What's the unhealthiest type of oil? A deep dive into dangerous fats

October 31, 2025 •

4 min read

According to the World Health Organization, more than 278,000 deaths are attributed to industrially produced trans fat every year. These dangerous fats, created through a process called hydrogenation, are the clear and definitive answer to the question: What's the unhealthiest type of oil?

Quick Summary

This guide examines the health dangers of partially hydrogenated oils (trans fats), including their negative impact on cholesterol levels, inflammation, and chronic disease risk. It compares different types of fats and provides actionable tips for choosing healthier cooking oils.

Key Points

Industrially produced trans fats are the unhealthiest type of oil. They are found in partially hydrogenated oils and have no known health benefits.
Trans fats raise 'bad' LDL cholesterol and lower 'good' HDL cholesterol. This significantly increases the risk of heart disease and stroke.
Hydrogenation is the chemical process that creates trans fats. It is used to solidify liquid vegetable oils and extend shelf life, particularly in processed foods.
Highly refined vegetable oils (seed oils) contain high levels of pro-inflammatory omega-6s. An unbalanced ratio of omega-6 to omega-3 is common in modern diets and linked to inflammation.
Healthier alternatives include extra virgin olive oil and avocado oil. These offer heart-healthy monounsaturated and polyunsaturated fats.
Reading ingredient lists is crucial. Look for terms like 'partially hydrogenated oil' or 'vegetable shortening' to spot hidden trans fats in packaged foods.

The Unquestionable Villain: Industrially Produced Trans Fats

When it comes to the world of cooking oils, the most dangerous and unhealthy offender is industrially produced trans fat, found in partially hydrogenated oils (PHOs). Unlike other fats that can offer health benefits in moderation, trans fats have no known health benefits and are actively harmful. The U.S. Food and Drug Administration (FDA) banned the use of partially hydrogenated oils in food manufacturing, a move that phased out most of this artificial fat from the food supply. However, trace amounts can still be present in some products, and their legacy serves as a warning against chemically altered oils.

The Manufacturing Process of Hydrogenation

Trans fats are the byproduct of hydrogenation, a process used to turn liquid vegetable oils into solid fats to increase their shelf life. It involves heating liquid oil to high temperatures and injecting it with hydrogen gas in the presence of a metal catalyst. This process alters the oil's chemical structure, creating trans fatty acids that are thick and stable at room temperature. This is why they were historically used in products like margarine, shortening, and commercial baked goods.

Health Consequences of Trans Fat Consumption

The health risks associated with consuming trans fats are serious and well-documented:

Cholesterol Havoc: Trans fats significantly increase low-density lipoprotein (LDL), or "bad" cholesterol, while simultaneously lowering high-density lipoprotein (HDL), or "good" cholesterol. This dual-action is particularly harmful for cardiovascular health, promoting the buildup of plaque in arteries.
Inflammation: Chronic inflammation is a known contributor to numerous health problems, and trans fats are a key trigger. This systemic inflammation is linked to heart disease, stroke, and type 2 diabetes.
Increased Chronic Disease Risk: Beyond heart disease, high consumption of trans fats is associated with an increased risk of stroke, insulin resistance, obesity, and certain cancers. Even small amounts can have a detrimental effect; for every 2% of daily calories from trans fat, the risk of heart disease rises by 23%.

Other Oils to Approach with Caution

While trans fats are the worst, other oils can also pose risks depending on their processing and how they are used. Highly refined vegetable and seed oils, while not containing trans fats, often come with their own set of potential drawbacks.

Highly Refined Vegetable and Seed Oils:

High Omega-6 Content: Oils like soybean, corn, and sunflower are high in omega-6 polyunsaturated fatty acids (PUFAs). While omega-6s are essential, a typical Western diet already contains a high amount, creating an imbalanced ratio with anti-inflammatory omega-3s. A healthy ratio is typically 1:1 to 4:1, but many modern diets have ratios as high as 15:1 or more, which promotes inflammation.
Instability: The high PUFA content in these oils makes them susceptible to oxidation when heated, especially for deep frying. This creates free radicals and other harmful compounds like acrolein, linked to cellular damage.
Heavy Processing: Many mass-produced vegetable oils are chemically extracted and highly refined, stripping them of beneficial compounds and potentially leaving trace chemicals.

