods just make matters worse.

The fat-vanishing trick that P&G hope to pull off is performed by an oil called Olestra which has had its molecules tinkered with so that it passes straight through the gut leaving not a calorie behind. But because, unlike other low-fat products, it actually contains fat molecules, it has the all-important "mouthfeel" of fat and it can also be heated to the high temperatures needed for frying without breaking down.

After years of deliberation, the American Food and Drug Administration looks set to grant it an initial licence to be used in savoury snacks - a prospect that fills many nutrition experts, including Myra Karstadt of Ralph Nader's Centre for Science in the Public Interest, with alarm. Earlier this month she took the unusual step of sending documents explaining precisely why to the Ministry of Agriculture, which is considering it for approval here, and to British food campaigning groups. "It would be a huge uncontrolled experiment with public health. It is just not worth the risk," she says.

Her concern centres on the way the fat-like Olestra absorbs fat-soluble vitamins and nutrients and then rushes them out of the body. Carotenoids, the nutrients found in vegetables that many researchers now believe boost the immune system and protect against some cancers, are especially vulnerable. "A third of a bag of Olestra-cooked potato chips could leach out 40 per cent of your lycopene, which is used by the prostate gland," says Karstadt.

But the real interest of the battle over Olestra is that it raises the whole issue of fat in our diet and whether the lucrative commercial attempts to reduce it aren't a huge mistake.

In food engineering terms, Olestra is a genuine breakthrough. Reducing fat and keeping food palatable is a tricky business. Most of the low-fat products that currently beckon to us from the supermarket shelves are produced by tinkering with existing foodstuffs. Something called Simplesse, found in salad-dressings and cakes, consists of concentrated whey protein chopped up into microscopic balls. McDonald's, for instance, uses an extract of red algae in their reduced-fat hamburgers. But none of them is terribly convincing.

This doesn't stop the food engineers from trying. Waiting in the wings for a licence are such delights as Salatrim and Caprenin, both poorly absorbed by the body, so 40 per cent less in calories, but both high in harmful saturated fats.

The trouble is that we have no way of knowing what these chemical marvels are doing to us. Karstadt's point about artificial foods constituting a public health experiment is echoed by the eminent nutritionist Joan Dyegussow who sits on the Food and Nutrition Board of the National Academy of Sciences.

"We don't even fully understand what fibre does and we have been researching it since the beginning of the century," she says. "For instance, we think oat bran helps to reduce cholesterol because it has soluble fibre. But rice bran is just as effective and it has no soluble fibre. So what hope have we of predicting the effect of foods that have only been around for a few years?"

Not only that, but when it comes to helping weight loss, low-fat foods don't seem to do the business. Professor Barbara J Rolls of the Johns Hopkins University School of Medicine gave healthy volunteers either a high-fat or a low-fat lunch, without revealing which was which. They were free to eat whatever they wanted the rest of the day. On low-fat days, the subjects had made up the calorie difference by dinner time.

When dieters consciously have something low in fat they feel free to have a little treat later. In one recent study, dieters who knew that they had eaten a low-fat yoghurt ate more at lunch than those who thought it was an ordinary one. In fact, as researchers at the Chemical Senses Centre in Philadelphia found, the problem with fat-substitutes is that people who eat them still want fat, so as soon as the diet has stopped or their guard is down it is the full-fat things they turn to. On the other hand, people who cut out both high- and low-fat substitute foods and go for naturally lower fat foods such as bread, vegetables and fruit, reduce their craving.

But does fat deserve its unhealthy reputation? Certainly children, major consumers of savoury snacks such as crisps, need it. A recent issue of Nutrition Today warned that avoiding giving certain foods to children because they had a high fat content could lead to "nutritional deficits". What's more, a recent study at McGill University in Montreal, Canada, found that reducing saturated fat in the diet to recommended levels may add no more than three extra months to life, and possibly as little as an extra 3.5 days.

A venerable fat heretic in this country is Dr Malcolm Caruthers who believes the interaction between stress, hormones and cholesterol is far more influential in determining blood-fat levels. "It's not so much a question of what you are eating," he says, "but what's eating you." Yet another perspective comes from Dr Artemis Simopoulos, director of the Centre for Genetics, Nutrition and Health in Washington.

She has amassed evidence to show that it is your genes that determine whether you need to worry about fat. "If you come from a family with no history of major diseases you should feel comfortable about eating anything you fancy. There is no reason to avoid eggs, butter or red meat, say, providing that you don't become overweight."