The link between trans fatty acids and coronary heart disease (CHD) is so strong that cities have banned them in restaurants, but now comes even more damning evidence: Women with the highest blood levels of trans fats have triple the risk of heart disease as those with the lowest levels, according to an NIH-sponsored study at the Harvard School of Public Health in Boston.

Previous trans-fat research used self-reported dietary information, which can be unreliable. The new study is based on an objective measure of trans fat intake—trans fat levels in RBCs.

The Harvard team examined blood samples collected from 32,826 women participating in the ongoing Nurses’ Health Study. During a six-year period, 166 of the women developed CHD. These women were compared with 327 healthy controls. Women in the highest quartile of trans-fatty-acid content in their erythrocytes were 3.3 times more likely to develop CHD than those in the lowest quartile. Even those in the second and third quartiles were at a 60% increased risk of CHD (Circulation. 2007;115:1858-1865).

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Found primarily in partially hydrogenated vegetable oils, trans fats increase the shelf-life of food products and enhance the stability of frying oils. “Trans fats are the only fatty acids that not only increase LDL cholesterol but also decrease HDL,” says the study’s senior author, Frank Hu, MD, PhD. That means on a gram-for-gram basis, trans fatty acids are substantially more damaging than saturated fat, Dr. Hu added.