It’s pretty much step one in the handbook of preventing heart disease: Swap artery-clogging saturated fats for their healthier, unsaturated counterparts. Except not all unsaturated fats will necessarily get the job done—and could even hurt your heart, according to new research.

Despite the fact that they help lower cholesterol, the polyunsaturated fats in certain vegetable oils could still actually increase the likelihood of death from coronary artery disease, finds a new Canadian Medical Association Journal analysis of more than 20 studies.

But wait, you ask, isn’t high cholesterol the thing that causes heart disease? In part, yes, but that’s not the whole picture: “There are several factors that can increase the risk of heart disease death, including diabetes, obesity, inflammation, and oxidative stress,” says study author Richard Bazinet, PhD, a professor of nutritional sciences at the University of Toronoto. And as it turns out, certain polyunsaturated fats are able to oxidize in our bodies relatively easily—specifically, ones that consist largely of omega-6 fatty acids, like corn and safflower oils. (Compared to others, like canola and soybean oils, which are mostly made up of omega-3 fatty acids and don’t easily oxidize).

Here’s the catch: Like omega-3s, omega-6 fatty acids are essential, which means they’re not produced by the body and therefore need to be obtained by food. The problem comes when we consume too many omega-6s—which is easy to do, since vegetable oils show up in tons of processed foods (even relatively healthy ones!) like cereal, energy bars, crackers, frozen foods, and more. “Experts recommend we should consume omega-6s to omega-3s in a 10:1 or 5:1 ratio, but most of us are closer to 20:1,” Dr. Bazinet says.

To bring those numbers back into balance, that means consuming more omega 3-rich fats and fewer omega 6-rich ones. “The oils containing higher level of omega-3s tend to show more protective effects in these coronary heart disease studies. The oils that have no omega-3s tend to show an increased risk,” Dr. Bazinet says.

In terms of vegetable oils, that means opting for canola or soybean over corn or safflower (be sure to choose organic varieties, since most conventional canola and soybean oils contain GMOs). Olive oil, too, is a smart choice, since its monounsaturated fats are made up of a third type of fatty acid, omega-9s, and won’t have any affect on your omega 3 to 6 ratios.

More from Prevention: 10 Minutes To A Stronger Heart

Marygrace Taylor Marygrace Taylor is a health and wellness writer for Prevention, Parade, Women’s Health, Redbook, and others.

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