The Truth About Fat

Fats, or in the biochemical world more often referred to as lipids, are an essential part of everyone's diet. They are the most energy rich food available and pack about 9 calories per gram. That's about twice as many calories for an equivalent amount of carbs or protein. When talking about diet and nutrition, fats seem to be a hot topic for debate. By breaking down the chemical composition of fats I hope to elucidate the facts and bust the myths about fat metabolism. I'll also provide some answers to commonly misunderstood questions regarding fat.

You'd be surprised how many people don't know what fat is actually made of. To keep things relevant everything talked about will be describing humans, unless otherwise noted. When we talk about fat we are referring to the deposits of triglycerides located in adipocytes (fat cells). In order to properly understand fat let's get down to the molecular level and work our way up.

Let's start with fatty acids, the building blocks of triglycerides. A fatty acid is simply a long chain of carbons with a carboxyl group (-COOH) as a head group. The carbon chain typically contains 12-24 carbon atoms. This carbon chain is hydrated with hydrogen atoms, which determines whether it is saturated or unsaturated. The number of carbon atoms in the chain determines the type of fatty acid the molecule is. For example palmitic acid contains 16 carbons and linoleic acid has 18. Fatty acids can be divided into two major groups; saturated and unsaturated:

Saturated Fat: The carbon chain is fully saturated with hydrogens meaning a hydrogen atom is bonded to every available spot on the chain. This causes all of the carbons to be held together by single bonds. Saturated fats can be more tightly packed together because of this. Saturated fat is almost always solid at room temperature and extremely insoluble in water. (i.e. lard, margarin, fats from meats).

Unsaturated Fat: The carbon chain is not fully saturated with hydrogen atoms. The lack of hydrogen causes the chain to kink as the carbons form double bonds between one another. These double bonds and kinks are rigid, which prevents unsaturated fatty acids from being tightly packed together. Unsaturated fat is almost always liquid at room temperature and also insoluble in water. (i.e. vegetable oil, olive oil, avocados, nuts).

Now that we know about fatty acids let's make a triglyceride; the stored form of fat. A triglyceride is made of three fatty acids bonded to a glycerol atom (see the picture above). Triglycerides make up the composition of fat found in the body. The average human has enough fat to provide the body's energy needs for up to three months! This is an enormous amount of stored energy when compared to sugar, which the average person only stores enough to last for about a day. Lipids (another word for fats) have may uses other than just providing energy. They make up the cell membrane in every single cell in your body and even provide a protective sheath on nerve cells. Certain types of lipids can also be used for chemical signaling within the body.

What makes fats so energy dense? The answer lies in the strength and number of carbon to carbon bonds the fat molecules contain. In order for the body to use food as energy it must first be converted into a compound called acetyl-CoA (Acetyl Coenzyme A). Fats, carbohydrates, and proteins are all first broken down into acetyl-CoA before they are used for energy. The details of the process are not important here, but what is important is the length of a fatty acid chain. Every 2 carbons in the chain produces 1 acetyl-CoA molecule. So a 16 carbon fatty acid will be able to make 8 acetyl-CoAs! A sugar molecule (carbohydrate) can only produce a maximum of 2 acetyl-CoAs.

1 fatty acid (containing a 16 carbon chain*) = 8 acetyl-CoA

1 triglyceride (contains 3 fatty acid chains) = 24 acetyl-CoA

vs.

1 glucose molecule = 2 acetyl-CoA

*As stated earlier not all fatty acids contain 16 carbons, this number is usually between 12 and 24. The amount of acetyl-CoA produced will change accordingly.

I will not go into detail exactly how acety-CoA is oxidized (broken down), but it continues into the citric acid cycle (also called the Krebs Cycle) where it donates electrons to drive the electron transport chain. The end result is the production of ATP; the form of energy the body can use.

All fats are either saturated or unsaturated. If you are unclear of the difference see the first section above. Unsaturated fat can be in the cis or trans structural formation (see the picture below). Almost all natural unsaturated fats are in the cis formation. In food manufacturing it's possible to take an unsaturated fat and make it saturated by hydrogenating it. This is adding hydrogen atoms to get rid of the double bonds by a chemical reaction. This process can also be reversed; change a saturated fat into an unsaturated fat by removing hydrogen atoms (dehydrogenation). However, doing so can create trans double bonds, rather than the naturally occurring cis bonds. Trans fats are actually unsaturated fat, but not natural. This process is typically done to create a more heat stable fat for use in cooking since trans fat has a higher melting point.

Trans fats are not good for you. The FDA says so and even passed a law requiring all food manufacturers to specify how much trans fat is in the product. This is why on the nutrition facts label there can be a whole list of different types of fats with their specific quantities. Studies have shown that consuming trans fats can increase LDL cholesterol levels ("bad cholesterol") and decrease HDL cholesterol levels ("good cholesterol").

You really should not eat trans fat at all. They are easily packed together the same way that saturated fats can be, but also contain one or more double bonds. Cis fats have a favorable kink, which prevents this tight packing together.

No. Eating fat will not make you any fatter than eating carbs. However, you should not eat an excessive amount of fat. If consuming a normal diet you want to get roughly 1/3 of your calories from fat. Notice that in terms of weight (usually grams) this will not be that much because eat gram of fat contains ~9 calories, which is twice as much as carbs and protein. 

You should not eliminate fat completely from your diet. It's a common mistake that to lose weight you should stop eating fat because it makes you fat. Fat is important, in fact certain organs such as the heart and kidneys rely primarily on the oxidation of fat to provide their energy needs.

If you have any questions feel free to ask me or leave a comment.