Dietary fat is one of the three macronutrients your body needs to function, alongside protein and carbohydrates. It provides 9 calories per gram, more than double the 4 calories per gram from protein or carbs, making it the most energy-dense macronutrient. Far from being something to avoid, fat plays essential roles in building cells, producing hormones, and absorbing vitamins.
What Fat Does in Your Body
Fat is the primary structural material of every cell membrane in your body. Without it, cells couldn’t maintain their shape or communicate with each other. Beyond that basic architecture, fat serves as a long-term energy reserve, insulates organs, and helps regulate body temperature.
Your body also relies on dietary fat to absorb four key vitamins: A, D, E, and K. These are fat-soluble, meaning they dissolve in fat rather than water. Without enough fat in a meal, those vitamins pass through your digestive system largely unused. Fat is also a precursor for steroid hormones and sex hormones, and it plays a role in vitamin D synthesis.
The Four Main Types of Dietary Fat
Saturated Fat
Saturated fat has no double bonds in its chemical structure, which means the carbon chain is fully packed with hydrogen atoms. This tight packing is why saturated fats tend to be solid at room temperature. Butter, cheese, red meat, and coconut oil are common sources. Saturated fat raises LDL (“bad”) cholesterol in the blood, though it also raises HDL (“good”) cholesterol. The net effect on heart disease risk is why health guidelines still recommend limiting it.
Monounsaturated Fat
Monounsaturated fat has one double bond in its structure, which creates a small bend in the molecule. That bend prevents the fat molecules from packing tightly together, keeping them liquid at room temperature. Olive oil is the classic example. Avocados, almonds, hazelnuts, peanuts, pecans, and canola oil are also rich sources. Replacing saturated fat with monounsaturated fat lowers LDL cholesterol without lowering HDL, producing a favorable ratio between the two.
Polyunsaturated Fat
Polyunsaturated fat has two or more double bonds, making it even more liquid and flexible. This category includes the omega-3 and omega-6 fatty acids, which are discussed below. You’ll find polyunsaturated fats in fatty fish like salmon, mackerel, herring, and trout, as well as in walnuts, flaxseeds, sunflower seeds, and oils like corn, soybean, and sunflower oil. Swapping saturated fat for polyunsaturated fat reduces LDL cholesterol and improves the total cholesterol-to-HDL ratio, which is a key marker for cardiovascular risk.
Trans Fat
Trans fats occur when hydrogen atoms end up on opposite sides of a double bond, creating a straighter, stiffer molecule that behaves more like saturated fat. Small amounts exist naturally in meat and dairy from ruminant animals like cows and sheep. The bigger concern has been artificial trans fat, created by partially hydrogenating vegetable oils to make them solid and shelf-stable.
Artificial trans fat raises LDL cholesterol and is strongly linked to heart disease. In 2015, the FDA determined that partially hydrogenated oils were no longer “Generally Recognized as Safe,” and manufacturers were required to remove them from foods by January 2021. In 2023, the FDA issued a final rule removing outdated references to these oils from food regulations. While artificial trans fats have been largely eliminated from the U.S. food supply, the naturally occurring versions in animal products remain unaffected by these rules.
Essential Fatty Acids: Omega-3 and Omega-6
Your body can manufacture most of the fats it needs from other nutrients. Two exceptions are linoleic acid (an omega-6 fat) and alpha-linolenic acid (an omega-3 fat). Because your body cannot produce these, they must come from food. Linoleic acid is found in soybean, safflower, and corn oils, as well as nuts, seeds, and some vegetables. Alpha-linolenic acid is concentrated in flaxseeds, walnuts, canola oil, and their respective oils.
Both omega-3 and omega-6 fats are important, but the ratio between them matters. Humans evolved eating roughly equal amounts of each. Modern Western diets contain 15 to 17 times more omega-6 than omega-3, largely because of the prevalence of soybean and corn oils in processed foods. This imbalance promotes inflammation and has been linked to higher rates of cardiovascular disease, certain cancers, and autoimmune conditions. Research has found that a ratio of 2 to 3 parts omega-6 for every 1 part omega-3 suppressed inflammation in people with rheumatoid arthritis, while a ratio of 5 to 1 showed benefits for asthma. A ratio of 10 to 1 already showed adverse effects.
Shifting this balance doesn’t require precise math. Eating fatty fish a couple of times a week, cooking with olive or canola oil instead of corn or soybean oil, and adding flaxseeds or walnuts to your diet can meaningfully increase your omega-3 intake.
How Much Fat You Should Eat
The World Health Organization recommends that adults get no more than 30% of their total calories from fat. Within that, saturated fat should account for less than 10% of calories, and trans fat should stay below 1%. These same thresholds apply to children over age 2. For someone eating 2,000 calories a day, that works out to roughly 65 grams of total fat and no more than 22 grams of saturated fat.
The emphasis in current guidelines is less about cutting total fat and more about choosing the right kinds. Most of your fat intake should come from unsaturated sources: olive oil, nuts, seeds, avocados, and fish. Replacing saturated fat with polyunsaturated or monounsaturated fat consistently improves cholesterol markers linked to heart disease.
Practical Food Sources at a Glance
Knowing which fats fall into which category makes grocery shopping and cooking simpler.
- Monounsaturated fat: avocados, olive oil, canola oil, peanut oil, almonds, hazelnuts, pecans, pumpkin seeds, sesame seeds
- Polyunsaturated fat: salmon, mackerel, trout, herring, tuna, walnuts, pine nuts, flaxseeds, sunflower seeds, corn oil, soybean oil
- Saturated fat: butter, cheese, whole milk, red meat, coconut oil, palm oil
Many foods contain a mix of fat types. An avocado is mostly monounsaturated but also contains some saturated and polyunsaturated fat. Salmon delivers polyunsaturated omega-3s alongside smaller amounts of saturated fat. The goal isn’t to eat only one type but to tilt the overall balance toward unsaturated sources while keeping saturated and trans fats low.
Fat and Cholesterol
Your body produces its own cholesterol, which it uses to build cell membranes, synthesize vitamin D, and make hormones. Dietary cholesterol (found in egg yolks, shellfish, and organ meats) has a more modest effect on blood cholesterol than once believed. The type of fat you eat has a larger influence. Saturated fat consistently raises LDL cholesterol, while polyunsaturated and monounsaturated fats lower it. That shift in LDL is the primary mechanism by which swapping fat types reduces cardiovascular risk.
One nuance: saturated fat raises both LDL and HDL cholesterol, so the overall ratio of total cholesterol to HDL doesn’t always change. That’s partly why the relationship between saturated fat and heart disease is more complex than “all saturated fat is bad.” Still, the weight of evidence supports replacing it with unsaturated alternatives when you can.