Macro Nutrients Part 2: Lipids (aka Fats)

The previous post introduced the macro nutrients. This post looks at lipids, one of the macro nutrients. I’ll explain what saturated, unsaturated, omega-3, and trans mean when they describe fats.

Lipids are a broad group of chemical compounds that are insoluble (do not dissolve) in water. Fats, oils, waxes, phospholipids, and sterols are all lipids. This post will focus on fats and oils.

Fats are lipids that are solid at ambient temperature and oils are lipids that are liquid at ambient temperature. This distinction works fairly well, but there is at least one exception. Coconut oil is solid if you find it in an air conditioned grocery store. It is called an oil because it is a liquid in temperatures slightly higher than ambient. Since coconuts grow in the tropics, people have been using coconut oil (in liquid form) for generations. So it is called an oil.

 

Triglycerides

Fats and oils are composed of triglycerides. Triglycerides have three fatty acids attached to a glycerol backbone. The combination of fatty acids is what differentiates one fat from another. Triglycerides are great energy storage molecules. The breakdown of fatty acids generates more than twice as much energy as carbohydrates do, gram for gram.

 

Fatty Acids

The building blocks of triglycerides are fatty acids. They are long chains of carbons with a carboxyl group on the one end where the fatty acid attaches to the glycerol. There are several different fatty acids, most of which have chains 12-24 carbons long.

Fun fact about butyric acid: it was discovered and named by French scientist Michel Eugene Chevreul. The name comes from the Latin word for butter which is the food he extracted it from. In its pure form as a free fatty acid, butyric acid smells like rancid butter or baby spit up. Good thing it is normally in a triglyceride!

 

Saturated vs Unsaturated Fats

Saturated fats are sometimes labeled as “bad fats”. I’m not a fan of this label, but it is a good idea to consume these fats in moderation. Saturated fats are simply triglycerides that have only saturated fatty acids. Saturated fatty acids have no double bonds in their carbon chains because all the carbons are saturated with hydrogen. The carbon chain has a regular zigzag pattern without any bends. This allows the fatty acids to pack tightly together. Solid fats, like butter, normally have higher amounts of saturated fatty acids than oils have.

Unsaturated fats have double bonds between some of the carbons in the fatty acid chains. This creates bends in the fatty acid as shown below. The fatty acids can’t stack together as closely. Most oils have more unsaturated fats.

Below is a series of unsaturated fatty acids that all have 18 carbons. (Stearic acid, shown above, also has 18 carbons.) Double bonds are represented by a double line between two circles.

This next image is of the three 18 carbon unsaturated fatty acids, but from a different angle. In the image below shows the fatty acids head on, with the head being the carboxyl group. See the carboxyl groups with the red oxygen atoms in the front? I like this orientation because it shows relatively how much space unsaturated fatty acids take up.

 

Omega-3 Fatty Acids

You have probably heard of omega-3 fatty acids. But what are they? The name gives a hint. Omega is the last letter in the Greek alphabet. The end of the carbon chain on a fatty acid, opposite from the carboxyl group, is call the omega end. The number refers to the number of carbons away from the omega end at which a double bond starts. So omega-3 oil is an oil with a high concentration of fatty acids that have double bonds three carbons away from the omega end.

Omega-3 fatty acids are polyunsaturated, so they have several double bonds. This makes them a bit healthier than saturated fatty acids. The omega-3 fatty acids are found in marine foods and plant oils.

 

Trans Fats

Trans fats are unsaturated fatty acids that do not have bends like other unsaturated fatty acids because they are the trans-isomer. The unsaturated fatty acids that are shown in the rest of this post are all cis-isomers. Because of their structure, the health issues attached to saturated fatty acids apply to trans fatty acids as well.

 

Wrap Up

This post was a bit technical, but hopefully it answered some questions about fats and oils. One macro nutrient down, two to go! Next on the list is proteins.

 

Images: Wolfgang Schaefer, 3D models created with JMol

Sources:

• Principles of Biochemistry, 5th Edition, by David L. Nelson and Michael M. Cox
• “Giants of the Past: Michel Eugène Chevreul” by Gary R. List, last updated Jan 22, 2010 on the AOCS Lipids Library website