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What is a fatty acid? Why is it important whether it is saturated or unsaturated? Why are trans fats bad? In this article we explore these questions, introduce the structure of fatty acids and discuss unsaturated fatty acids with examples and illustrations.


In 1869, a French chemist by the name of Hippolyte Mège-Mouriès, received an award from Napolean III for his invention – that tasty substitute for butter that we call margarine. His original creation was produced by churning beef tallow with milk. For the people of 19th century France, his creation provided a cheap substitute for butter.

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Today we know that while margarine is higher than butter in unsaturated fats, which are normally better for you than saturated fats, those unsaturated fats are trans fats. That is, they contain trans, rather than cis, double bonds. Scientists have shown that trans fatty acids raise the concentration of low-density lipoproteins (LDLs), which contributes to cardiovascular disease.

You might be wondering what the difference is between trans and cis double bonds. You might also wonder why trans double bonds make a difference for dietary health. For that matter, what is saturated or unsaturated fat? Is one wet and the other dry? Read on for the answers to these questions and others.

What is a Fatty Acid

What we often refer to as fat is actually composed of large bio-molecules known as fatty acids among other classes of fats – and in general are referred to as lipids. Fatty acids are composed of long hydrocarbon chains connected to a carboxyl head group.

Hydrocarbon chains are a series of carbon atoms connected by single or double bonds. Each carbon is also bound to enough hydrogen atoms to make a total of four bonds to the carbon atom. Figure 1, below, shows a model of a 6 carbon chain. The black balls represent carbon atoms and the white balls represent hydrogen atoms connected by single bonds (connecting lines).

Notice that all carbons have four total bonds. One hard and fast rule for carbon is that it always has four total bonds.Figure 1:

Hydrocarbon Tail

A carboxyl group, often written as COOH in chemical formula, consists of a single carbon with a double bond to one oxygen and single bond to the another oxygen. The singly bound oxygen is bound to a single hydrogen atom.

Figure 2, below, is a model of a carboxyl group with the carbon atom, in black, the oxygen atoms in red, and the hydrogen atom in white. The off white ball labeled with the capital letter R, represents an ‘R group,’ which, in the case of fatty acids, is a hydrocarbon tail.Figure 2:

Carboxyl Head Group

A fatty acid is formed when a hydrocarbon tail ends in a carboxyl group, as shown below (Fig. 3).

This fatty acid, decanoic acid, has a 10-Carbon chain with all single bonds. When a fatty acid chain has all single bonds it is saturated, because each carbon is attached to as many atoms as possible.Figure 3:

Decanoic Acid

However, many fatty acids contain one or more double bond. These are unsaturated fatty acids. Each double bond is a point of unsaturation, where the carbons on either side of the double bond now are bound to one less hydrogen atom.

The image below depicts Vaccenic acid, a fatty acid with a single double bond, or point of unsaturation.Figure 4:

Vaccenic Acid

Cis and Trans:

Whether a double bond is cis or trans has to do with the orientation of the atoms on either side of the bond. In the case of double bonds in a fatty acid chain these are the carbon atoms attached to the carbons on either side of the double bond. If the adjacent carbons are on opposite sides of the double bond, this is a trans double bond. If the adjacent carbon atoms are both on the same side, this is a cis double bond.

This is illustrated in the figure below. In this figure the lines represent covalent bonds between carbon atoms with a single carbon atom located at each bend, where two or more lines meet, and at the end of the last line on either side.Figure 5:

cis vs trans

Naturally occurring unsaturated fatty acids normally have cis double bonds.

Cis double bonds causes a ‘kink’ or bend in the carboxy tail of a fatty acid. These bends create spacing between the fatty acids when they are grouped together. These are better for heart-health, because they can be more easily broken up and are less likely to form clogs in arteries.In man-made products, such as margarine, however, the double bonds formed are trans double bonds.

These double bonds allow the fatty acid carboxyl tails to maintain the zig-zag pattern in a straight chain. Studies have shown that trans fatty acids leas to elevated LDLs (or bad cholesterol) which are unhealthy because they contribute to clogs forming on artery walls. Essentially trans-fatty acids are just as bad as saturated fatty acids for heart health.

Below are examples of three fatty acids, all with the same number of carbons in the fatty acid tail group. The first, stearic acid, is saturated, having no double bonds. The second, elaidic acid, has a single trans double bond. Note the continuation of the straight chain with no bend due to the trans nature of the double bond. The third, oleic acid, has a single cis double bond which causes the tail to kink.Figure 6:

Stearic, Elaidic, and Oleic Acids

Lesson Summary

Let’s put this all together:; Fatty acids are composed of a hydrocarbon tail connected to a hydroxyl head group.; The carbons in a saturated fatty acid’s hydrocarbon tail are connected by single bonds.• In the tail of an unsaturated fatty acid there is one or more double bond, or point of unsaturation.• The cis or trans arrangement of the carbons on either side of this double bond has important health implications.• Trans double bonds contribute to LDL formation, which is bad for heart-health.

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