Fat
What it does
Fat is a concentrated source of energy and a major structural component of the body. Fatty acids are built into cell membranes, stored in adipose tissue, and used to make signaling compounds. Cholesterol is the starting material for steroid hormones, while specific polyunsaturated fatty acids are used to make compounds involved in inflammation, clotting, and blood-vessel function.
Dietary fat helps absorb vitamins A, D, E, and K and supplies the two fatty acids that humans cannot make in sufficient amounts.
Fat also slows gastric emptying and can increase how long a meal feels filling. The effect depends on the rest of the meal as well as the amount and type of fat.
How much fat do adults need?
There is no RDA for total fat in healthy adults. The adult Acceptable Macronutrient Distribution Range is 20 to 35 percent of total calories.
| Daily calories | 20% from fat | 35% from fat |
|---|---|---|
| 1,600 | ~36 g | ~62 g |
| 2,000 | ~44 g | ~78 g |
| 2,500 | ~56 g | ~97 g |
| 3,000 | ~67 g | ~117 g |
The range is broad because diets with substantially different fat intakes can meet nutritional needs. The foods supplying that fat and the foods replacing it when intake changes are also important.
The types of fat
Most dietary fat is made of triglycerides: three fatty acids attached to a glycerol backbone. Foods contain mixtures of fatty acids, so terms such as saturated, monounsaturated, and polyunsaturated describe the predominant pattern rather than an exclusive ingredient.
Saturated fatty acids have no carbon-carbon double bonds. Major sources include butter, cheese, fatty meats, coconut oil, palm oil, and cocoa butter. Individual saturated fatty acids can affect blood lipids differently.
Monounsaturated fatty acids contain one double bond. Oleic acid is the most common and is abundant in olive oil, avocado, almonds, peanuts, canola oil, and cocoa butter.
Polyunsaturated fatty acids contain two or more double bonds. They include the omega-6 and omega-3 families. Common sources include plant oils, nuts, seeds, flax, chia, walnuts, and fatty fish.
A food’s fatty-acid profile matters more than whether it came from a plant or an animal. Coconut is a plant food rich in saturated fat. Salmon is an animal food rich in long-chain omega-3 fats.
The two essential fatty acids
Linoleic acid (LA) and alpha-linolenic acid (ALA) are the two fatty acids with established essential dietary requirements for adults. Humans lack the enzymes needed to place double bonds at the positions required to make them from other fatty acids.
| Group | Linoleic acid (omega-6) | ALA (omega-3) |
|---|---|---|
| Men 19–50 | 17 g/day | 1.6 g/day |
| Men 51+ | 14 g/day | 1.6 g/day |
| Women 19–50 | 12 g/day | 1.1 g/day |
| Women 51+ | 11 g/day | 1.1 g/day |
Linoleic acid is widely available in soybean, corn, sunflower, safflower, and other plant oils, as well as nuts and seeds.
ALA is concentrated in flax, chia, walnuts, canola oil, and some other plant foods.
The body can convert ALA into the longer-chain omega-3 fatty acids EPA and DHA, but conversion is limited. Fatty fish and seafood provide EPA and DHA directly. Algae are the original source in the marine food chain and are used to produce vegan EPA and DHA oils. The United States has not established an RDA or AI specifically for EPA or DHA.
Vitamin E helps protect polyunsaturated lipids from oxidation. Nuts, seeds, and many plant oils provide both polyunsaturated fat and vitamin E.
Where different fats come from
| Food or fat | Typical fatty-acid pattern |
|---|---|
| Olive oil | Mostly monounsaturated, especially oleic acid |
| Avocado | Mostly monounsaturated |
| Almonds and peanuts | Mostly monounsaturated with some polyunsaturated fat |
| Walnuts | High in polyunsaturated fat, including ALA |
| Flax and chia | Rich in ALA omega-3 |
| Soybean and canola oils | Mixture of polyunsaturated and monounsaturated fats |
| Salmon, sardines, and other fatty fish | EPA and DHA plus other fats |
| Butter and cheese | Saturated plus monounsaturated fat |
| Coconut oil | Predominantly saturated, particularly lauric acid |
| Cocoa butter | Stearic and palmitic saturated fats plus substantial oleic acid |
How dietary fat becomes energy
Bile disperses dietary fat into small droplets in the intestine, and pancreatic enzymes break triglycerides apart so their components can be absorbed.
Inside intestinal cells, most long-chain fatty acids are rebuilt into triglycerides and packaged into chylomicrons. These particles travel through the lymph and bloodstream, delivering fatty acids to muscle and other tissues or to adipose tissue for storage.
Between meals, during exercise, and during fasting, stored triglycerides are split through lipolysis, releasing fatty acids and glycerol.
Inside cells, fatty acids enter mitochondria and are broken down through beta-oxidation. The resulting acetyl-CoA enters oxidative metabolism and contributes to ATP production.
Fat oxidation requires oxygen and supplies ATP more slowly than carbohydrate metabolism. Fat can therefore supply a large share of energy at rest and during lower-intensity activity, while carbohydrate generally supplies a larger share as exercise intensity rises.
The glycerol released from triglycerides can travel to the liver and contribute to glucose production.
