Which Fats Are Good and Bad for Your Heart?

Updated July 2026

There was a time when all fat was labeled as evil and we were taught to avoid it as much as possible. But not all fat is created equal, and some is actually good for us. Unsaturated fats from plants and fish are good for your heart, trans fats are bad and should be avoided entirely, and saturated fats sit in the middle, where what you replace them with matters more than simply cutting them. Here's what you really need to know.

What are the different types of dietary fat?

Fats exist in four separate formats, and the differences come down to chemistry.

At the most basic level, a fat molecule consists of a long chain of carbon and hydrogen atoms. The way those atoms are bonded determines whether the fat is characterized as saturated, monounsaturated, polyunsaturated or trans, and how the fat behaves in your body.

What constitutes a saturated fat?

A saturated fat has a straight chain of carbon atoms linked entirely by single bonds, with every remaining spot filled, or "saturated", with hydrogen. The molecular structure looks something like this:

That tidy structure is what makes saturated fats solid at room temperature. Butter is the classic example, but the marbling in beef, fat in chicken, cheese, and cream are all part of this group. You might notice that these fats mostly come from animal sources, but coconut oil and palm oil also fall into the saturated fat category because they, too, are solid at room temperature (even though they come from plants).

Saturated fat intake has been associated with increased heart and vascular risk. This is at least in part due to the impact on LDL ("bad") cholesterol, which tends to go up when saturated fat intake increases. Some people are very sensitive to this effect. They go on a keto diet and their LDL goes through the roof.

How are monounsaturated and polyunsaturated fats different from saturated fats?

Monounsaturated fats have one double bond in the carbon chain, while polyunsaturated fats have more than one. The molecular structure of a polyunsaturated fat would look something like this (this one has two double bonds):

The lower number of hydrogen atoms allows these fats to remain liquid at room temperature. Examples of monounsaturated fats include olive oil and the oil in avocados. Polyunsaturated fats come primarily from fish but are also found in walnuts, chia and flax. Intake of unsaturated (either mono or poly) fats has been shown repeatedly to be associated with lower LDL cholesterol, higher HDL ("good") cholesterol and lower triglycerides, as well as a lower risk of heart disease.

Note one additional and important fact about the illustration above: in a healthy unsaturated fat, the hydrogen atoms around each double carbon bond sit on the same side of the chain. That is the shape nature intended.

What are trans fats and why are they so bad?

Trans fats, historically listed on food labels as "hydrogenated" or "partially hydrogenated" oils, are made by chemically forcing hydrogen atoms onto opposite sides of the carbon chain. The structure of a trans fat might look something like this:

That small change in how hydrogen atoms are oriented around the carbon atoms turns a liquid oil into a shelf-stable solid, and a cheap and convenient fat source for processed food manufacturers.

It also makes these fats harmful. Trans fats raise LDL, lower HDL, and increase inflammation, a worse combination than saturated fat ever produced. In fact, there is really no amount of trans fat that is considered safe for human consumption, which is why the U.S. Food and Drug Administration banned artificial trans fats outright. Manufacturers could no longer add partially hydrogenated oils as of 2018, and by 2021 they had been cleared from the food supply, a change estimated to prevent thousands of heart attacks and deaths a year.

There is real irony here. In the 1970s, when saturated fat was considered the main dietary villain, we were told to switch from butter to margarine, which was full of trans fat. The "healthier" choice turned out to be the more dangerous one.

This is largely a battle already won. Artificial trans fat has been almost entirely removed from what you'll find on store shelves. Two footnotes remain worth knowing: a product with less than half a gram per serving can still be labeled "0 grams trans fat," and small amounts of trans fat occur naturally in some meat and dairy. But the era when you had to scan every ingredient list for "partially hydrogenated" oils has, thankfully, largely passed.

Are saturated fats actually bad for you?

This is where the old advice was too simple. For years the message was just "eat less saturated fat," but it turns out that what you replace it with is what actually determines your risk. Cutting saturated fat only helps if you put something better in its place.

A large 2015 study followed nearly 130,000 adults for decades and showed exactly this. Replacing just 5% of your calories from saturated fat with an equal amount of polyunsaturated fat was associated with a 25% lower risk of heart disease. Swapping in monounsaturated fat lowered risk by 15%, and swapping in whole grains by 9%. But replacing that saturated fat with refined carbohydrates, the white bread and potatoes most people actually reach for (and what we did with gusto during the low fat craze), produced no benefit at all.

An easy way to picture it: if you swap the cheese on your plate for walnuts, you cut cardiovascular risk about 25%. Swap it for almonds, about 15%. Swap it for oatmeal, about 9%. Swap it for white bread, and you have gained nothing. It is not about eliminating a food group. It is about strategic substitution.

One last point. Turns out there are subtypes of saturated fat as well. Some saturated fat that comes from plants (like the saturated fat found in cocoa beans) is neutral when it comes to cholesterol, but accompanied by so many other beneficial compounds (antioxidants, flavonoids) that on balance these ingredients are actually cardioprotective. Hence the positive health outcomes seen in people who regularly eat small amounts of dark chocolate.

All to say that saturated fat is not something to panic over. You do not have to swear off steak forever. If you eat animal fats, choose high quality sources, keep portions reasonable, eat them less often, and surround them with the right foods, the salad or the steamed vegetables instead of the hash browns. And dark chocolate (70% cacao or greater) is, happily, good for you and can be enjoyed in moderation.

So what should you actually do about fat?

Keep it simple:

• Don't worry much about trans fats anymore. They've been largely removed from the food supply, though it's still worth skipping the rare product that lists anything "hydrogenated."
• Make unsaturated fats from whole plant foods your default, olive oil, nuts, seeds, and the omega-3s in walnuts, flax, chia, and fish.
• Do not fear saturated fat, but be strategic. When you cut back, replace it with unsaturated fats or whole grains, never refined carbs.
• Remember that fat is essential. Your body needs it, so the goal is better fat, not no fat.

This is exactly the thinking we built into Step One Foods. Our foods are rich in the monounsaturated and polyunsaturated fats and omega-3s shown to protect the heart, delivered through real whole-food ingredients rather than anything hydrogenated or processed. When you swap a couple of your usual daily foods for Step One servings, you are not just removing something harmful, you are adding something genuinely protective, which is the whole point of smart substitution.

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