Linoleic acid (LA), an $\omega$ -6 fatty acid, found in vegetable oils (corn, safflower, soybean), nuts, and seeds
$\alpha$ -Linolenic acid (ALA), an $\omega$ -3 fatty acid, found in flaxseed, chia seeds, walnuts, and canola oil

Both are required for normal growth, development, and maintaining cell membrane integrity. Because the body lacks the enzymes to insert double bonds at the $\omega$ -3 and $\omega$ -6 positions, diet is the only source.

Omega-3 Fatty Acid Benefits and Sources

Omega-3 fatty acids are polyunsaturated fatty acids (PUFAs) with their first double bond at the third carbon from the methyl end of the chain. The three main types are:

ALA (18 carbons, 3 double bonds) — the plant-based precursor found in flaxseed, chia, and walnuts
EPA (eicosapentaenoic acid) (20 carbons, 5 double bonds) — found in fatty fish like salmon, mackerel, and sardines
DHA (docosahexaenoic acid) (22 carbons, 6 double bonds) — also found in fatty fish and fish oil supplements; a major structural component of brain and retinal tissue

The body can convert ALA to EPA and DHA, but the conversion rate is very low (typically under 5–10%), which is why direct dietary sources of EPA and DHA matter.

Omega-3s have anti-inflammatory properties and are associated with reduced risk of cardiovascular disease, improved cognitive function, and support for eye health. They work in part by competing with $\omega$ -6 fatty acids for the same metabolic enzymes, shifting the balance away from pro-inflammatory eicosanoids.

Fat-Soluble Vitamin Absorption and Transport

Fat-soluble vitamins (A, D, E, and K) require dietary fat for absorption in the small intestine. During digestion, these vitamins are incorporated into mixed micelles along with bile salts and lipid digestion products, which allows them to cross the intestinal wall.

Once absorbed, they're transported in the bloodstream by lipoproteins and stored in adipose tissue and the liver.
Very low-fat diets or fat malabsorption disorders (such as celiac disease or cystic fibrosis) can lead to deficiencies in these vitamins.
This is one reason why eliminating fat entirely from the diet is nutritionally counterproductive.

Essential Fatty Acid Functions and Sources, D1: Human Nutrition (Core) - AMAZING WORLD OF SCIENCE WITH MR. GREEN

Unhealthy Fats

Trans Fat Health Risks and Sources

Trans fats are unsaturated fats with at least one double bond in the trans configuration. That trans geometry makes the fatty acid chain straighter and more rigid, so it packs tightly like a saturated fat. This is why partially hydrogenated oils are solid or semi-solid at room temperature.

The health effects are particularly harmful because trans fats do two things simultaneously: they raise LDL ("bad") cholesterol and lower HDL ("good") cholesterol. No other type of dietary fat has this double negative effect, which is why trans fats carry a greater cardiovascular risk gram-for-gram than saturated fats.

Industrially produced trans fats come from partial hydrogenation of vegetable oils and have historically been used in baked goods, fried foods, and snack foods.
Small amounts of naturally occurring trans fats exist in dairy and meat from ruminant animals, but these are not considered a significant health concern at typical intake levels.
The FDA banned partially hydrogenated oils from the U.S. food supply in 2018, and many other countries have enacted similar restrictions.

Cholesterol Metabolism and Dietary Recommendations

Cholesterol is a waxy lipid that serves essential functions: it's a structural component of cell membranes, a precursor for steroid hormones (estrogen, testosterone, cortisol), and the starting material for bile acid synthesis.

The liver synthesizes the majority of the body's cholesterol. Dietary cholesterol (from eggs, shellfish, organ meats) contributes a smaller and variable amount, and most people's blood cholesterol responds more strongly to the types of fat they eat than to cholesterol itself.

High blood levels of LDL cholesterol are associated with increased risk of atherosclerosis, heart disease, and stroke.
Current dietary guidelines emphasize limiting saturated fats and trans fats because these have the greatest impact on raising LDL cholesterol.
The 2020–2025 Dietary Guidelines for Americans recommend keeping saturated fat below 10% of total daily calories.

Essential Fatty Acid Functions and Sources, Linoleic acid - Wikipedia

Fat Replacers and Low-Fat Foods

Types and Functions of Fat Replacers

Fat replacers are ingredients designed to mimic the sensory and functional properties of fat (creaminess, mouthfeel, moisture retention) while providing fewer calories. They fall into three categories:

Carbohydrate-based — gums, modified starches, cellulose gels, and polydextrose. These bind water to create a fat-like texture and typically provide 1–2 kcal/g compared to fat's 9 kcal/g.
Protein-based — microparticulated whey protein or egg white protein. The proteins are processed into tiny spherical particles that feel creamy on the tongue. These work well in dairy products and dressings but break down at high temperatures, so they can't be used for frying.
Fat-based — chemically modified lipids like structured triglycerides or emulsifiers that provide fewer absorbable calories. Some pass through the digestive tract partially or fully undigested.

Some fat replacers offer additional nutritional benefits. For example, inulin (a carbohydrate-based replacer from chicory root) and pectin also function as dietary fiber and prebiotics, supporting gut health.

Olestra as a Fat Substitute

Olestra (brand name Olean) is a synthetic fat substitute made by esterifying sucrose with 6–8 fatty acids. The resulting molecule is too large for digestive lipases to break down, so it passes through the GI tract undigested and contributes zero calories.

It has sensory properties similar to regular fat and is heat-stable, making it suitable for fried snack foods like potato chips and crackers.
Because olestra is not absorbed, it can carry fat-soluble vitamins (A, D, E, K) out of the body, reducing their absorption. To compensate, manufacturers are required to fortify olestra-containing products with these vitamins.
Olestra can cause gastrointestinal side effects including abdominal cramping and loose stools. FDA labeling previously required a warning about these effects on the package, though this requirement was removed in 2003 after further review.

Nutritional Considerations for Low-Fat Foods

By regulation, a "low-fat" food must contain no more than 3 g of fat per serving, while a "reduced-fat" product must have at least 25% less fat than the reference food. These are distinct claims worth knowing.

A common pitfall with low-fat products: manufacturers often compensate for the loss of flavor and texture by adding sugars, refined starches, or sodium. The result can be a product with similar or even more total calories than the original.

Always check the Nutrition Facts label and ingredient list, not just the front-of-package claim.
Compare total calories, added sugars, and sodium between the low-fat and regular versions.
A balanced dietary approach that includes moderate amounts of healthy fats (from nuts, seeds, avocados, olive oil, and fatty fish) tends to be more nutritionally sound than strictly avoiding fat. Dietary fat supports satiety, fat-soluble vitamin absorption, and essential fatty acid needs.