Fats and lipids
Biochemistry and nutrition
AssessmentsFats and lipids
Fats and lipids
USMLE® Step 1 style questions USMLE
Fats and lipids exam links
Content Reviewers:Rishi Desai, MD, MPH, Yifan Xiao, MD
Contributors:Stephanie Hung, MSc, MScBMC, Tanner Marshall, MS, Kaylee Neff, Viviana Popa, MD
Fats are an essential part of a healthy diet. They contribute to the taste and texture of foods, like the smoothness of guacamole and the flakiness of a croissant. Fats are also a major source of energy and a critical component of cells and tissues, and they also help absorb essential vitamins, and can be converted into other molecules like prostaglandins which help cells communicate with each other.
Fats have a three-carbon backbone called glycerol, as well as fatty acid chains. The fatty acid chain is basically a string of carbon and hydrogen atoms. When an “OH” group from the glycerol molecule binds to a Hydrogen from the fatty acid, an “H20” or a water molecule - gets released, and the two molecules link up.
If this happens once, the result is a monoglyceride. If it happens twice, it’s a diglyceride, and three times makes a triglyceride.
Now, there are various types of fatty acid chains, and one way to categorize them is by their length, in other words, how many carbons they have. Short chain fatty acids have 2 to 5 carbons, medium chain fatty acids have 6 to 12 carbons, and long chain fatty acids have 13 or more carbons.
Fatty acid chains are also categorized by the bonds connecting the carbons in the chain. A single bond is just one bond between the carbon atoms, and when a fatty acid chain has only single bonds, it’s called a saturated fatty acid - because it has as many hydrogen atoms as possible or it’s saturated with them.
Triglycerides with saturated fatty acids are nice and straight so they pack together really well, and as a result they’re usually solid at room temperature. And the longer the saturated fatty acid chain, the more likely it will be solid at room temperature.
Carbons can also have double bonds between them though, and when a fatty acid has one or more double bonds, it’s called an unsaturated fatty acid because it’s not saturated with hydrogen atoms - for every double bond there are two fewer hydrogen atoms. Also, a double bond causes a kink in the molecule so the unsaturated fats don’t pack together as nicely as saturated fats. As a result, unsaturated fats are usually liquid at room temperature.
Unsaturated fatty acids can be further classified, according to the number of their double bonds. Monounsaturated Fatty acids are unsaturated fatty acids with just one double bond. Polyunsaturated fatty acids have two or more double bonds.
Also, they can be classified according to their location as well, since all these hydrogens can get kinda crazy-looking, we’ll just take them away for now. So, another name for the methyl end is the omega end, and then we can count the number of carbons until the first double bond. Since this one’s three, it would be an omega-3 fatty acid. If the double bond is 6 carbons from the end, it’s omega-6, and if it’s 9 carbons from the end, it’s called omega-9.
Now, to make things even easier when looking at these molecules, I’m just going to show the bonds. Alright, so omega 3’s are usually polyunsaturated fatty acids, and include alpha-linolenic acid, or ALA, eicosapentaenoic acid, or EPA, and docosahexaenoic acid, or DHA.
EPA and DHA are marine sources of omega-3’s. They’re produced by microalgae, and end up in the tissues of fish like anchovies, mackerel, salmon, and sardines. ALA is found in plants like flaxseed, walnuts, and canola and soybean oils. Our bodies can convert ALA into EPA and DHA, but it’s an inefficient process that yields only small quantities, and that’s why dietary recommendations include foods that have EPA and DHA.
Omega-6 fatty acids are also usually polyunsaturated, and include linoleic acid and arachidonic acid. Linoleic acid is found in oils like safflower, corn, and soybean oils. Arachidonic acid is found in animal sources like fish, meat, and eggs. Our bodies can convert linoleic acid into arachidonic acid, but once again the process is inefficient. Because ALA and linoleic acid can only be obtained in the diet, they are considered essential fatty acids.
Omega-9 fatty acids are typically monounsaturated fatty acids, and an example would be Oleic acid, and these can be made by the human body. Foods like canola and olive oil, as well as almonds contain omega-9s.
Now, looking at the double bond of this unsaturated fatty acid, like most unsaturated fats, it’s got a cis configuration. In a cis configuration, the two functional groups are on the same side of the double-bonded carbons. Now when this happens, the fatty acid chain naturally bends. A molecule that bends does not pack tightly together, so it’s a lot more fluid - think about cooking oils, which are liquid at room temperature.