2.5: Lipids - Lipids and Fatty Acid Structure
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Lipids, commonly referred to as fats, have a poor reputation among some people, in that "fat free" is often synonymous with healthy. We do need to consume certain fats and we should try to incorporate some fats into our diets for their health benefits. However, consumption of certain fats is also associated with greater risk of developing chronic disease(s). In this section we will dive deeper into fats and why they do not need to be feared altogether.
How Does Fat Differ From Lipids?
The answer you receive from this question will depend on who you ask, so it is important to have an understanding of lipids and fats from a chemical and nutritional perspective.
To a chemist, lipids consist of:
- Triglycerides
- Fatty Acids
- Phospholipids
- Sterols
These compounds are grouped together because of their structural and physical property similarities. For instance, all lipids have hydrophobic (water-fearing) properties. Chemists further separate lipids into fats and oils based on their physical properties at room temperature:
- Fats are solid at room temperature
- Oils are liquid at room temperature
From a nutritional perspective, the definition of lipids is the same. The definition of a fat differs, however, because nutrition-oriented people define fats based on their caloric contribution rather than whether they are solid at room temperature. Thus, from a nutrition perspective:
- Fats are triglycerides, fatty acids, and phospholipids that provide 9 kcal/g.
The other difference is that from a caloric perspective, an oil is a fat. For example, let's consider olive oil. Clearly, it is an oil according to a chemist definition, but from a caloric standpoint it is a fat because it provides 9 kcal/g.
The following sections will discuss the different lipid classes introduced above in detail.
Fatty Acids
Fatty acids are lipids themselves, and they are also components of triglycerides and phospholipids. Like carbohydrates, fatty acids are made up of carbon (\(\ce{C}\)), hydrogen (\(\ce{H}\)), and oxygen (\(\ce{O}\)).
On one end of a fatty acid is a methyl group (\(\ce{CH3}\)) that is known as the methyl or omega end. On the opposite end of a fatty acid is a carboxylic acid (\(\ce{COOH}\)). This end is known as the acid or alpha end. Figure \(\PageIndex{1}\) shows the structure of fatty acids.
There are a number of fatty acids in nature that we consume that differ from one another in three ways:
- Carbon chain length (i.e. 6 carbons, 18 carbons)
- Saturation/unsaturation
- Double bond configuration (cis, trans)
Carbon Chain Length
Fatty acids differ in their carbon chain length (number of carbons in the fatty acid). Most fatty acids contain somewhere between 4-24 carbons, with even numbers (i.e. 8, 18) of carbons occurring more frequently than odd numbers (i.e. 9, 19). Fatty acids are classified as short-chain fatty acids, medium-chain fatty acids, and long-chain fatty acids based on their carbon chain length using the criteria shown in the table below.
| Classification | # of carbons |
|---|---|
| Short-Chain Fatty Acid | < 6 |
| Medium-Chain Fatty Acid | 6-12 |
| Long-Chain Fatty Acid | > 12 |
Carbon chain length also impacts the physical properties of the fatty acid. As the number of carbons in a fatty acid chain increases, so does the melting point as illustrated in the figure below.
Thus, shorter chain fatty acids are more likely to be liquid, while longer chain fatty acids are more likely to be solid at room temperature (20-25ᐤC, 68-77ᐤF).
Saturation/Unsaturation
A saturated fatty acid is one that contains the maximum number of hydrogens possible, and no carbon-carbon double bonds. Carbon normally has four bonds to it. Thus, a saturated fatty acid has hydrogens at every position except carbon-carbon single bonds and carbon-oxygen bonds on the acid end. Two examples of the same 18 carbon saturated fatty acid (stearic acid/stearate) are shown in Figures \(\PageIndex{1}\) and \(\PageIndex{3}\). Figure \(\PageIndex{3}\) is the simplified view of this fatty acid.
Unsaturation means the fatty acid doesn't contain the maximum number of hydrogens on each of its carbons. Instead, unsaturated fatty acids contain a carbon-carbon double bond and only 1 hydrogen off each carbon. The simplest example of unsaturation is a monounsaturated fatty acid. Mono means one, so these are fatty acids with one degree of unsaturation, or one double bond (shown below).
Any fatty acid that has two or more double bonds is considered a polyunsaturated fatty acid. As you may remember from the polysaccharide section, poly means many. A simple example of a polyunsaturated fatty acid is linoleic acid (shown below).
Double Bond Configuration (Shape)
Double bonds in unsaturated fatty acids are in one of two structural configurations: cis or trans. In a trans configuration, the hydrogens on the carbons involved in the double bond are opposite of one another. A trans fatty acid is one that contains at least one trans double bond (even if other bonds in it are in the cis configuration). In the cis configuration the hydrogens are on the same side of the bond. Steric hindrance in the cis orientation causes the chain to take on a more bent shape or if the depicted in a linear manner causes the bond to flatten out and take on a mesa or plateau look rather than allowing the zig-zag confirmation to continue.
Most natural unsaturated fatty acids are in the cis configuration. As can be seen in Figure \(\PageIndex{7}\), the cis fatty acids have a more of kinked shape, which means they do not pack together as well as the saturated or trans fatty acids. As a result, the melting point is much lower for cis fatty acids compared to trans and saturated fatty acids. To illustrate this difference, the figure below shows the difference in the melting points of saturated, trans-, and cis-monounsaturated 18 carbon fatty acids.
There are some naturally occurring trans fatty acids, such as conjugated linoleic acid (CLA), in dairy products. However, for the most part, trans fatty acids in our diets are not natural; instead, they have been produced synthetically. The primary source of trans fatty acids in our food supply is partially hydrogenated vegetable oil. The 'hydrogenated' means that the oil has gone through the process of hydrogenation. Hydrogenation, like the name implies, is the addition of hydrogen. If an unsaturated fatty acid is completely hydrogenated it would be converted to a saturated fatty acid as shown below.
However, this isn't/wasn't always desirable, thus partially hydrogenated vegetable oil became widely used. To visualize the difference in the amount of hydrogenation consider the difference between tub margarine and stick margarine.
Stick margarine is more fully hydrogenated leading it to have a much harder texture. This is one of the two reasons to hydrogenate, to get a more solid texture. The second reason is that it makes it more shelf-stable, because the double bond(s) of unsaturated fatty acids are susceptible to oxidation, which causes them to become rancid.
Partial hydrogenation causes the conversion of cis to trans fatty acids along with the formation of some saturated fatty acids. Originally, it was thought that trans fatty acids would be a better alternative to saturated fat (think margarine vs. butter). However, it turns out that trans fat is actually worse than saturated fat in altering biomarkers associated with cardiovascular disease. Trans fat increases low-density lipoprotein and decreases high-density lipoprotein levels, while saturated fat increases low-density lipoproteins without altering high-density lipoprotein levels. The FDA revoked Generally Recognized as Safe (GRAS) status of partially hydrogenated vegetable oil as described in the link below, and is requiring its use to be phased out by 2018. After that point, permission will need to be requested to use them in foods.