Why the body stores so much fat
Fat is well suited to long-term energy storage because it is energy-dense and stored with very little water. Glycogen holds substantially less energy per gram and is stored with roughly several grams of water for every gram of glycogen.
A typical adult stores only a few thousand calories as glycogen. Body-fat stores contain tens of thousands of calories and can exceed 100,000 calories depending on body size and composition.
Living off the reserve
As food intake stops and glycogen falls, the body releases more stored fatty acids. The liver converts increasing amounts of fat-derived acetyl-CoA into ketone bodies.
The brain does not use long-chain fatty acids directly as a major fuel, but it can use ketones. During prolonged fasting, ketones provide an increasing share of the brain’s energy and reduce the amount of glucose that must be produced from other sources.
Some glucose is still required. The liver makes it from glycerol, lactate, and amino acids. Ordinary even-chain fatty acids do not provide meaningful net glucose in humans.
This shift in fuel use allows stored body fat to cover energy needs long after glycogen stores have fallen. A published medical case from the 1960s documented a medically supervised fast lasting more than a year in a patient with severe obesity who received water, vitamins, electrolytes, and close medical monitoring.
Stored fat does not supply a complete diet. Lean tissue continues to be lost, and prolonged food deprivation can cause dangerous electrolyte, vitamin, protein, and organ problems even while substantial body fat remains. Restarting food after prolonged starvation can also cause severe electrolyte shifts known as refeeding syndrome.
This section describes human fasting physiology, not a fasting protocol. Prolonged fasting can be dangerous and requires medical supervision.
Saturated fat and what replaces it
The FDA Daily Value for saturated fat is 20 g on a 2,000-calorie diet. Current federal dietary guidance recommends keeping saturated fat at about 10% of calories or less.
Replacing saturated fat with polyunsaturated fat lowers LDL cholesterol and is associated with lower cardiovascular risk. Replacing it with monounsaturated fat also tends to improve the lipid profile. Replacing saturated fat with refined starch or added sugar does not produce the same effect.
Replacing butter with olive oil is therefore a different dietary change from replacing butter with refined carbohydrate.
Individual saturated fatty acids also differ. Stearic acid, prominent in cocoa butter, generally has less LDL-raising effect than palmitic, myristic, or lauric acid. Cocoa butter also contains palmitic acid and substantial oleic acid, so its fatty-acid profile differs markedly from coconut fat, which is especially rich in lauric acid.
Trans fat
Trans fats are unsaturated fatty acids with a different molecular configuration around one or more double bonds.
Industrially produced partially hydrogenated oils were once common in shortening, margarine, baked foods, and frying fats. The FDA removed partially hydrogenated oils from generally recognized as safe status, and they are no longer a major permitted source of industrial trans fat in the U.S. food supply.
Small amounts of naturally occurring trans fatty acids remain in meat and dairy from ruminant animals. There is no dietary requirement for trans fat.
Fat and fat-soluble vitamins
Vitamins A, D, E, and K are absorbed as part of normal fat digestion. Carotenoids and other fat-soluble compounds are handled similarly.
A meal containing some fat generally improves their absorption compared with an otherwise similar fat-free meal.
Pancreatic insufficiency, cystic fibrosis, impaired bile flow, significant intestinal disease or resection, and other causes of severe fat malabsorption can reduce absorption of both fat and fat-soluble vitamins.
Persistent greasy or unusually pale stools, unexplained weight loss, or diagnosed malabsorption deserve medical evaluation rather than simply adding more dietary fat.
Who may need to pay closer attention
- people eating very low-fat diets, where essential fatty-acid intake can become harder to cover
- people with elevated LDL cholesterol who are reviewing saturated-fat sources and what could replace them
- people who eat no fish or seafood and want a direct source of EPA or DHA
- vegans who need reliable ALA sources and may choose algae-derived EPA or DHA
- people with pancreatic insufficiency, impaired bile flow, cystic fibrosis, significant intestinal disease, or other conditions that impair fat absorption
- people who have had bariatric or intestinal surgery that changes nutrient absorption
- people taking medications that deliberately reduce fat absorption
Familial hypercholesterolemia, severe hypertriglyceridemia, disorders of fatty-acid oxidation, and other metabolic conditions can require individualized dietary-fat guidance.
What to check in your routine
Start with both the amount of fat and the foods supplying it.
Look at the fats used repeatedly for cooking and the fats built into regular meals. Olive oil, nuts, seeds, avocado, fish, dairy, meat, coconut, and cocoa provide different mixtures even when the total grams are similar.
Check whether the diet supplies both essential fatty acids. Plant oils, nuts, seeds, flax, chia, and walnuts provide linoleic acid and ALA. Seafood or algae provide EPA and DHA directly.
If saturated fat is being reduced, look at what replaces it. Unsaturated fats from foods such as olive oil, nuts, seeds, avocado, and fish produce a different dietary pattern from replacing the same calories with refined starch or added sugar.
The information on this page is educational and does not constitute medical advice. Talk to a qualified healthcare provider or dietitian if you have a condition affecting fat digestion or absorption, a significant lipid disorder, or have been given a specific dietary-fat prescription. Prolonged fasting is discussed only to explain human metabolism and can carry serious risks without medical supervision.