Fatty Acids: Saturated, Unsaturated and Essential

The difference between butter and olive oil, between a stick of lard and a bottle of fish oil, comes down to the structure of their fatty acids. These long molecules are the main building blocks of fats, oils and cell membranes. A small change in a fatty acid, adding one double bond or flipping its geometry, can change whether a fat is solid or liquid, how it behaves in your body and even whether it’s good or bad for your heart.

The basic structure

A fatty acid is a carboxylic acid with a long hydrocarbon tail:

CH₃–(CH₂)ₙ–COOH

• One end has a carboxyl group (–COOH), which is polar and acidic. In the body, at pH around 7.4, it’s usually ionised to –COO⁻ (see pKa).
• The rest is a hydrocarbon chain, which is non-polar and hydrophobic.

Natural fatty acids usually:

• have an even number of carbon atoms, typically 12 to 24, because the body builds them two carbons at a time;
• are unbranched;
• occur mainly as parts of triglycerides and phospholipids rather than free.

Saturated fatty acids

In a saturated fatty acid, every carbon in the chain is joined to its neighbours by single bonds and carries as many hydrogen atoms as possible. It is “saturated” with hydrogen.

Single bonds allow free rotation, and the lowest-energy shape is a straight zigzag. Straight chains can pack closely alongside each other, maximising London dispersion forces. So saturated fats have higher melting points and tend to be solid at room temperature.

Common examples:

Fatty acid	Carbons	Melting point (approx.)	Found in
Lauric acid	12	44 °C	Coconut, palm kernel oil
Myristic acid	14	54 °C	Butter, coconut
Palmitic acid	16	63 °C	Palm oil, meat, butter
Stearic acid	18	70 °C	Meat, cocoa butter

Melting point rises with chain length, because longer chains have more surface area for dispersion forces.

Unsaturated fatty acids

An unsaturated fatty acid contains one or more C=C double bonds.

• Monounsaturated: one double bond (e.g. oleic acid, the main fatty acid in olive oil).
• Polyunsaturated: two or more (e.g. linoleic acid in sunflower oil; α-linolenic acid in flaxseed; EPA and DHA in fish oil).

Cis double bonds put kinks in chains

In natural unsaturated fatty acids, double bonds are almost always cis: the two hydrogen atoms on the double-bonded carbons are on the same side, so the chain bends by about 30° at each double bond. Kinked chains can’t pack tightly, so intermolecular forces are weaker and melting points are lower. That’s why fats rich in unsaturated fatty acids are liquid oils at room temperature.

Fatty acid (all 18 carbons)	Double bonds	Melting point (approx.)
Stearic acid	0	70 °C
Oleic acid	1 (cis)	13 °C
Linoleic acid	2 (cis)	−5 °C
α-Linolenic acid	3 (cis)	−11 °C

Each extra cis double bond lowers the melting point dramatically, even though the chain length stays at 18 carbons.

Trans fats

In a trans double bond, the hydrogen atoms are on opposite sides, and the chain stays nearly straight, like a saturated chain. Trans fatty acids therefore pack well and have relatively high melting points: elaidic acid, the trans isomer of oleic acid, melts at about 45 °C compared with oleic acid’s 13 °C.

Small amounts of trans fats occur naturally in meat and dairy from ruminant animals. Much larger amounts were created industrially by partial hydrogenation of vegetable oils (see below). Industrial trans fats raise LDL (“bad”) cholesterol and lower HDL (“good”) cholesterol, and they’re strongly linked to heart disease. Many countries have banned or severely restricted them.

Naming fatty acids: shorthand and omega numbers

Chemists use a compact code: number of carbons : number of double bonds.

• Stearic acid: 18:0
• Oleic acid: 18:1
• Linoleic acid: 18:2
• α-Linolenic acid: 18:3

The position of double bonds can be counted two ways:

• From the carboxyl end (the Δ system): oleic acid is 18:1 Δ9, with its double bond starting at carbon 9.
• From the methyl end (the omega, ω or n system): the last carbon is called omega. A fatty acid whose first double bond is three carbons from the methyl end is an omega-3 (n-3); six carbons, an omega-6 (n-6).

Fatty acid	Shorthand	Family
Oleic	18:1	Omega-9
Linoleic (LA)	18:2	Omega-6
α-Linolenic (ALA)	18:3	Omega-3
EPA	20:5	Omega-3
DHA	22:6	Omega-3

Essential fatty acids

Humans can make many fatty acids, including saturated and some monounsaturated ones, from other nutrients. But we lack the enzymes to put double bonds close to the omega end of the chain. So two fatty acids must come from the diet: these are the essential fatty acids.

• Linoleic acid (omega-6): from sunflower, corn and soybean oils, nuts and seeds.
• α-Linolenic acid (omega-3): from flaxseed, walnuts, chia and rapeseed (canola) oil.

The body can convert ALA into the longer omega-3s EPA and DHA, but only inefficiently, so oily fish and algae are the best direct sources. DHA is a major component of cell membranes in the brain and retina. Omega-3 and omega-6 fatty acids are also starting materials for signalling molecules that help control inflammation and blood clotting.

Hydrogenation: turning oil into solid fat

Adding hydrogen across C=C double bonds, using a nickel catalyst at high temperature, turns unsaturated fatty acids into saturated ones. This is hydrogenation, and it was used for over a century to turn cheap liquid vegetable oils into solid spreads and shortenings.

When hydrogenation is partial, some double bonds aren’t fully saturated but are converted from cis to trans on the catalyst surface. That’s how industrial trans fats formed. Modern alternatives, such as full hydrogenation followed by blending, or interesterification (rearranging which fatty acids sit on each glycerol), make solid fats without creating trans fats. The catalysis involved is discussed in reaction rates and catalysts.

Fatty acids and health: a balanced view

Nutrition science on fats is complex and still developing, but some points are well supported:

• Replacing saturated fats with unsaturated fats (especially polyunsaturated) tends to lower LDL cholesterol.
• Industrial trans fats are harmful and are best avoided.
• Omega-3 fatty acids from fish are linked with heart health, although supplements show more mixed results.
• All fats are energy-dense (about 37 kJ g⁻¹), whether saturated or unsaturated.

Common misconceptions

• “Unsaturated means healthier in every case.” Most evidence favours unsaturated fats, but trans fats are technically unsaturated and are harmful.
• “Saturated fats contain no double bonds anywhere.” The carboxyl group has a C=O double bond; “saturated” refers to the carbon–carbon bonds in the chain.
• “Oils have fewer calories.” Oils and solid fats have almost identical energy content.
• “Omega numbers count double bonds.” They give the position of the first double bond from the methyl end, not how many there are.

Key takeaways

• A fatty acid is a long hydrocarbon chain with a carboxyl group, usually with an even number of carbons.
• Saturated chains are straight and pack tightly → higher melting points, solid fats.
• Cis unsaturated chains are kinked → lower melting points, liquid oils. Trans chains are straight and are harmful in large amounts.
• Omega-3 and omega-6 name the position of the first double bond from the methyl end; linoleic and α-linolenic acids are essential.
• Fatty acids build triglycerides and membranes: see lipids explained.