Dietary fats provide 9 kcal per gram — more than twice the energy of carbohydrates or proteins. Beyond energy, fats are essential for cell membrane structure, absorption of fat-soluble vitamins, production of hormones, insulation and protection of organs, and signaling molecules.
Chemical Classification
Saturated fatty acids (SFAs). No double bonds between carbon atoms — the chain is fully saturated with hydrogen. Solid at room temperature. Found primarily in animal products (butter, cheese, whole milk, red meat, poultry skin, lard) and tropical oils (palm oil, coconut oil). Dietary SFAs raise LDL cholesterol and are associated with increased cardiovascular disease risk. The recommendation is to limit SFA intake to less than 10% of total calories.
Unsaturated fatty acids. One or more double bonds. Liquid at room temperature. Monounsaturated fatty acids (MUFAs) have one double bond. Found in olive oil, canola oil, avocados, nuts (almonds, cashews, peanuts), and sesame seeds. MUFAs reduce LDL cholesterol without lowering HDL. The Mediterranean diet, rich in olive oil, is associated with reduced cardiovascular disease. Polyunsaturated fatty acids (PUFAs) have two or more double bonds. Two essential PUFAs must be obtained from the diet because humans lack the enzymes to introduce double bonds at the omega-3 and omega-6 positions.
Essential fatty acids. Linoleic acid (LA, omega-6) and alpha-linolenic acid (ALA, omega-3) are the parent essential fatty acids. LA is found in vegetable oils (soybean, corn, sunflower, safflower), nuts, and seeds. ALA is found in flaxseed, chia seeds, walnuts, and canola oil.
Omega-3 fatty acids. ALA is converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the body, though conversion efficiency is low (5–15%). EPA and DHA are found directly in fatty fish (salmon, mackerel, sardines, herring, anchovies, tuna) and fish oil. EPA and DHA have anti-inflammatory effects, reduce triglycerides, lower blood pressure, improve endothelial function, and support brain and retinal health. The American Heart Association recommends at least two servings of fatty fish per week.
Omega-6 fatty acids. LA is converted to arachidonic acid, a precursor to pro-inflammatory eicosanoids. Modern Western diets typically have a high omega-6 to omega-3 ratio (15:1 to 20:1) compared to ancestral diets (approximately 2:1 to 4:1). A lower ratio is associated with reduced inflammation.
Trans fatty acids. Unsaturated fatty acids with trans configuration (hydrogens on opposite sides of the double bond). Small amounts occur naturally in ruminant fat (dairy, beef). Industrially produced trans fats (partially hydrogenated oils) are created by hydrogenating vegetable oils to increase solidity and shelf life. Trans fats raise LDL cholesterol, lower HDL cholesterol, promote inflammation, and increase cardiovascular disease risk. The FDA determined that partially hydrogenated oils are not generally recognized as safe (GRAS), and they have been largely eliminated from the US food supply.
Digestion and Absorption
Fat digestion begins in the stomach with gastric lipase, which digests a small portion of triglycerides. In the small intestine, bile salts (produced by the liver, stored in the gallbladder) emulsify fat globules into micelles, increasing surface area for pancreatic lipase. Pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids. These, along with bile salts, form mixed micelles that transport lipids to the enterocyte surface. Within enterocytes, monoglycerides and fatty acids are reassembled into triglycerides and packaged into chylomicrons (lipoprotein particles) for transport via the lymphatic system. Chylomicrons enter the bloodstream at the thoracic duct and deliver triglycerides to peripheral tissues (adipose, muscle).
Dietary Recommendations
The AMDR for total fat is 20–35% of total calories. Saturated fat: less than 10% of total calories (American Heart Association recommends less than 7% for those with elevated LDL). Trans fat: as low as possible (ideally 0 g/day). Omega-3s: at least two servings of fatty fish per week (providing approximately 250–500 mg EPA/DHA daily). Cholesterol: no specific daily limit, but intake should be minimal from trans and saturated fat sources.
Food Sources
Unsaturated fat sources: olive oil, canola oil, avocado, nuts (almonds, walnuts, pistachios), seeds (flaxseed, chia, sunflower, pumpkin), fatty fish (salmon, mackerel, sardines), and nut butters.
Saturated fat sources: butter, cheese, whole milk, cream, red meat, processed meats (sausage, bacon), poultry skin, coconut oil, palm oil, and many baked goods.
Trans fat sources: largely eliminated from processed foods but may still be present in some baked goods, fried foods, and stick margarines. Check ingredient lists for partially hydrogenated oils.
Health Effects
Replacing saturated fats with unsaturated fats (particularly PUFAs) reduces cardiovascular disease risk. Replacing SFAs with refined carbohydrates does not reduce risk and may increase triglycerides. The Mediterranean diet — rich in olive oil, nuts, fish, and whole foods — is the most extensively studied dietary pattern for cardiovascular health, reducing major cardiovascular events in the PREDIMED trial by 30%.
Summary
Dietary fats are essential for energy, membrane structure, and signaling. Unsaturated fats (MUFAs and PUFAs) are beneficial, while saturated and trans fats increase cardiovascular risk. Omega-3 fatty acids have anti-inflammatory and cardioprotective effects. The Mediterranean diet pattern emphasizes unsaturated fat sources and is associated with reduced cardiovascular disease. Fat intake should account for 20–35% of total calories, with emphasis on unsaturated sources.