Trans fats are formed when food manufacturers add hydrogen to vegetable oil to make it more solid. Americans get much of their trans fat from the partially hydrogenated oils found in cookies, crackers, pastries, and fried foods. Among dietary fats, trans fats are uniquely hard on the heart, as they raise “bad” LDL cholesterol while lowering “good” HDL cholesterol.

By now you’ve probably heard that trans fat is one of the unhealthiest fats in the American diet. That’s because trans fat not only raises low-density lipoprotein (LDL, or “bad”) cholesterol but trans fat also lowers the “good” high density lipoprotein (HDL) cholesterol. In fact, trans fat is so harmful to your health that the Institute of Medicine says there is no safe level of intake and recommends consuming as little trans fat as possible. The American Heart Association has put a number on “as little as possible,” advising Americans to keep their trans fat intake below 1% of total calories. On a 1,800-calorie-a-day diet, that’s less than 2 g of trans fat a day.

To make it easier for you to reduce trans fat intake (it’s virtually impossible to consume no trans fat at all), the U.S. Food and Drug Administration now requires food manufacturers to list the amount of trans fat on the Nutrition Facts panel. (The amount is listed beneath the entry for saturated fat.)

Recently a large study reported in the journal Circulation (Volume 115, page 1858) confirmed the dangers of consuming trans fats.

Among nearly 33,000 U.S. women in the Nurses Health Study, those with the highest levels of trans fatty acids in their circulating red blood cells were three times more likely to develop coronary heart disease over the next six years than those with the lowest levels.

The link remained when age, overall diet, and other lifestyle factors were weighed — suggesting that the trans fats themselves contributed to coronary heart disease development.

This study bolsters the case against trans fats. Instead of simply relying on reported diet habits, it measured trans fat levels in participants’ blood — a more accurate indicator of the intake of trans fat, which the body cannot synthesize.

The bottom line on trans fats: Instead of processed snack foods, reach for whole foods, like fruits, vegetables, and whole grains. And when it comes to fat, choose the healthy forms found in fish, nuts, and olive oil.