The Big Question: What are the properties of trans fats, and should they be banned?

Why are we asking this question now?

The fast-food chain Kentucky Fried Chicken scored a publicity coup in the US on Monday by announcing that it was removing trans fats from the cooking of all but a handful of items on sale in its 5,500 restaurants in the US. Monday, hardly by coincidence, was also the day that public hearings began in New York on a proposed city-wide ban on the fats. The KFC decision is the biggest concession yet by the fast food industry to the growing campaign against trans fats on both sides of the Atlantic that has accelerated since the summer.

New York City announced in September that it would ban all but tiny quantities of trans fats in the food served in its 24,000 restaurants. Under the plans, which are out to consultation until December, all restaurants, cafés and street stalls will be required to limit the trans fats in any item served from their menus to no more than half a gram. Currently one serving of chips can contain up to eight grams.

What are trans fats?

They are produced when liquid oil is turned into solid fat during a high-temperature process called hydrogenation. The method was used a century ago by Procter and Gamble to make a vegetable fat, Crisco, from cottonseed oil, which was cheaper than the lard it replaced and lasted for up to two years at room temperature. It was an immediate hit in US households.

Since then the use of hydrogenated fats has penetrated all sectors of the food industry. It grew in popularity during the two world wars when butter was in short supply and hydrogenated fats were used to make margarine. Hydrogenated fats also mimicked the properties of pork and beef fats so they went into halal, kosher and vegetarian foods. They are found in baked goods, biscuits, snack foods and anything made with or fried in partially hydrogenated oils. Their main selling point is that they are cheap.

What is wrong with them?

Put that another way: what is right with them? According to food experts they have no nutritional value and are an artificial toxic fat that we don't need. The UK Food Standards Agency says they are "harmful and have no known nutritional benefits... They raise the type of cholesterol in the blood that increases the risk of coronary heart disease. Some evidence suggests that the effects of these trans fats may be worse than saturated fats."

Anti trans-fat campaigners put it more colourfully. "Would you melt tupperware and put it on your toast," they ask.

The New England Journal of Medicine, which published a scientific review of trans fats earlier this year, said that "from a nutritional standpoint, the consumption of trans-fatty acids results in considerable potential harm but no apparent benefit." trans fats, like saturated fats, increase levels of LDL cholesterol (so called "bad" cholesterol) but unlike saturated fats also lower levels of HDL cholesterol (good cholesterol).

The British Medical Journal said in a review in July that a 2 per cent rise in our consumption of trans fats - five grams a day - was associated with a 23 per cent increase in the incidence of coronary heart disease. Consumption of trans fats has also been linked with prostate cancer, diabetes, obesity and liver problems, but there is no consensus on these effects.

Which British food products contain trans fats?

trans fats are the hidden fats in Britain, often not listed on food labels. The only way of calculating them is to subtract the sum of the saturated, polyunsaturated and mono-unsaturated fats from the total fats. What is left is the trans fats. So unless you shop with a calculator it is difficult to know how much you are eating.

In the US, since January, food labels have had to state trans-fat quantities. The campaign against them is more advanced there and many products carry banners saying "Now with no trans-fat!" This has fooled some consumers into thinking that products such as ice creams and cakes are fat free when they may contain large amounts of saturated fat. The result has been rising sales. Fast food, pub food and fish and chips don't have to signal the presence of trans fats. Campaigners in the UK argue that there is no pressure to get rid of trans fats because consumers do not know they are there.

What is being done in Britain?

Four British supermarkets - Tesco, Sainsbury's, the Co-op and Asda - have pledged to phase out trans fats from their own-label products. Marks and Spencer and Waitrose have already done so. Kellogs, United Biscuits, Nestlé, Cadbury Schweppes have also promised to reduce or remove trans fats from their products. But Britain has yet to follow Denmark's example which banned trans fats in 2003.

In the US, the burger chain Wendy's has removed trans fats from its french fries and chicken. McDonalds is still using them, though it says it is gradually reducing them. A total ban on trans fats is impractical because they occur naturally in some foods.

A survey by Danish researchers, published in the New England Journal of Medicine in August, found levels of trans fats vary widely, even in identical products. One large serving of chicken nuggets and french fries contained 10 grams of trans fats in New York, 6 grams in the UK, 5 grams in France and 1 gram in Denmark.

Is there any alternative to trans fats?

There is. Under pressure of law suits and legislation, manufacturers have been developing substitute oils and fats. It is this that has persuaded KFC and others to abandon opposition to removing them and concede gracefully. Only in June KFC was still protesting that it had no intention of changing Col Sanders' 50-year-old "finger-lickin' good" recipe. But a new refining process called interesterification has meant that trans fats-containing oils can be replaced at no extra cost.

Unfortuantely, however, some manufacturers are returning to the saturated fats used 20 years ago and abandoned when concern about their link with heart disease was at its height. If that were to occur generally, we would be exchanging one unhealthy food for another.

Should trans fats be banned from all foods in Britain?

Yes...

* Although they occur naturally in small quantities in a few foods, they are mostly artificially produced and have no nutritional benefits

* They are harmful, increasing the risk of coronary heart disease, and suspected of contributing to other illnesses

* There are safer alternatives available, some of which have been researched and put into use by KFC

No...

* Replacing trans fats with saturated fat would be almost as bad for our health

* Unlike smoking, eating products containing trans fats does not threaten the health of others

* Provided products are properly labelled it should be up to consumers to decide what they eat