What Are Trans Fatty Acids (Trans Fats)?Natural foods (that is, unprocessed foods) contain two main types of fatty acids - saturated and unsaturated. Saturated fatty acids - which come from animal fats (meat, lard, dairy products) and tropical oils such as coconut and palm oils - can raise your blood levels of LDL cholesterol. Unsaturated fats - which come from vegetable oils - in general do not increase cholesterol levels, and may reduce them.
Trans fatty acids (trans fats) are a third form of fatty acids. While trans fats do occur in tiny amounts in some foods (particularly foods from animals), almost all the trans fats now in our diets come from an industrial process that partially hydrogenates (adds hydrogen to) unsaturated fatty acids. Trans fats, then, are a form of processed vegetable oils. This means that in our diets, trans fats are almost exclusively found in the processed foods we eat.
The advantage of trans fats to the food processing industry is that partial hydrogenation solidifies and stabilizes vegetable oils, which otherwise tend to turn rancid relatively quickly. Because they exist in solid form instead of liquid form, trans fats can be used as substitutes for saturated fats in food products that are meant to have a long shelf life.
Trans fats were invented in the 1890s and began entering the food supply in the 1910s. However, the use of trans fats in food processing really took off in the 1970s and 1980s, when it was learned that saturated fats can be bad for your health.
Because trans fats were derived from the more healthy vegetable oils, it was assumed that they, too, would be healthy food products.
What Is Unhealthy About Trans Fats?Unfortunately, as it turns out (and as we were relatively slow to learn), trans fats increase total cholesterol levels and LDL cholesterol levels; worse (and in contrast to saturated fats), they reduce HDL cholesterol levels. Trans fats also appear to interfere with the body's usage of omega-3 fatty acids, which are important for heart health.
In other words, trans fatty acids are bad for cardiovascular health.
Which Is Worse - Saturated Fats or Trans Fats?Both. Saturated fats and trans fatty acids are bad for you. Saturated fats are almost always found in foods that also contain cholesterol, so saturated fats offer a "one-two" punch to heart health. On the other hand, trans fatty acids not only increase LDL cholesterol, they also decrease HDL cholesterol. While nobody can say yet definitively which is worse, it does appear that both are bad.
Which Foods Contain Trans Fats?Fortunately, today it is relatively easy to identify foods that contain substantial amounts of trans fats. The easiest way is to check the food labels, since by law food manufacturers must now disclose how much trans fats they are putting into their products.
Also, there are certain types of foods that are very likely to contain trans fats. These include margarines. (The more solid the margarine, the more the trans fatty acids. So, stick margarines contain the most, tub margarines less and semi-liquid margarines the least trans fats).
Also, high-fat baked goods (especially doughnuts, cookies, cakes, chips and crackers); most deep-fried foods; and any product that has "partially hydrogenated vegetable oils" contain trans fats.
What Are the Good Fats?Unsaturated vegetable oils from canola, peanuts, olive, flax, corn, safflower and sunflower (as long as they have not been subjected to the process of hydrogenation) are heart healthy. These oils contain monounsaturated or polyunsaturated fatty acids that can reduce total cholesterol and increase HDL cholesterol levels. These oils also contain the essential fatty acids - specific fatty acids necessary for life but which the body cannot make itself - such as omega-3 and omega-6 fatty acids.
What Should the Health-Conscious Consumer Do?There are three basic steps to reducing the amount of "bad" fat in your diet and substituting "good" fat.
- First, avoid the saturated fatty acids found in meat, dairy products and tropical oils (palm and coconut).
- Second, avoid trans fats by reading the food labels of processed foods.
- Third, whenever possible substitute one of the natural unsaturated vegetable oils in recipes calling for stick margarine, butter or lard (pig fat).
Mozaffarian D, Katan MB, Ascherio A, et al. Trans fatty acids and cardiovascular disease. N Engl J Med 2006; 354:1601.