Calorie-free crisps and tasty vegetables will one day make society slimmer and food industry profits fatter. But are they safe? Aisling Irwin reports.

Picture the fat American, that stereotype in tartan trousers, with waistband stretched to the limits of its elasticity. Now picture him throwing his outsize garments into the bin and buying new versions that will clothe a thinner figure - a shape that is gradually forming as he reabsorbs the saggy flesh of his former self.

The transformation is possible because of biotechnology. In the future, scientists will have redesigned the food supply so that millions of obese shoppers will be able to buy foods that are good for them. They will pile their trolleys up with crisps cooked in no-calorie fat and vegetables engineered to taste irresistible.

The wrong kind of food is a principal culprit in most western illnesses: heart disease, diseases of obesity and many cancers. The affluent nations are eating too much fat, particularly saturated fat; too many calories; and insufficient fruit and vegetables. Telling them so does not seem to work. Doctoring their food supply might. Now there is a bewitching variety of ways in which our foods could be improved. Bacon can be made ultra-lean by artificially speeding up selective breeding of pigs; vegetables can be made more digestible, or higher in vitamins, with the flick of a genetic switch.

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Gerald Gaull, of the Georgetown Centre for Food and Nutrition Policy, says: "The role of food in the aetiology of chronic diseases is increasingly recognised by the public. The next generation of biotechnology products promises to respond to consumer demands for more healthy foods."

Scientists working on the food supply believe they can banish illness. Nicholas Frey, plant physiologist at Pioneer-Hi-Bred International, told a meeting on functional foods at the American Association for the Advancement of Science earlier this year about his genetic manipulation of soya bean plants. He says his plant could vastly reduce Americans' susceptibility to heart disease.

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Most soya oil is hydrogenated to make it more stable. The trouble is that hydrogenation is the same as saturation. The more saturated a fat, the more it raises cholesterol levels in the body. Frey has altered the soya plant so that the oil it produces is "naturally" stable while remaining unsaturated. He says that the American food industry uses 12 billion pounds of soya bean oil each year, of which half is partially hydrogenated. "800 million pounds of saturated fat could be removed from consumer diets with no change to eating habits - that's three pounds per person. We can achieve up to 50 per cent lower saturated fat without dietary change. The science is done," he says.

Similar excitement exudes from Ganesh Kishore, director of Ceregen, a Monsanto company, who also spoke at the American Association debate. He claims tomatoes can now be altered so that their salt content rises, thereby improving their flavour.

"You can literally make ketchup out of the extract. It increases the flavour of the tomatoes so that people will be eating tomatoes instead of cardboard." Eating more tomatoes equals eating more healthy food. There have also been breakthroughs in altering starches in potatoes and tomatoes to make them more digestible. Again, more goodness, this time because more passes through our gut walls. And to accompany our healthier vegetables - some very lean meats.

Lawrence Schook, director of the Food Animal Biotechnology Centre at the University of Minnesota, described a new technique which allows scientists to decipher all the characteristics that a pig may be carrying in its DNA - genetic material - and which it may pass on to the next generation. The breeder can then breed only those pigs with desirable genes. The United States has been immersed in a tradition of big, fat pigs, says Schook. Now things will change. In his brochure is a photo of a litter of pigs. On each has been superimposed a barcode. So the future dinner plate of the obese westerner will be piled with tasty vegetables, lean meat and absorbable potatoes all fried in a fat that is benign towards heart disease.

Alternatively, for fat that is low in calories rather than low in saturation, there is now olestra, the miracle fat-free fat, approved by the US Food and Drug Administration in January. It passes through the body undigested, taking its calories with it. You can enjoy the taste of the fat without absorbing it. You can eat two-and-a-half bags of crisps made with olestra for every bag made with ordinary fat.

But there were a small minority of dissenters in the FDA task force that advised on whether olestra should be approved. One was Joan Dye Gussow, emeritus professor of nutrition and education at Teachers College, Columbia University and long-time thorn in the side of the food industry. Gussow argues that we must not credit scientists with knowledge about social science and that we must not credit the food industry with a mission to improve public health.

