Oils provide your body with fatty acids it literally cannot make on its own, deliver the building blocks for hormones, and unlock the absorption of several critical vitamins. Without adequate fat in your diet, these processes slow down or stall entirely. The World Health Organization recommends that adults get up to 30% of their daily calories from fat, and the type of oil you choose matters as much as the amount.
Your Body Cannot Make Its Most Important Fats
Two fatty acids are classified as “essential” because human cells lack the biological machinery to produce them: linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3). You can only get them from food, and oils are among the most concentrated sources. Linoleic acid is abundant in sunflower, soybean, and corn oils. Alpha-linolenic acid is found in flaxseed, walnut, and canola oils.
These two fats serve as raw material for longer-chain fatty acids your body uses for brain function, immune response, and cardiovascular health. But the conversion process is inefficient. Your body converts less than 5% of alpha-linolenic acid into DHA, the omega-3 most critical for the brain, and only 5 to 10% into EPA, which helps regulate inflammation. That low conversion rate is why many nutrition experts recommend also eating fatty fish or algae-based sources that supply DHA and EPA directly.
Vitamins A, D, E, and K Need Fat to Work
Four vitamins dissolve only in fat, not water. When you eat foods containing vitamins A, D, E, or K, your small intestine packages them into tiny lipid clusters called micelles. These clusters have a fat-friendly interior that carries the vitamins across the intestinal wall and into your bloodstream. Without enough dietary fat present in the same meal, this transport system doesn’t form properly and the vitamins pass through you largely unabsorbed.
This is why a fat-free salad dressing on a plate of vegetables can actually reduce the nutritional value of the meal. A drizzle of olive oil or a few slices of avocado gives your gut the fat it needs to extract the full benefit from those greens. The process also depends on bile and pancreatic enzymes, both of which are triggered by the presence of fat in your digestive tract.
Hormones Start With Cholesterol
Every steroid hormone in your body, including estrogen, testosterone, progesterone, and cortisol, is built from cholesterol. The synthesis begins inside your cells’ mitochondria, where cholesterol is chemically modified into pregnenolone, the precursor molecule for all other steroid hormones. From there, different organs shape pregnenolone into the specific hormones they need: the ovaries and testes produce sex hormones, while the adrenal glands release cortisol and aldosterone.
Your liver produces most of its own cholesterol, but dietary fats influence the overall supply and the balance of cholesterol types circulating in your blood. Chronically low fat intake can disrupt hormonal balance, which may show up as irregular menstrual cycles, low energy, poor stress tolerance, or reduced bone density over time.
Your Brain Is Mostly Fat
Lipids make up about 60% of the brain’s dry weight, primarily in the form of phospholipids that build the membranes surrounding every neuron. DHA, the omega-3 fatty acid your body struggles to produce efficiently, plays a particularly important structural role. Its long carbon chain and six double bonds give neuronal membranes their flexibility, allowing receptors, ion channels, and signaling proteins to move freely across the cell surface. That fluidity is what makes efficient communication between brain cells possible.
Research links omega-3 intake to support for cognitive function across the lifespan, from fetal brain development through aging. Adequate omega-3 consumption is associated with lower risk of neurodegenerative conditions like Alzheimer’s disease and dementia, though the strength of that protection likely depends on long-term dietary patterns rather than short-term supplementation.
How Oils Affect Cholesterol and Heart Health
Not all fats treat your cardiovascular system the same way. Saturated fats, found in butter, coconut oil, and animal fats, raise LDL cholesterol by increasing the production of LDL particles in the blood while simultaneously slowing the rate at which your liver clears them. Trans fats have a similar detrimental effect.
Monounsaturated fats (olive oil, avocado oil) and polyunsaturated fats (found in many seed and nut oils) work in the opposite direction. They increase the number of LDL receptors on liver cells, which pulls more LDL cholesterol out of circulation and breaks it down. This is one reason the Mediterranean diet, which relies heavily on olive oil, consistently shows cardiovascular benefits in large studies.
The ratio of omega-6 to omega-3 fats in your diet also matters for heart health and inflammation. Humans likely evolved eating these two types in roughly equal amounts, but the modern Western diet delivers them in a ratio of about 15 to 1, heavily skewed toward omega-6. In studies on cardiovascular disease prevention, bringing that ratio down to 4 to 1 was associated with a 70% decrease in total mortality. For inflammatory conditions like rheumatoid arthritis, a ratio of 2 to 3 to 1 suppressed inflammation effectively. You can shift your ratio by using more omega-3-rich oils and eating fatty fish regularly, while reducing heavy use of corn and soybean oils.
Oils and Your Skin Barrier
Your skin’s outermost layer is waterproofed by a precise mixture of lipids: roughly 50% ceramides, 25% cholesterol, and 10 to 20% free fatty acids. These lipids fill the gaps between skin cells and form layered sheets that prevent water from escaping. When this barrier breaks down, skin becomes dry, cracked, and more vulnerable to irritants and infection.
Dietary omega-3 fatty acids support this barrier in several ways. EPA has been shown in animal studies to influence ceramide production in skin. DHA can increase the production of filaggrin, a protein essential for healthy skin cell development, while also reducing inflammation. GLA, an omega-6 fat found in evening primrose and borage oils, is linked to increased ceramide synthesis and improved barrier function. While the research is still developing, these findings suggest that the oils you eat have a direct impact on how well your skin holds moisture and resists damage.
Fat Provides Dense, Sustained Energy
Fat is the most calorie-dense macronutrient, delivering 9 calories per gram compared to 4 for protein or carbohydrates. This caloric density is not a drawback when consumed in appropriate amounts. It means oils and other fats provide a concentrated source of energy that your body can store efficiently and draw on between meals or during prolonged physical activity.
Dietary fat also slows digestion. Meals containing fat take longer to leave the stomach, which contributes to the feeling of fullness after eating. This slower transit helps moderate blood sugar spikes that can follow high-carbohydrate meals eaten alone. Including a moderate amount of oil in a meal, whether as a cooking fat or in a dressing, can help stabilize your energy levels over the hours that follow.
Physical Protection for Organs
Beyond its metabolic roles, the fat tissue your body builds from dietary fats serves a purely mechanical function. Subcutaneous fat beneath the skin acts as both a shock absorber and thermal insulation. Omental fat, a sheet of fatty tissue draped over the intestines, cushions and protects internal organs from physical injury. These protective fat deposits require ongoing dietary fat intake to maintain, which is one reason extremely low body fat levels carry health risks of their own.
Choosing the Right Oils
The goal is not simply to eat more fat but to prioritize the types that support these biological functions. Olive oil and avocado oil are rich in monounsaturated fats that lower LDL cholesterol. Flaxseed oil and walnut oil supply alpha-linolenic acid, the plant-based omega-3. Fatty fish and fish oil provide the preformed DHA and EPA your brain and cardiovascular system rely on most.
Limit oils high in saturated fat and avoid partially hydrogenated oils, which contain trans fats. Keep your overall fat intake at or below 30% of daily calories, and within that budget, shift the balance toward unsaturated sources. Even small changes, like cooking with olive oil instead of butter or adding ground flaxseed to a smoothie, move the ratio of fats in your diet in a more favorable direction.