Harvard Health Letter

Where the worst type of fat is hiding in supermarket foods

Trans fats are undeniably bad for health, and they’re still in many foods.

Lurking on supermarket shelves, within colorful, seemingly harmless packages, is something that can cause serious harm to your health: trans fat. “No amount of trans fat is acceptable, from a health standpoint,” says registered dietitian Kathy McManus, director of the Department of Nutrition at Harvard-affiliated Brigham and Women’s Hospital.

About trans fat

Trans fats in the diet come mostly from partially hydrogenated oils. They’re made by a process that uses hydrogen gas to turn vegetable oils into solids. This new form of fat extends a food’s shelf life and improves flavor and texture. It’s been a mainstay in processed foods—such as pastries, crackers, margarine, and corn chips—for decades.

But artificial trans fats are the worst fats you can eat. They increase “bad” LDL cholesterol, decrease “good” HDL cholesterol, raise the risk of blood clots, and boost inflammation—all of which increase the risk of heart disease, stroke, and diabetes. Trans fats are so bad for you that the FDA is essentially banning them in processed food starting in 2018.

Still a danger

Meanwhile, food manufacturers are allowed to use partially hydrogenated oils in their products, and so are restaurants. And if you’re not savvy about reading Nutrition Facts labels, you may not detect the trans fat in your food. “The FDA doesn’t require trans fat to be listed until there’s a half gram or more per serving,” explains McManus, “so the label may show zero grams of trans fat, even if a serving contains almost half a gram.”

Are small amounts of trans fat dangerous? “It adds up, especially if you eat several foods with trans fat each day,” says McManus. Based on FDA estimates, researchers at the CDC report it is possible that eliminating trans fats in the diet may prevent as many as 10,000 to 20,000 heart attacks and 3,000 to 7,000 deaths from heart disease each year.

Become a detective

Start reading the ingredient lists on Nutrition Facts labels. If partially hydrogenated oil is among the ingredients, you’ll know the food contains trans fat, even if the label states that a serving has zero grams of trans fat.

All foods that aren’t fresh are suspect for trans fat. It’s hiding in many types of supermarket products, not just baked goods and margarines. Partially hydrogenated oils can be in frozen foods (fish sticks, pizza), cake and brownie mixes, ready-to-use frosting, ready-to-use dough (biscuits, cinnamon rolls), snacks (granola bars, microwave popcorn), coffee creamer, vegetable shortening, cereals, soup, salad dressing, dips, sauce mixes, peanut butter, taco shells, cocoa mix, and even low-fat ice cream.

Trans fats are also in many fast foods, especially French fries, and they may be in fresh bakery items, such as breads, cakes, pies, and cookies, even at a neighborhood bakery.

What about other fats?

All fat is high in calories (nine calories per gram of fat, versus four calories per gram of carbohydrate, for example). A high-calorie diet can lead to weight gain, which can lead to chronic health problems.

An excess of saturated fats (such as those found in whole milk, butter, and red meat) can increase “bad” LDL cholesterol and lead to heart disease. Limit saturated fats to less than 7% of your total daily calories or less than 12 grams in a 1,500-calorie diet.

Some fats, within calorie limits, are good for you. Such "good" fats include monounsaturated fat (such as those in olive and canola oils, most nuts, peanut butter, and avocados) and polyunsaturated fat (for instance, in salmon, mackerel, walnuts, and safflower oil). Both are associated with lower LDL cholesterol and total cholesterol when substituted for saturated fats.

Surprising sources of trans fat that list 0 grams on the Nutrition Facts label

Product type


Identifying ingredient

Frozen fish fillets

Sea Cuisine Potato-Crusted Cod

Partially hydrogenated soybean oil

Coffee drink mix

Hills Bros. Double Mocha Cappuccino

Partially hydrogenated coconut oil

Breakfast cereal

Kellogg’s Apple Jacks

Partially hydrogenated soy-bean and/or cottonseed oil

Seasoned bread crumbs


One or more partially hydrogenated oils (soybean, cottonseed, corn, canola)

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