She illustrates her point with the tale of artificial sweetener, which shows the unpredictable ways in which people react to the tweaking of their food supplies. A long-running study monitored the health of US nurses. Researchers examined the data to find the biggest single predictor of obesity. They found that it was not an obvious factor, such as lack of exercise. It was use of artificial sweetener. The reason is simple. If you like the taste of sugar, and you know that your sweet-tasting coffee is low on calories, you are more likely to also have to a doughnut.

Americans increased their use of artificial sweeteners five-fold between 1971 and 1991. Over that period, the proportion of the population that is overweight increased from a quarter to a third. Artificial sweetener, Gussow told the meeting, has not helped Americans to lose weight.

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Gaull, a former vice-president of the nutritional science division of the company that produces the artificial sweetener, NutraSweet, accepts Gussow's point. But he adds that artificial sweeteners "gave people a chance for calorific flexibility".

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If something as simple as low-calorie sweetener has arguably led to something as unpredictable as weight-gain, what will be the result of releasing other new foods onto the market? If you know your olestra-fried crisps are less fattening will you help yourself to another of those doughnuts?

Gussow says: "I have been in the presence of scientists working on the biochemical redesign of the food supply. My task is to raise a few alarms about a food supply that is increasingly driven by the politics of profit. You have to start from the basic assumption that the food industry is not a charity. No products are usually introduced for the purposes of improving health. The food industry's ideal is the fat person who eats diet foods and fattening foods - that's the perfect consumer."

But what do consumers, perfect or not, actually want to put in their supermarket trolleys? Gaull believes they want functional foods: "Products are not put on the market because someone dreams them up but because people have carefully tested what people want." When he said this at the AAAS meeting, Gussow was incredulous: "You mean the consumers asked for a blue cereal called Norman?" she asked. She is not convinced that consumers have really demanded some of the more esoteric products foisted on them over the past decade.

The argument evolves into that eternal question about whether the consumer citizen should be given what she wants or what is good for her. That cereal named Norman may be blue and anthropomorphic but at least it is probably made of classic foodstuffs such as sugar and cereals, about which we have generations of experience.

Gussow's second worry about altering foods is that change is outpacing knowledge of its consequences. We have only a fragmentary understanding of the effects of different components of food. We do not know how important the ratios of food components are. Yet we are removing and adding fragments. This is why Gussow objected to olestra. As the fat slips through the gut it drags with it substances known as carotenoids. "The reason olestra got through the committee was that they could convince the panel that there was no evidence that carotenoids have anything to do with health," she says.

There probably is no evidence, but that is because there has been so little research into carotenoids which are antioxidants. Several studies have shown that a diet rich in fruit and vegetables is linked to a low rate of heart disease and cancer. It is believed that the high levels of antioxidants in fruit and vegetables neutralise the free radicals that trigger cancer. But a Finnish study confused the issue last year by reporting that smokers who received an antioxidant supplement were more likely to get cancer than smokers who took a placebo.

Gussow has examined the study in detail. The antioxidant that the subjects received was beta-carotene, which they took in high doses. "The beta-carotene was probably interfering with the uptake of the relevant substances," she thinks. But there is another interesting fact about this trial. Before it started, some of the participants already ate food rich in carotenes (there are several). These people registered lower levels of cancer at the end of the study. So, the trial has not discredited the theory that carotenes protect against cancer. It has discredited the idea that you can pull a single chemical from a food and use it to protect against disease.

Similarly, we do not know the effect of removing one substance, such as carotenoids, from the diet. There is evidence that a very high level of vitamin C, for example, can increase the risk of cancer rather than reduce it. So should we eat tomatoes genetically engineered to produce a higher level of one antioxidant? If such tomatoes, which are already being worked on, were to go on sale, what instructions would we give to people who already eat a lot of tomatoes? Gussow wants to know whether we should tell them to eat fewer.

In a world full of complicated food products it is hard being a nutritionist. Food has become too confusing for the public to choose to eat healthily, says Gussow.

Nicholas Frey is frustrated that the public has not shown interest in his soya bean work. "Consumers are not asking for these products," he says. "They need to be made aware of improved oils. They may not be knowledgeable enough about dietary concerns to make the right choices".

The consumer, says Gussow, is not to blame. As food becomes less and less what it seems, we are being detached from our natural feel for a healthy balance. The sad fact is that "we really do know what people can eat that will make them relatively healthy". The healthy foods, she says, approximate to a stone-age diet. They are fresh and unprocessed - the kinds of foods that do not make much profit.

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