You've probably heard that fat isn't the villain anymore. For a long time, the 1990s basically told us that if you touched a drop of oil, your heart would explode. We ate SnackWells cookies and plastic-tasting fat-free cheese. It was a dark time for flavor. But then the pendulum swung. Suddenly, everyone was putting sticks of butter in their coffee and eating bacon by the pound.
So, where does that leave us?
Honestly, it’s confusing. If you’re trying to figure out what are the bad fats, you have to look past the marketing. Fat is a macro-nutrient. You need it to absorb vitamins A, D, E, and K. You need it for your brain. But not all of it is your friend. Some fats are basically chemical sludge that your body doesn't know how to process, while others are just... okay in moderation but easy to overdo.
The Absolute Worst: Artificial Trans Fats
If there is a "black hat" in the world of nutrition, it’s trans fat. Specifically, industrial trans fats. You might see them on a label as "partially hydrogenated oils."
These are weird. They don't really exist in nature in high amounts. Scientists figured out that if you pump hydrogen into vegetable oil, it turns from a liquid into a solid. This was a massive win for food companies because it meant crackers could sit on a shelf for three years without going rancid. It gave pie crusts that perfect crumble.
But your arteries? They hate it.
The Harvard T.H. Chan School of Public Health has been pretty vocal about this for decades. Trans fats are a double whammy of bad news. They raise your LDL (the "bad" cholesterol) and simultaneously lower your HDL (the "good" stuff). Most other fats at least leave your good cholesterol alone. Not these guys. They actively sabotage the cleanup crew.
Even though the FDA basically banned added trans fats in the US a few years ago, they still linger. Some products can legally claim "0g trans fat" if there's less than 0.5 grams per serving. If you eat five servings of something "0g" but it has "partially hydrogenated" in the ingredients, you’re still getting hit. Fried foods in some restaurants might still be using older batches of oil or different blends. It's sneaky.
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Saturated Fat: The Great Debate
This is where things get messy. For years, saturated fat was the primary answer when people asked what are the bad fats. It’s the stuff that’s solid at room temperature—think butter, lard, the white strip on a New York strip steak, and coconut oil.
The old-school thinking was simple: saturated fat clogs your pipes.
But recent science, like a massive meta-analysis published in the British Medical Journal, suggests the link between saturated fat and heart disease isn't as "open and shut" as we thought. Some people can eat a fair amount of it and their blood markers stay perfect. Others eat a little bit and their LDL skyrockets. It's often down to your genetics and what else you're eating.
If you're eating a high-saturated fat diet along with a ton of refined sugar and white bread—the classic "Western Diet"—you’re basically asking for inflammation. Saturated fat seems to be a lot more "bad" when it's paired with high-carb processed junk.
It's also about the source.
- Saturated fat from highly processed sausage or pepperoni? Generally bad.
- Saturated fat from a piece of grass-fed beef or a bit of full-fat Greek yogurt? Probably fine for most people.
- Coconut oil? It’s high in lauric acid, which acts a bit differently than animal fats, but it's still 80-90% saturated. Don't drink it by the gallon.
The Problem with Pro-inflammatory Seed Oils
This is a controversial one. If you hang out on "Wellness Twitter" or TikTok, you’ll hear people screaming about "seed oils" like they're poison. We're talking soybean oil, corn oil, cottonseed oil, and sunflower oil.
Are they "bad fats"? Kinda. It depends on who you ask and how they're used.
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These oils are high in Omega-6 fatty acids. We need Omega-6s, but humans evolved on a diet where the ratio of Omega-6 to Omega-3 was roughly 1:1. Today, thanks to processed snacks and fast food, most of us are eating a ratio closer to 15:1 or even 20:1.
When you have that much Omega-6 floating around, it can trigger systemic inflammation. Dr. Chris Knobbe, a physician who has researched this extensively, argues that these refined oils are actually more responsible for modern chronic diseases than sugar is. That’s a bold claim, and not every doctor agrees, but the sheer volume of these oils in our food supply is definitely a red flag.
Also, these oils are very unstable. When they're heated over and over again in a restaurant deep fryer, they oxidize. You're essentially eating "stressed" fats that create free radicals in your body. That's never a good thing.
Identifying the Culprits in the Wild
You're at the grocery store. You’re looking at a box of frozen biscuits or a bag of "veggie" chips. How do you actually spot what are the bad fats in the real world?
Look for the word "hydrogenated." If it's there, put the box back. It’s a relic of a food system that didn't care about your heart.
Check the "Refined" label. If an oil is "highly refined," it’s been treated with chemicals like hexane to get the oil out of the seed, then bleached and deodorized. It’s a far cry from an olive being pressed into juice.
Think about stability. If you're cooking at high heat, using a delicate fat like unrefined walnut oil is actually a bad idea. It'll smoke and break down into toxic compounds. For high heat, you want something stable like avocado oil or even ghee (clarified butter), which has the milk solids removed so it doesn't burn as easily.
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Why "Fat-Free" is Often Worse
We can't talk about bad fats without talking about the replacement. When food companies took the fat out of cookies and salad dressings, they had to make it taste like something.
They used sugar. Tons of it.
Your body actually knows what to do with most fats. It knows how to burn them or store them. But when you hit your liver with massive amounts of high-fructose corn syrup from a "low-fat" snack, it triggers de novo lipogenesis. Basically, your liver turns that sugar into fat—specifically, a type of fat called triglycerides that circulates in your blood and increases your risk of a stroke.
So, ironically, eating a "fat-free" diet can often lead to having more "bad fat" in your bloodstream than eating a moderate-fat diet would.
Actionable Steps for a Greener Arterial Path
Stop stressing about every single gram of fat. It’s exhausting and honestly doesn't help much. Instead, focus on the big wins.
- Purge the pantry of "Partially Hydrogenated" anything. This is the easiest win. It’s the one fat everyone agrees is objectively harmful.
- Swap your "vegetable oil" for extra virgin olive oil. Use the good stuff for dressings and low-heat cooking. It’s packed with polyphenols that actually protect your heart.
- Be wary of the "Deep Fryer Cycle." Most fast food is fried in soybean or corn oil that has been heated and cooled for days. If you're going to eat fried food, make it a rare treat, not a Tuesday night staple.
- Watch the pairings. If you’re having a high-fat meal (like a steak or a cheesy dish), skip the massive pile of fries or the giant soda. Saturated fats are much harder on your body when your insulin is spiked from sugar.
- Cold-water fish is your best friend. Salmon, sardines, and mackerel are loaded with Omega-3s. These are the "anti-bad fats." They help balance out the inflammation from the other oils you inevitably consume.
The reality is that "bad fats" aren't just one thing. It’s a spectrum. It’s about how processed the fat is, how much heat it’s been exposed to, and what you’re eating alongside it. Your body is resilient, but it’s not designed to run on industrial lubricants and chemically altered oils. Stick to things that look like where they came from—an olive, an avocado, a piece of fish—and you’ll be ahead of 90% of the population.
Focus on whole, single-ingredient sources. If the ingredient list looks like a chemistry final, the fats inside it probably aren't doing you any favors. Pay attention to how you feel after a meal; your body usually tells you when you've loaded up on the wrong kind of fuel. Use avocado oil for your high-heat searing, save the butter for a bit of flavor at the end, and keep the processed seed oils to a minimum. Small shifts in your cooking oil choices today pay massive dividends in your cardiovascular health ten years from now.