What are omega-3 and omega-6 fatty acids?

In this Article (Index)

• What are Fatty Acids
• Saturated and Unsaturated Fatty Acids
• Saturated Fats
• Unsaturated Fats
• Hydrogenated Fats
• Mono and Poly Unsaturated Fatty Acids (MUFAs & PUFAs)
• Omega (ω) Fatty Acids
• Omega-9 (ω-9)
• Omega-6 (ω-6)
• Omega-3 (ω-3)
• Sources of Fatty Acids
• Recommended intake
• The role of omega-3s and omega-6s in the body
• Omega 6 to omega 3 ratio

What are Fatty Acids

A fatty acid is an organic molecule, meaning it is synthesized by living beings, that is composed of a backbone of carbon atoms (C) linked to each other in a linear configuration by double (═) or single (―) bonds, with hydrogen atoms (H) attached to the carbon chain.
The type of bond has an important role in their properties as we will see below.

The Carboxyl Group makes it a Fatty Acid

Fatty acids have a carboxyl group at one tip of the molecule.

A carboxyl group consists of an oxygen atom (O) linked by a double bond (═) to the terminal carbon atom of the molecule's backbone, which is also bonded to a hydroxyl group, composed of one atom of oxygen (O) and one atom of hydrogen (H). The image shows a carboxyl group, whose general formula is R―COOH where ―COOH is the carboxyl group and R is the remainder of the molecule, which is a linear chain of carbon atoms bonded to each other.

They are called "acids" because they have a pH that is lower than 7; pH is a measure of acidity. But as you know, fatty acids (think of flax oil or olive oil) aren't as acidic as lemon juice or as tarty as berries. Fatty acids are much weaker than most organic acids but they are, nevertheless, acidic.

Saturated and Unsaturated Fatty Acids

Further up we mentioned that the carbon atoms in the backbone of the molecule could be bonded together by single (―) or double (═) bonds. This is what defines whether a fat is saturated or unsaturated. If all bonds in the fatty acid chain are single, the compound is a Saturated fatty acid.

Saturated Fats

The type of bond shapes the atom. Since carbon atoms can have four bonding sites, using two to link up with the previous and consecutive carbon atom in the chain leaves them with another two available to bond with hydrogen atoms. This forms a straight chain.

They are called "saturated" because their carbon atoms have the maximum possible quantity of hydrogen atoms they can bind to, and are therefore "saturated".

As the molecules are straight, they can line up and pack closely together, and the electric attraction between close chains enhances this effect. The outcome is a dense substance known as solid fat. Most animal fats are saturated fats, and some plant oils are also saturated, like coconut or palm oils.

Unsaturated Fats

These fatty acids have some double bonds (═) in their chains linking carbon (C) atoms together. When this happens, it leaves only one bond available for a single hydrogen atom (H), instead of 2 bonds as in saturated fats.

There are two possible configurations for the single hydrogen atoms: transposed (trans) with one on each side of the chain or side-by-side (cis), a word that comes from Latin and means "on this side." Below we depict both configurations.

Most vegetable and plant-based fats are of the unsaturated type.

"Trans" chains are straight, like those of saturated fats, but the "cis" double bonds bend the chain causing a kink in it.

This means that the cis-unsaturated fatty acid molecules can't stack neatly together as saturated fats or trans-unsaturated fats do, so they occupy more space, and being less dense, they are liquid at room temperature and have a lower melting point than solid fats.

Mono and Poly Unsaturated Fatty Acids

MUFAs and PUFAs

Depending on the number of double bonds (═) the unsaturated fatty acids can be classified as mono-unsaturated (only one double bond) also called MUFAs, or poly-unsaturated (several double bonds) or PUFAs.

The following image shows these different kinds of fatty acids: Saturated, MUFA, and PUFA. Simple bonds are shown as C―C and double bonds as C═C.

Most natural unsaturated fatty acids have the "cis" configuration, while man-made fats obtained through hydrogenation are "trans." There are some natural "trans" fats found in the milk and meat of cows and sheep which are manufactured by bacteria that live in the stomachs of these animals.

Omega (ω) Fatty Acids

The placement of the double bonds along the chain is important and defines the fatty acid's biochemical properties. Chemists devised a terminology to describe molecules (a nomenclature) so that everyone can understand the exact shape of a molecule. It is quite simple.
The first carbon atom is called alpha (α) and it is the one located next to the carboxyl group. We marked it with a blue "C" in the image below. The atom at the other tip of the chain is the omega (ω) carbon (red "C" in the image).

The first double bond is in position 9, and the second is in position 12 counting from the α carbon (blue numbers). However, nutritionists and biochemists prefer to count from the ω carbon (red numbers), in which case the double bond starts at the 6th carbon from the omega tip, followed by another in position 9.

Using the "omega" system the first double bond classifies the fatty acid, in this case, it is on the 6th carbon, hence it is an omega-6 fatty acid.

Oleic acid, pictured further up, has the first double bond on its 9th carbon, which makes it an omega-9 fatty acid.
Below is an omega-3 fatty acid, known as α-linolenic acid or ALA.

Omega-9

Human beings can only synthesize omega-9 fatty acids, that is, we can only add the first double bond on the 9th carbon atom counting from the omega tip.

Omega-6

The first double bond is located on the 6th carbon atom, and they are considered essential fatty acids, as they must be obtained from food. Linoleic acid with 18 carbons and 2 double bonds is an omega-6 polyunsaturated fatty acid, which can be written as C18:2n-6 (indicating chain length, number of double bondsm, and omega type). Another example is arachidonic acid (C20:4n-6).

Omega-3

They are also essential fatty acids. Below are some examples of ω-3 fatty acids:

• ALA (α-linolenic acid) pictured above, with 18 carbon atoms in its chain and 3 double bonds, making it a polyunsaturated fatty acid or PUFA (C18:3n-3).
• EPA (eicosapentaenoic acid ), with 20 carbons in its chain, and 5 double bonds, is a "long-chain" PUFA (C20:5n-3).
• DHA (docosahexaenoic acid), with 22 carbons and 6 double bonds is also a long-chain PUFA (C22:6n-3).

The human body can convert ALA into EPA and DHA in the liver but isn't very efficient in doing so. EPA and DHA must be obtained from the diet.

Sources of Fatty Acids

Vegetable oils and animal fats contain a mix of different amounts of saturated fats, MUFAs, and PUFAs, and among the PUFAs, a variety of omega oils.

The following image (click on it to enlarge it) shows the proportions of fatty acids in selected animal and plant-based oils and fats.

Click on the image to enlarge it.

Omega-3 content is high in plant oils like flaxseed, soybean, and canola, where ALA is the main component. Fish also contain significant amounts of omega-3s such as DHA and EPA, like salmon, herring, sardines, and cod liver oil. Fish with a lower fat content have less omega-3s. (2)

The omega-3s in fish are not produced by the fish, instead, they accumulate in fish tissue as they are ingested by the fish when consuming microalgae or fish that have fed on them. (3)

Recommended intake

Currently, there is an Adequate Intake (AI) level as there isn't sufficient evidence to define an RDA. There is no AI for EPA, DHA, or long-chain omega-3s except for the following (values as ALA) (2)

The U.S. Food and Drug Administration (FDA) has defined a Daily Value (DV) of 65 g for total fat (saturated, MUFAs, and PUFAs), but not for omega-3s. (2)

The role of omega-3s and omega-6s in the body