Trans-fats

Concerns about trans-fats

In recent years, trans-fats have risen to the top of the food-and-health agenda. A number of studies have linked trans-fats to coronary heart disease and, as a consequence, food manufacturers, governments and consumers are increasingly concerned about trans-fats: what they are, which foods they appear in and how consumption of them may affect health.

Trans-fats: a brief history

About 100 years ago the discovery was made that liquid oils could be converted to solid fats (which were more useful in food manufacture) by a procedure called hydrogenation. As hydrogen was added to liquid unsaturated oil it gradually became a solid saturated fat (also called a hydrogenated fat).

After the Second World War the process for making hydrogenated and hardened fats from cheaper sources of vegetable oils was widely adopted. Margarines were developed and marketed as alternatives to butter, and vegetable shortenings increasingly replaced the animal fats in cooking.

As early as 1975 a group of scientists led by Mr Leo Thomas at what is now the University of Glamorgan in South Wales suspected that eating partially hydrogenated fats had a connection with death from coronary heart disease. The suspected link between the consumption of trans-fats and this illness was subsequently investigated at the Harvard School of Public Health in the US. It is now generally accepted that trans-fats are actually worse for the health than the saturated animal fats they were designed to replace.

Food-labelling and trans-fats

There has been increasing acceptance by governments that the risks to consumers of eating trans-fats in quantity cannot be ignored. In 2003 Denmark became the first country to introduce laws to control the sale of foods containing trans-fats. In the same year, Canada required that the presence of trans-fats be shown on food labels, and in the following year the Canadian government essentially banned the use of trans-fats in food altogether.

In January 2006 it became law in the US that the content of trans-fats has to be specifically listed on food labels. There is a complication to this, however, because foods containing less than 0.5g of trans-fats per serving can be labelled as being free from them. Furthermore, the regulations only apply to food labels: food sold in restaurants and canteens are not covered by this law.

However, in December 2006 New York City’s Board of Health 'banned' many trans-fats from the city’s restaurants, prompting similar moves in Philadelphia, Montgomery County in Maryland, and the Boston suburb of Brookline. The first phase of the regulation applies to oils, shortening and margarines used in cooking and as spreads, in recipes that contain more than a half-gram of trans-fat per serving.

The second phase, in July 2008, extended the ban to include trans-fats used in bread and cakes, prepared foods, salad dressings and oils used for deep-frying or in dough or cake batter. Similar 'bans' are being proposed in a growing number of cities, towns and states across the country.

Q&A: trans-fats

Food scientist Prof Tony Blake answers your questions about trans-fats.

What are trans-fats?

"All fatty foods consist of chain-like molecules. In animals these chains are very flexible. By contrast, the molecules in vegetable oils have rigid kinks which restrict their flexibility. The more flexible the chain the more easily it can become a hard fat on cooling, so vegetable oils are more likely to stay liquid at room temperature. The artificial hardening of vegetable oils by hydrogenation would have been fine were it not for the fact that some of the kinks unexpectedly change shape during the process and form unhardened trans-fats, which cannot be properly digested by the body."

How can I tell if a product contains trans-fats?

"In the UK and many other European countries the situation is complicated. Although there is no specific requirement for the labelling of trans-fats on food labels, some manufacturers have started to do so voluntarily. Most of the margarines and vegetable shortenings on supermarket shelves now show the products as being 'virtually free of trans-fats'; on the other hand products such as cakes and biscuits that include hydrogenated fats in the ingredient lists do not often mention trans-fats. Food manufacturers and suppliers are increasingly turning the absence of trans-fats into a marketing claim for their products. In the UK, in February 2006, Marks and Spencer announced in full page adverts in the national press that they had removed all hydrogenated fats from their ready meals."

How can I avoid eating trans-fats?

"The simplest advice in the absence of any specific labelling is to avoid all foods that list hydrogenated or partially hydrogenated fats or shortenings on their labels. When cooking at home use liquid vegetable oil for frying and only margarines and shortenings that are labelled trans-fat free. If you can check these details with the managers of restaurants or takeaways then even better for you."

Are trans-fats 'dangerous'?

"They are not dangerous in the sense that they are acutely toxic but, as we now know that they are not digested normally, we should avoid eating them if possible."

Can food labelled as containing 'vegetable shortening' or 'vegetable fat’ contain trans-fats?

"Fats that have been partially hardened by hydrogenation usually contain trans-fats but complete hardening eliminates them. A new strategy being explored by some manufacturers is to completely harden part of the vegetable oil and then to blend this with untreated oil to make a composite of the correct texture but with no trans-fats present. Thus vegetable shortening or vegetable fat need not necessarily contain trans-fats."

Is it possible for 'good' oils to be transformed into trans-fats, eg. by frying?

"No; trans-fats are a peculiarity of the hydrogenation process used for hardening vegetable oils."

What are the long-term effects of eating trans-fats?

"Very much the same as eating an excess of saturated fats, although some work suggests that trans-fats are even more likely to increase 'bad cholesterol' and thus the risk of coronary heart disease."

Saturated fat content is usually included in the nutritional information panel [on food packaging]. Wouldn't it be advisable to itemise the trans-fat content too?

"Yes, and this is now a legal requirement in some countries. In the UK some food manufacturers have started to do this voluntarily."

What does Prof Blake think has sparked the recent salience of the evils of hydrogenated vegetable fat?

"Initial concern was first raised in the UK as early as the 1970s and further scientific research was needed to substantiate these early worries. The fact that there are now pressure groups to highlight this issue and that several countries have taken specific action to limit or ban the presence of trans-fats in foods has raised general awareness of the problem."

Are trans-fats produced by the margarine-making process? Are we better off eating butter instead of margarine?

"Modern margarine processing does not now produce trans-fats and many margarines are specifically formulated for their health-promoting benefits."

Reviewed July 2009