Omega fatty acids are in vogue as health products, and in the health claims of various foods, but what are they?

Omega (ω) is a letter in the Greek alphabet, the last one in fact. Omega is often used in sciences as shorthand for last, or end. Thus Omega refers to the last carbon atom in the fatty acid chain furthest from the carboxyl group.

Omega is usually followed by a number, often Omega-3, Omega-6 or Omega-9. These numbers refer to the carbon atom counted back from the end of the carbon chain. Thus an Omega-3 fatty acid refers to the third carbon from the end of the fatty acid chain (Figure 1).

Figure 1: A saturated fatty acid with the terminal residue marked with the Greek letter omega (ω). The carbons in fatty acids with double bonds are counted from the terminal carbon atom being called ω and the bond from this carbon atom to the adjacent carbon atom atom towards the carboxyl group of the fatty acid. The bonds are counted in this direction from ω 1 onwards.

What the omega number when describing a fatty acid refers to is that this is where a carbon-carbon double bond is found when counting from the last carbon atom of the fatty acid chain. In this case an Omega-3 fatty acid has the first carbon-carbon double bond counted from the end of the fatty acid chain between the third and fourth carbon atoms from the end of the fatty acid chain. Similarly, the Omega-6 and Omega-9 fatty acids have the carbon-carbon double bonds further away, at between six to seven and nine to ten carbon atoms away from the end of the fatty acid chain (Figure 2).

Figure 2: Left: Α-linolenic acid, an omega 3 fatty acid, which contains three carbon-carbon double bonds in a cis conformation, with the closest carbon-carbon double bond being the ω-3 bond, the third bond from the end of the carbon chain furthest from the carboxyl group. It is prominently found in flaxseed oil. Right: Linoleic acid, an omega 6 fatty acid which contains two carbon-carbon double bonds in a cis conformation, with the closest carbon-carbon double bond being the ω-6 bond, the sixth bond from the end of the carbon chain furthest from the carboxyl group. This fatty acid is most prominently found in safflower oil, where it is bar far the most abundant fatty acid. Both of these fatty acids are considered essential fatty acids, as humans are not able to synthesize fatty acids with double bonds in these places, and so must obtain them from diet.

The significance of Omega-3 and Omega-6 fatty acids is that this close to the end terminal of the fatty acid, the human body is unable to add double bonds, as humans do not have the enzymes which can add double bonds in these parts of fatty acids. This means that fatty acids which have double bonds in these regions that are essential for human biology and can only be obtained from diet. Humans cannot synthesize these from other fatty acids, only from fatty acids that already contain these double bonds. This is why Omega fatty acids are associated with healthy diets, and why Omega-3 and Omega-6 fatty acids are called essential fatty acids.