Palm Oil:

Saturated Fat: Palm oil is high in saturated fat and can increase LDL cholesterol levels when consumed in excess. However, it also contains some unsaturated fats and antioxidants. Its health impact is debated, but moderation is key.
Refined Versions: As with other oils, heavily refined versions like palmolein are considered less healthy due to their processing.

Comparative Overview of Common Oils


Feature	Trans Fats (PHOs)	Refined Seed Oils	Saturated Fats (e.g., Palm, Coconut)	Healthy Unsaturated Fats (e.g., EVOO, Avocado)
Primary Fat Type	Artificial Trans Fat	Polyunsaturated (Omega-6)	Saturated Fat	Monounsaturated & Polyunsaturated (Omega-3)
Effect on Cholesterol	Increases LDL, Decreases HDL	Unbalanced Omega ratio can be pro-inflammatory	Increases LDL, though some nuance exists	Lowers LDL, boosts HDL
Heat Stability	Very stable, but toxic when created	Poor at high heat, prone to oxidation	Fairly stable for cooking	Good stability, especially refined versions
Health Impact	Highly detrimental, linked to major chronic diseases	Can promote inflammation in unbalanced diets	Best in moderation, can increase heart disease risk with high intake	Protective against heart disease and inflammation
Processing	Highly chemical, creates trans fat	Heavy refining, chemical extraction common	Varies, can be minimally processed (red palm oil) or refined	Varies, extra virgin is minimally processed

How to Choose the Healthiest Oils

To ensure your diet includes healthy fats, it's important to make informed choices at the grocery store and in the kitchen. Healthline provides a great resource for understanding healthy cooking oils.

Prioritize minimally processed oils: Look for labels like "cold-pressed," "unrefined," or "extra virgin." This indicates the oil retains more of its natural nutrients and antioxidants.
Consider smoke point: For high-heat cooking like deep frying, opt for oils with a high smoke point, such as refined avocado oil or refined coconut oil. For sautéing or salad dressings, extra virgin olive oil is an excellent choice.
Read ingredient lists: Always check the ingredients on packaged and processed foods. The inclusion of "partially hydrogenated oil," "hydrogenated oil," or "vegetable shortening" is a major red flag, regardless of the trans fat count listed on the nutrition label.
Vary your intake: Incorporate a variety of healthy fats into your diet. Sources like nuts, seeds, and fish offer beneficial polyunsaturated and monounsaturated fats that support overall health.

Conclusion

While navigating the world of dietary fats can seem complex, the most vital takeaway is clear: industrially produced trans fats are the unhealthiest type of oil and should be completely eliminated from your diet. By learning to identify and avoid partially hydrogenated oils in processed foods, minimizing the use of heavily refined vegetable oils, and opting for healthier alternatives like extra virgin olive oil and avocado oil, you can make significant strides toward protecting your heart and improving your overall nutrition.

Frequently Asked Questions

Not all saturated fats are created equal, and some health experts argue that their impact depends on the food source. While a diet high in saturated fat can raise LDL cholesterol, some fats like those in coconut oil also contain medium-chain triglycerides (MCTs) with potential benefits. However, a high intake can still be problematic for heart health, so unsaturated fats are generally preferred.

Partially hydrogenated oil contains harmful trans fats, which is why it was banned by the FDA. Fully hydrogenated oil has been processed to a point where it is fully saturated, and while it doesn't contain trans fats, it is very high in saturated fat.

A healthy omega-6 to omega-3 ratio is crucial for controlling inflammation. Omega-6 fatty acids can be pro-inflammatory, while omega-3s are anti-inflammatory. The typical Western diet is very high in omega-6s, leading to an imbalance that promotes chronic inflammation.

You don't need to avoid all vegetable oils, but be mindful of the type and how you use them. Highly refined seed oils (like corn and soybean) are often high in omega-6s and more prone to oxidation when heated repeatedly. Healthier plant-based options like olive oil and avocado oil are excellent alternatives.

Yes, in trace amounts. The FDA ban phased out the use of partially hydrogenated oils, but food manufacturers can still label products as having "0g trans fat" if a serving contains less than 0.5g. These small amounts can accumulate if you consume multiple servings.

The best oils for cooking are typically rich in monounsaturated fats and have a high smoke point. Excellent choices include extra virgin olive oil for low to medium-heat cooking, and avocado oil for high-heat applications like frying or roasting.

When an oil is heated past its smoke point, it starts to break down, burn, and produce harmful compounds like free radicals and acrolein. This not only affects the flavor of your food but can also contribute to cellular damage and inflammation.