Fats: Good, Bad and Confusing

The short version

Fat is essential, but with 9 calories per gram, it adds up quickly.
Unsaturated fats, like those in olive oil, nuts, avocado and fish, are good for your heart.
Limit saturated fat to under 10% of calories. Avoid trans fats completely.
Claims that seed oils are toxic aren't supported by human research.

Why you need it

Fat provides energy, helps build your cells, and lets you absorb vitamins A, D, E and K. It also makes food taste good, which helps you stick with your plan. Very-low-fat diets aren't necessary for most people.

The types

Type	Where it's found	What to do
Monounsaturated	Olive oil, avocado, nuts	Favor it
Omega-3 (polyunsaturated)	Salmon, sardines, walnuts, flax, chia	Favor it. Aim for fish twice a week
Other polyunsaturated	Canola, soybean and sunflower oils	Fine, and better than saturated fat
Saturated	Fatty meat, butter, cheese, coconut oil, palm oil	Limit it
Trans	Some shelf-stable baked goods and fried foods, older products	Avoid it

The arguments, sorted out

Saturated fat. The 2025 to 2030 US Dietary Guidelines kept the limit of 10% of daily calories, while also encouraging whole-fat dairy and some animal fats, which critics, including Harvard nutrition researchers, say is contradictory, since those foods make the limit hard to meet. What is well established is that replacing saturated fat with unsaturated fat lowers LDL cholesterol and heart disease risk. If your LDL is high, your limit may be lower. The American Heart Association has suggested about 6% for people who need to lower LDL.

Butter vs. olive oil. Studies consistently favor olive oil and other plant oils over butter.

Seed oils. Some online voices say canola and soybean oils are "toxic" or inflammatory. Human studies don't support this. When people replace saturated fat with these oils, LDL falls and heart disease risk tends to fall too. Deep-fried and ultra-processed foods made with them are a problem, mainly because of the foods themselves.

Coconut oil. It's high in saturated fat and raises LDL. It's fine occasionally, for flavor, but it isn't a health food.

Eggs. Most healthy people can eat them regularly. If you have diabetes or high LDL, ask your clinician.

How much to eat while losing weight

There's no single target, but 20 to 35% of calories is a common guideline. On 1,800 calories, that's about 40 to 70 grams.

Fat calories are easy to miss. One tablespoon of oil is about 120 calories, and an ounce of nuts is about 170. Keep eating them, but measure them for a week or two to calibrate your eye.

Practical swaps

Cook with olive or canola oil instead of butter.
Top toast with avocado or nut butter.
Pick fish twice a week, even canned.
Choose lean cuts of meat, and have cheese as a flavoring and not the main event.
Use nuts as a snack or salad topper in place of chips or croutons.

Sources

This guide is general information, not medical advice. Please check with a clinician before making big changes, especially if you have a health condition, take medication, or are pregnant. Read the full disclaimer.