They can cause heart attacks and diabetes, yet most of us know nothing about them - let alone how to avoid them. Catherine Nixey lifts the lid on trans fats

Oh, for the days when fat was fat. Over the years, what we used to think of as a single substance has fractured into a bewildering chemical confusion of varieties: monounsaturated fats, polyunsaturated fats, saturated fats, hydrogenated fats and now trans fats. Earlier this month, McDonald's paid out $8.5m (£4.5m) to settle a law suit in San Fransisco over using trans fats in its cooking. And health experts now warn that these "killer" fats are the most dangerous foodstuffs we might consume. But what exactly are they?

While trans fatty acids occur naturally in small quantities in dairy products and the meat of ruminant animals, the ones that are causing alarm are those created as a by-product of processing hydrogenated vegetable oils. Trans fats are found in their highest quantities in processed foods that contain large quantities of hydrogenated oils, such as ready-made cakes, chips, pastries, pies, chocolates, sweets and ice cream.

"According to the experts, they are best avoided," says Tessa Russell, of Which? magazine, which recently conducted an investigation into trans fats. "The European Food Safety Authority has said that they may cause more damage than saturated fats. Some think that as little as 1g a day will significantly increase your risk of heart disease." Trans fats raise the levels of "bad" (LDL) cholesterol in the blood, and lower the levels of "good" (HDL) cholesterol, so increasing your risk of coronary heart disease. They have also been linked to type-2 diabetes.

But trans fats almost never appear on labels - food manufacturers are not obliged to say whether or not a food contains them, or in what quantities they might occur. All that manufacturers are required to say is that a product contains hydrogenated fat or oil. It is this opacity in labelling that has led to them being called "stealth fats".

Given the dearth of available data, Which? decided to analyse a wide variety of foods on sale in Britain to see whether and at what levels they contained these "killer fats". "We were surprised at the quantities we found," says Russell. "The UK recommendation is that you should eat no more than 4.4g of trans fats a day if you are a woman, and 5.6g a day if you are a man. But a regular KFC meal contained 4.4g of trans fats. A McDonald's nuggets and fries meal contained 3g." Other offenders were Tesco Free From toffee fudge shortbread (2.5g per portion), Saxby's fresh ready-rolled short pastry (2.5g per portion) and Cadbury's Boost bars (1.2g per portion).

Russell is also quick to point out that the levels of trans fats that are considered safe are "questionable" in any case. "The recommendations vary from country to country," she says, pointing out that Denmark has already banned oils and fats that contain more than tiny quantities of the fats. "The World Health Organisation has recommended that we try to eliminate them from our diet completely. We would like the British government to tell manufacturers to stop using them."

However, the Government doesn't feel that there is anything much to worry about. "We don't have any plans to ban trans fats in this country," says a spokesperson from the Food Standards Agency, "partly because the consumption of them is on the decline anyway. The UK recommendations are that trans fatty acids should provide no more than two per cent of dietary energy. In 1986 the average was just that, but by 2000 it had gone down to 1.1 per cent." For similar reasons, the FSA has no plans to make labelling of trans fats compulsory.

But as Oliver Tickell, the founder of tfX, The Campaign Against Trans Fats in Food, points out, this is only an average figure. "Within thatyou will have certain people who are consuming very large amounts of trans fats indeed. Generally, I think, [these will be] poor people eating a poor diet - a diet rich in chips, pizzas and the cheaper kinds of junk food. They will be getting very large doses of trans fats."

But trans fats continue to be used because they are extremely beneficial to the food industry. The hydrogenated oils and fats they are found in are cheap, give a good "crumbly" texture to pastry and - crucially - prolong the shelf-life of the product. "If you make a nice simple pastry with butter, it won't last more than four or five days," says Shane Osborn, the chef and patron of the Michelin-starred restaurant Pied à Terre. "Make one with margarine, and it might last for four or five weeks."

But while they might have a preservative effect on food, they do not have a preservative effect on us: research conducted by the Harvard School of Public Health concluded that: "by our most conservative estimate, replacement of partially hydrogenated fat in the US diet with natural unhydrogenated vegetable oils would prevent 30,000 premature coronary deaths a year." And whatever benefits hydrogenated oils and their attendant trans fats might have, it seems that taste is not one of them. "I never use them," says Osborn. "Pastries made with hydrogenated fats will always have an insipid, even a manufactured flavour."

So how can you avoid trans fats? "You can take a guess that if the ingredients list says 'hydrogenated' or 'partially hydrogenated fats' on the label, then that product will contain trans fats," says Russell. Earlier this year, the Co-op announced that it is going to start labelling products that contain the fats. Waitrose, meanwhile, has embarked on a programme to eliminate them from its own-brand products - although, says a Waitrose spokeswoman, "it will probably take about three or four years before we can absolutely say that we haven't got trans fats in our products."

One clear way to avoid trans fats is to buy organic food. "It is forbidden to include hydrogenated fat in organic food," says Tickell. "And I don't think that organic manufacturers would want to include it anyway: an industrial process like hydrogenation goes so completely against the organic ethic."

And despite the reluctance of the Government to act, campaigners, especially the more litigious ones, are likely to ensure that trans fats become an increasingly problematic ingredient for food companies. McDonald's is not the only company to have come under scrutiny. Kraft Foods, the maker of Oreo cookies, has also had a law suit brought against it. The controversy seems unlikely to go away.

"Something that is essentially a poison is habitually included in tens of thousands of processed foods," says Tickell. "Trans fats are one of the major public health scandals of the 20th, and now the 21st century."


BAD: Tesco Free From toffee fudge shortbread (2.5g per portion)

BAD: Asda milk chocolate flavour caramel shortcakes (0.65g)

BAD: Cadbury's Boost bar (1.2g)

BAD: Chips fried in hydrogenated oils

BAD: Margarine

GOOD: McVitie's Go Ahead caramel crunch bars (<0.1g)

GOOD: Asda Custard Creams (<0.1g)

GOOD: McVitie's Taxi biscuits (<0.1g)

GOOD: Chips fried in animal fat

GOOD: Butter