Trans fatty acids are unsaturated fatty acids that have at least one double bond in the trans configuration. They are produced commercially by a process known as hydrogenation, in which a vegetable oil is heated in the presence of a metal catalyst and hydrogen. These hydrogenated fatty acids are more saturated than natural vegetable oils and also more linear, due to the presence of trans double bonds in place of cis double bonds. This enables trans fatty acids to pack together more tightly. As a result, trans fatty acids are more solid at room temperature and behave more like saturated fatty acids.
Where are trans fatty acids found in food?
The most concentrated sources of trans fatty acids are margarine, vegetable shortening and other partially hydrogenated fats. However, they are also present in many other foods, including crackers, cookies, potato chips, doughnuts, cakes, puddings, French fries and other fried foods. It is estimated that 3%-5% of calories in the average American diet come from trans fatty acids. It is difficult to know exactly how much trans fat is in the American diet since, at present, manufacturers are not required to include this information on food labels. As a result, the trans fatty acids in food are "hidden" within the levels reported for unsaturated fatty acids (including polyunsaturated fatty acids and monounsaturated fatty acids). This is very misleading, since unsaturated fatty acids are widely considered to be "healthful". As a result, the Food and Drug Administration (FDA) is currently considering changing the food labeling regulations to make it mandatory to include the trans fatty acid content of a product on all food labels.
Trans fatty acids also occur in nature as a result of anaerobic bacterial fermentation in ruminant animals and, thus, are found in small amounts in dairy products. Trans fats are also found in low levels in meats such as beef and lamb.
What are the effects of trans fatty acids?
Clinical studies have consistently demonstrated that trans fatty acids increase plasma total cholesterol and LDL cholesterol while decreasing HDL when they are substituted for unsaturated (unhydrogenated) fatty acids. This results in a significant increase in the total cholesterol/HDL ratio, thus putting a person at a greater risk for coronary heart disease (CHD). Although the data regarding the effects of trans fatty acids on triglyceride and very low density lipoprotein (VLDL) levels have not been as consistent, some studies have shown an increase in both when compared to diets high in unsaturated fatty acids. There is also evidence that trans fatty acids increase lipoprotein (a) (Lp[a]), another risk factor for CHD.
In order to put this information in perspective, it is important to compare the effects of trans fatty acids on cardiovascular risk factors to those of saturated fatty acids. Hydrogenated vegetable fats that contain trans fatty acids, such as shortening and margarine, are often used as alternatives to butter (a source of saturated fatty acids) in baked goods and fried foods. Relative to saturated fatty acids, trans fatty acids result in a lower blood cholesterol level. However, trans fatty acids may also decrease HDL. The long term impact of these findings is not yet fully understood. Suffice it to say, the butter versus margarine debate is not over yet. Of course, the best solution is to reduce or to eliminate the consumption of both types of fat.
The bottom line:
Trans fatty acids have negative effects on blood lipid profiles. Compared to unsaturated fatty acids that have not undergone hydrogenation, trans fatty acids:
- Increase total cholesterol
- Increase LDL
- Decrease HDL
- Possibly increase Lp(a)
The American Heart Association recommends that trans fatty acids should be replaced by unmodified (not hydrogenated) monounsaturated and polyunsaturated oils whenever possible.