Trans fats

Trans fatty acids occur in nature, where they are found in some plants (leeks, peas, spinach, lettuce), in the flesh of ruminant animals (e.g. beef and lamb) and in milk and dairy products (butter) as a result of bacterial fermentation. However, the major source in developed countries is from partially hydrogenated fats which are present in margarine, vegetable shortenings, cooking fats and salad oils often used by the food industry. A study in the 1980s revealed that a typical fast food meal (cheeseburger, french fries, milk shake and a pastry) contained 3-4 g trans fats.

Intake in the UK between 1980-1990 was about 7g/person/day; by 1995 this had fallen to 5g/person/day. Intakes in the US have remained constant at about 8g/person/day since the 1970s. Trans fats are formed during the hydrogenation process (uses heat and chemicals) of liquid oils; the fatty acid molecules straighten out which then allows them to stack closer together to form solid fats with more desirable textural and functional properties. For example, oleic acid (cis fatty acid) and elaidic acid (trans counterpart) have melting points of 16.2 and 43.7 C, respectively. Hydrogenated vegetable oils are also less susceptible to oxidation, have improved flavour stability and are spreadable. Hydrogenation also results in the saturation of some double bonds; i.e. converts polyunsaturated fats into less healthy saturated fats. Levels vary from 5% in margarines to 40% in commercial frying fats. Stick/hard margarines contain 16-36% and some soft margarines can contain 10-30% trans fats. Butter contains 2-9%. Non hydrogenated vegetable oils do not have any trans fats. Many margarines have been reformulated in the 1990s & 2000 containing only trace quantities of trans fats (e.g. I can't believe it's not butter).

The National Heart Foundation gives the ‘tick’ to margarines if they contain less than 28% saturated + trans fatty acids of the total fat content. However, the food industry still uses hydrogenated vegetable fats high in trans fats for frying and processed foods. Even if a margarine is low in trans fatty acids there is some evidence to suggest that cooking with margarine is not ideal since trans fatty acids can form when margarine melts.

Experimental evidence shows that trans fats can increase LDL and lipoprotein-a (important risk indicator for CHD) and reduce HDL. The carefully performed metabolic study of Mensink and Katan (NEJM 1990; 323: 439-45) found that trans fatty acids behave unlike any other type of fat as they raise LDL and depress HDL. The negative impact of trans fats on the total cholesterol/HDL ratio was almost twice as great as that of saturated fat. Several studies have confirmed that trans fats are worse than saturated fats with respect to total/HDL ratio. In the Nurses Health Study the women with the highest intakes of trans fatty acids had a 50% higher risk of CHD than women with low intakes of trans fats and there was a highly significant dose response relationship.

Lichtenstein et al (NEJM 1999; 340 (25): 1933-7) studied 18 men and women consuming 6 diets with increasing quantities of trans fatty acids from soy bean oil, semiliquid margarine, soft margarine, vegetable shortening, stick margarine, or butter. Although all butter substitutes reduced the level of LDL, the trans fatty acids sometimes drove down the HDL cholesterol. The stick margarine resulted in a worse blood lipid profile than butter because of its higher content of trans fats. None of this argues for a return to an all-butter diet since soft margarines still lower LDL cholesterol and butter does not. They found that consumption of fats low in trans fats and saturated fats had a beneficial effect on blood lipids.

Margarines need to disclose their trans fat content, but so too should fried fast foods like French fries, which account for up to 75% of the trans fats consumed in the US.

At face value you could argue that they have a more adverse effect than saturated fats, but it must be remembered that the level of saturated fat in our diets remain substantially higher than the amount of trans fat.

 

Update on Trans fatty acids 2002

Last Updated: June 2002