Of all the questions we're asked about dietary fat, none are more difficult to answer than those that concern trans fats. Without being overly dramatic, trans fats are like land mines--we are told to avoid them, but nobody knows exactly where they are or what they look like.
Some definitions might help.
* Dietary fats are just what you think they are--fats found in foods. Functionally, they add flavor, texture and energy to foods. They are essential in a healthy diet. It would not be healthy to consume absolutely no fat.
* Fatty acids are molecules composed mostly of carbon and hydrogen atoms. They are the building blocks of fats, in much the same way that amino acids are the building blocks of protein. Dietary fats contain a combination of saturated and unsaturated fatty acids. If every carbon atom in the fatty acid chain has a hydrogen atom attached, it is said to be saturated with hydrogen (hydrogenated). Saturated fatty acids are found mostly in animal products such as meat, cheese and whole milk. The degree of saturation determines whether a fat is solid or liquid.
* Unsaturated fatty acids are missing one (mono) or more (poly) pairs of hydrogen atoms. The more unsaturated the fat, the more liquid it is. Vegetable oils that are liquid at room temperature are considered unsaturated.
* Trans fats are formed when unsaturated fats are hydrogenated and made more saturated. By partially hydrogenating vegetable oils, food processors can make food products that stay fresh longer and have a better texture. Any food that contains these oils must list "partially hydrogenated vegetable oil" in the ingredients on the label. However, only a portion of partially hydrogenated oils actually become trans fats. Trans fats end up having a unique chemical structure. The amount of trans fat produced during hydrogenation depends on the process used. Since most food processors keep this a proprietary secret, it's almost impossible to know what percentage of fats in any food are trans fats.
Some trans fats also occur naturally in foods, like meat and dairy products, and they are found in any food made with hydrogenated oils, like baked goods and fried foods and some margarines. In general, trans fats make up less than 5% of our total calories, compared with 12% for saturated fats and 34% for total fat. About a fifth of trans fats comes from animal sources.
Why all the fuss over trans fats? Even though trans fats are not saturated, they may affect blood cholesterol levels in a way similar to that of saturated fats. Some studies suggest that trans fats may raise LDL (bad cholesterol) and total blood cholesterol in much the same way as saturated fats. However, other studies have shown they have less of an effect than saturated fats. Some animal studies show no atherosclerosis in animals fed diets high in trans fats compared with animals fed diets high in saturated fat.
Obviously, the jury is still out on this one.
Nonetheless, some people want to hedge their bets and reduce the amount of trans fats in their diets. The following suggestions for doing so come from the International Food Information Council.
* Reduce total fat intake, which will help lower your intake of both saturated and trans fats.
* Do not try to substitute food higher in saturated fats to reduce the amount of trans (unsaturated) fats. This is a bad trade-off.
* Most liquid vegetable oils are naturally lower in saturated fats and are trans fat-free. These include soybean, canola, corn, olive, safflower and sunflower oils.
* Margarine products contain significantly lower amounts of saturated fats than animal fats such as butter, tallow and lard or solid shortenings. And, many margarine products are low in trans fats or are trans fat-free. Unfortunately, liquid and lower fat versions of margarines do not substitute well in recipes in which shortening, stick margarine or butter is required.
* New fat processing technologies have produced some trans fat-free products. Additional products are likely to become available in the near future.
* To lower intake of saturated fats and trans fats, try reduced-fat, low-fat, fat-free and trans fat-free versions of frequently consumed foods.
Don't expect to find listings of trans fats in the nutrition facts panel on your food labels. Although the U.S. Food and Drug Administration is considering a rule for labeling of trans fats, one is not likely to pass any time soon.
Until we learn something to the contrary, there is probably no reason to be overly worried about trans fats in particular, and the same old advice about fat and cholesterol consumption still holds:
Try to limit your total fat intake to no more than 30% of your overall calories, and of this no more than 10% should come from saturated fat. Put another way, if you are eating 2,000 calories a day, you should have no more than about 60 grams of fat a day and no more than 6 grams of saturated fat. If you can reduce your fat intake even further, you're way ahead of the game. Try to make sure that as much of your fat intake as possible is unsaturated. Try to limit your cholesterol intake to no more than 300 milligrams per day.