In a study of female nurses, women with the highest levels of trans fats in red blood cells from stored samples had triple the risk of developing heart disease compared with women with the lowest levels.
Higher trans fat levels were associated with elevated "bad" LDL cholesterol and lower "good" HDL cholesterol. But the study's researcher tells WebMD that this did not fully explain the impact of trans fats on heart disease risk.
"We definitely believe that other mechanisms are involved, but we were only able to look at cholesterol," says Frank B. Hu, MD, PhD, of the Harvard School of Public Health.
"This research shows that trans fats are a very strong risk factor for coronary heart disease, and it serves to justify current efforts to get trans fats out of the American diet."
Banning Trans Fats
Those efforts include bans or considered bans on the use of trans fats in restaurant food in major cities like New York, Philadelphia, and Chicago, and pledges from a growing number of fast-food restaurant chains to make their products trans-fat free.
Trans fats are formed when liquid vegetable oils are converted into solids through a process called hydrogenation. Hydrogenated or partially-hydrogenated oils are used to increase the shelf life of foods and to improve their texture.
They are most often found in fried foods, vegetable shortenings, hard margarine, and processed cookies, crackers, baked goods, and chips.
The newly published investigation included 32,826 women participating in the larger Nurses' Health Study, an ongoing trial examining lifestyle and disease risk in women followed since 1976. All of the women in the trans fat study contributed blood samples between 1989 and 1990.
During six years of follow-up, 166 of the women developed coronary heart disease. The researchers compared trans fat levels in the stored blood samples of these women with samples from 327 women who did not develop heart disease.
Women with the highest blood levels of trans fats were three times as likely to develop coronary heart disease as women with the lowest levels.
Although the investigation included only women, Hu says there is no reason to believe that the risk would be any different for men.
The study will be published in the April 10 issue of the American Heart Association journal Circulation.
Good and Bad Fats
The American Heart Association (AHA) recommends limiting trans fats to no more than 1% of total calories and saturated fats to less than 7% of total calories.
While there is some suggestion that trans fats are more dangerous than saturated fats, AHA immediate past president Robert Eckel, MD, says both are associated with an increased risk of heart disease.
The major sources of saturated fats in the diet are meat, poultry, whole-fat dairy products, and butter. Some tropical oils such as palm and coconut oil are also high in saturated fats.
Monounsaturated and polyunsaturated fats, such as those found in nuts, fish, olive oil, and most other liquid vegetable oils, are better fat choices, Eckel says.
"Replacing trans fats with saturated fats would do little good," he tells WebMD. "People should avoid trans fats whenever possible and they should choose healthier fats."
Hu adds that the movement to eliminate or lower trans fat levels in the foods Americans eat should have a positive impact at the population level.
"Eliminating the use of partially hydrogenated oils and other sources of trans fats in the U.S. diet -- as long as saturated fat intake doesn't increase -- will likely help reduce the burden of cardiovascular diseases," he notes.