Page Banner

United States Department of Agriculture

Agricultural Research Service

Are dietary trans fats really bad for you?
headline bar

By Philip Reeves

By now, most of you have heard of trans fatty acids, or trans fats, in foods, and that they may not be good for your cardiovascular system. In fact, the health authorities in New York City recently decided that trans fats are so unhealthy that their levels will be rigorously regulated in NYC restaurants and processed foods sold in the entire state.

Exactly what are trans fats? Trans fatty acids are found in nature making up from 2 percent to 5 percent of the fat in dairy products and the meat of cattle and sheep. But most trans fat found in our food supply come from a process called partial hydrogenation, where hydrogen atoms are added to vegetable oils to convert them to semisolid fats.

At the turn of the 20th century, the original hydrogenated vegetable oil, or shortening, first was produced and contained relatively high amounts of trans fats. But that product soon was followed by various types of hydrogenated vegetable oils to make margarines and other fats used for deep-frying. Because of their stability upon heating and ease of use these hydrogenated oils soon became a boon to the food manufacturing industry.

Fatty acids are made up of long chains of carbon atoms stuck together with bonds. Each carbon has a hydrogen atom sticking out on each side of the chain. Most of the bonds between carbons are single, but in many cases, a single set of double bonds can be found, as in monounsaturated acids such as olive oil. Two or more sets of double bonds can be found in polyunsaturated fatty acids, which are found in sunflower and soybean vegetable oils. Both oils are liquid at room temperature.

Trans fats are made when relatively healthy oils are hydrogenated to make the oils more solid at room temperature - thus helping packaged foods stay on the shelf longer without going bad. The trans fatty acids are metabolized in the body similar to saturated fats.

What is the evidence that trans fatty acids might be bad for you? Over the past 30 to 40 years, numerous human studies have shown that the consumption of trans fats in the diet, compared with other fats, will increase serum LDL cholesterol, the bad cholesterol. They also increase the amount of serum triglycerides and the ratio of total serum cholesterol to HDL cholesterol, the good cholesterol. All of these effects are associated with an increased risk for coronary heart disease.

Other effects of consuming too many trans fats include disturbances in the function of cells that line the blood vessels and reductions in activities of enzymes that process HDL-C. These effects, too, are associated with diseases of the cardiovascular system.

What are the federal regulatory agencies doing to protect us from these trans fats? As of January 2006, the Food and Drug Administration ruled that the food industry must state on food labels the amount of trans fats found per serving. But if the food contains less than 0.5 grams of trans fats per serving, the food can be labeled as containing zero trans fats.

The labeling of some foods can be tricky, depending on how much trans fat is in a serving. For example, since dairy products such as skim milk, 2 percent milk and whole milk (3.7 percent fat) contain natural trans fats in small quantities, an 8-ounce serving can be labeled as having zero trans fats.

But even though cream contains about 18 percent fat, a serving of 1 tablespoon also could be labeled as zero trans fats. On the other hand, one serving (½ cup) of high quality “real” ice cream with 18 percent fat would have to be labeled as having at least 0.5 grams trans fat. A 1-ounce serving of high-fat cheese would not contain enough trans fat to be labeled.

Since most trans fat in foods comes from hydrogenated vegetable oils, the margarines and fats used for deep-frying would provide the highest intakes. For example, one serving (1 tablespoon) of some stick margarines contain as much as 2.5 grams of trans fat and must be labeled as such. But one serving of the soft, tub margarines is low enough to be labeled as having zero trans fat.

New formulation of some hydrogenated oils no longer contains trans fat; but some other shortenings do. French fries made with the latter could contain up to 6 grams of trans fat per serving.

Some of the undesirable effects of trans fat can be seen after consuming as little as 2 grams per day. Thus, even though a food might contain 0.499 grams of trans fat per serving and be labeled zero trans fat, 4 servings per day of that food would contain almost 2 grams of the unwanted fat. If we consume other foods containing even smaller amounts of trans fat, we easily could obtain more than 2 grams of these fats per day.

The current recommendation by some authorities is to limit trans fat intake to less than 0.5 percent of the total energy intake. This would amount to a little more than 1 gram per day for a 2000-calorie diet.

As a result, consumers are strongly encouraged to read food labels and try to limit their intake of trans fats as much as possible. Lowering the total amount of fat consumed per day also would aide in reducing the amount of trans fatty acids consumed.


Last Modified: 11/7/2007
Footer Content Back to Top of Page