Fats are one of three macronutrients your body needs to function, alongside protein and carbohydrates. At 9 calories per gram, fat is the most energy-dense macronutrient, more than double the 4 calories per gram that protein and carbohydrates each provide. That caloric density is part of why fats got a bad reputation in past decades, but they play roles in your body that no other nutrient can fill.
What Fats Do in Your Body
Every cell in your body depends on fats. Your cell membranes are built from fatty compounds called phospholipids, which act as gatekeepers controlling what enters and exits each cell. Cholesterol, another type of fat, provides structural support to those membranes. Without these lipids, your cells couldn’t maintain their shape or communicate with each other.
Beyond structure, fats serve as your body’s primary long-term energy reserve. When you eat more calories than you immediately need, your body converts the excess into triglycerides and stores them in fat tissue for later use. Fats also help your body produce steroid hormones, which send chemical signals between cells. And they’re essential for absorbing fat-soluble vitamins: A, D, E, and K. These vitamins dissolve in fat rather than water, so eating them alongside healthy fats (like avocados, olive oil, or nuts) helps your body actually absorb them. If you eat a varied diet with whole foods, you’re likely getting enough fat at meals for this absorption to happen naturally. If you take fat-soluble vitamin supplements, pairing them with a meal or snack improves uptake.
The Four Main Types of Dietary Fat
Saturated Fat
Saturated fats have no double bonds between the carbon atoms in their molecular chains. This structure makes them solid at room temperature, which is why butter, coconut oil, and the fat on a steak are firm when cool. Eating saturated fat tends to raise LDL cholesterol (the type linked to plaque buildup in arteries). The American Heart Association recommends keeping saturated fat below 6% of your total daily calories. On a 2,000-calorie diet, that’s about 13 grams, or roughly the amount in a tablespoon and a half of butter.
Monounsaturated Fat
Monounsaturated fats have one double bond in their carbon chain, which creates a slight bend in the molecule and keeps the fat liquid at room temperature. Olive oil is the classic example. Replacing saturated fats with monounsaturated fats can improve blood cholesterol levels and lower your risk of heart disease.
Polyunsaturated Fat
Polyunsaturated fats have more than one double bond, making them even more fluid. Canola oil, sunflower oil, and the fat in fish and walnuts fall into this category. This group includes two fatty acids your body cannot make on its own: linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3). Because your body lacks the enzymes to produce them, these are called essential fatty acids, and you must get them from food.
Trans Fat
Industrial trans fats are created by pumping hydrogen into liquid vegetable oil, turning it into a solid (partially hydrogenated oil). This process extends shelf life and improves texture in processed foods. It also creates a fat that is uniquely harmful. High trans fat intake increases the risk of death from any cause by 34% and raises the risk of coronary heart disease by 21%, according to data compiled by the World Health Organization. Many countries have banned or restricted partially hydrogenated oils. Small amounts of trans fat also occur naturally in meat and dairy, but the industrial form poses the greatest concern.
Essential Fatty Acids: Omega-3 and Omega-6
Your body can build saturated fats and some monounsaturated fats from the carbon in carbohydrates and proteins. It cannot, however, create omega-6 or omega-3 fatty acids from scratch. Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) serve as the starting materials your body uses to produce longer-chain fats involved in inflammation, blood clotting, and brain function.
Most Americans eat about 10 times more omega-6 fats than omega-3 fats. A low omega-3 intake is clearly linked to worse cardiovascular health, so narrowing that gap is worthwhile. But the best approach, according to both the American Heart Association and Harvard Health, is to add more omega-3s rather than cut omega-6s. Good omega-3 sources include fatty fish (salmon, mackerel, sardines), flaxseed, chia seeds, and walnuts. Omega-6 fats from whole foods like nuts, seeds, and vegetable oils are not harmful and don’t need to be restricted.
How Much Fat You Need
The World Health Organization recommends that adults get between 15% and 30% of their total daily calories from fat. On a 2,000-calorie diet, that works out to roughly 33 to 67 grams of fat per day. Going below 15% can impair hormone production and vitamin absorption. Going consistently above 30% without adjusting other calorie sources may contribute to unhealthy weight gain, largely because fat is so calorie-dense.
The type of fat matters as much as the amount. A practical approach: keep saturated fat under 6% of calories, avoid industrial trans fats entirely, and fill the rest of your fat intake with monounsaturated and polyunsaturated sources. That means cooking with olive or canola oil instead of butter, snacking on nuts rather than processed pastries, and eating fish a couple of times a week.
How Fat Affects Cholesterol
Your liver produces cholesterol on its own, but the types of fat you eat influence how much circulates in your blood and in what form. Saturated and trans fats tend to raise LDL cholesterol, the kind that contributes to arterial plaque. Unsaturated fats, when used to replace saturated fat in the diet, can improve your overall cholesterol profile. This is one of the most well-established findings in nutrition science, and it’s the basis for most dietary fat recommendations.
It’s worth noting that dietary cholesterol itself (from eggs, shellfish, and organ meats) has a smaller effect on blood cholesterol than the type of fat you eat. For most people, the saturated fat in a breakfast of bacon and buttered toast matters more than the cholesterol in the eggs beside it.
Common Fat Sources by Type
- Saturated: butter, cheese, red meat, coconut oil, palm oil
- Monounsaturated: olive oil, avocados, almonds, peanuts
- Polyunsaturated (omega-6): sunflower oil, soybean oil, corn oil, walnuts
- Polyunsaturated (omega-3): salmon, mackerel, sardines, flaxseed, chia seeds
- Trans (industrial): some margarines, fried fast food, packaged baked goods made with partially hydrogenated oil
Reading nutrition labels helps you track what you’re actually eating. In the U.S., labels list total fat, saturated fat, and trans fat. If the ingredient list includes “partially hydrogenated oil,” the product contains industrial trans fat, even if the label rounds the amount down to zero.