Importance of Fats

Fatty acids


Butyric acid

We are now ready to turn our attention to the fascinating world of fats. The term “fat” encompasses a wide group of compounds so we will start with the simplest, the “fatty acid.” What is it? Well perhaps the best way to explain it is to look at Figure 1. Shown in the figure is the structure of butyric acid, which is a fatty acid. All fatty acids contain a “carboxylic acid group” and a chain made up of carbons and hydrogens (a so-called hydrocarbon chain). A light blue circle highlights the carboxylic acid group in the figure to the right. Butyric acid smells terribly, and it is the cause of the awful taste of rancid butter. And you know those gym shoes that stink? It is probably because of the butyric acid, among other things, that is found in sweat.

Some of the more common fatty acids are shown below. They are from top to bottom lauric acid, palmitic acid, and arachidic acid, and are referred to as “saturated” fats because all the carbon-carbon bonds are single bonds. Lauric acid is found in coconut oil and in human breast milk, palmitic acid is a major component of the oil from palm trees, and arachidic acid is found in peanut and corn oils.

The next fatty acid shown is oleic acid, located to the right. Oleic acid is an abundant molecule in human fat tissue. It is called a monounsaturated fatty acid because it has one carbon-carbon double bond. When you see the term “unsaturated” it means that somewhere there is at least one double bond. There are two forms of oleic acid, one called cis and one called trans. What does that mean? For a cis bond, it means that the hydrogens attached to the carbons forming the double bond are located on the same side. For trans oleic acid, the hydrogens are located on the opposite side. Note how a cis double bond imparts a kink in the long hydrocarbon chain of the fatty acid.

 

 

The last two fatty acids are arachidonic acid and linoleic acid, shown to the right. Both of these are polyunsaturated fats because they have multiple double bonds (and the double bonds are cis). Linoleic acid is an essential fatty acid, which means you have to get it from your diet because your body cannot synthesize it. Dietary sources include, for example, sunflower, corn, sesame, peanut, wheat germ, and soybean oils. Linoleic acid is also found in egg yolks. Arachidonic acid is not an essential fatty acid because your body can make it starting from linoleic acid.

Saturated fats

You need fats in your diet, because as you will learn below, there are lots of places they are used in your body. But not all fats are equal in terms of our health. In recent years, nutrition experts have encouraged us to maintain diets that are low in saturated fats. Now you know what that means – fats with no double bonds. These experts even refer to them as “bad” fats. Sources of saturated or “bad” fats include red meat, cheese, sour cream, ice cream, and butter.

Unsaturated fats

The unsaturated fats, on the other hand, have been dubbed “good” fats. One in particular is called omega-3, and it is found in oily fish, nuts, seeds, and leafy green vegetables (shown on the left).  Unfortunately we have now learned through research that not all unsaturated fats are good for us. For example, there are the “trans” fats. They are called trans fats because they have carbon-carbon trans double bonds. Yes, they are unsaturated fats, but they are not good for us. Why are they not good? Well, most of the naturally occurring unsaturated fatty acids have cis double bonds, and your body knows how to metabolize them. The problem with trans fats is that your body cannot deal with them as effectively, and large levels of trans fats in your diet have been linked to all sorts of medical problems including an elevated risk of coronary heart disease.

Trans fats

If trans fats are not normally found in the human body in large concentrations, where do they come from? Turns out that there is a process in the food industry called hydrogenation, which leads to the production of trans fats. Specifically, hydrogenation is a process whereby hydrogens are added to vegetable oils, and the resulting hydrogenated fats are used in the preparation of such foods as crackers, cookies, cakes, doughnuts, and French fries. Why use these types of trans fats? Because foods made with them stay fresh longer and have a longer shelf life. Beware that some shortenings and margarines have very high trans fat contents. Next time you eat a cracker, take a look at the label!

Triacylglycerol

Now that we know what fatty acids are, how are they moved around in your body? They are transported as part of a larger molecule called a triacylglycerol. Now, that is a pretty big word, but it is not as complicated as you might think. A triacylglycerol is a compound that contains one molecule of glycerol and three molecules of a fatty acid (shown to the right). Different triacylglycerols result from the different fatty acids that are attached to the glycerol backbone. In fact most of the important fats and oils that come from animals and plants consist almost exclusively of triacylglycerols. These include the fats in corn, olives, lard, butter, etc. A high level of triacylglycerols in your bloodstream has been linked to heart disease and stroke. For that reason, the amount of triacylglycerols in your blood is often monitored by physicians. When thinking about proper food choices, keep in mind that there seems to be a correlation between diets high in carbohydrates (sugars) and increased blood triacylglycerol concentrations.


Difference between a triacyglycerol
and a glycerophospholipid

Do we need fats in our diets? Absolutely. The breakdown of fatty acids supplies us with much needed energy. Heart and skeletal muscle cells, for example, depend critically on the breakdown of fatty acids as a major source of their energy. We also need fats for the membranes that surround our cells. For the formation of membranes, the fatty acids are not part of a triacylglycerol molecule, however, but rather they are on another complex molecule called a glycerophospholipid (shown to the right).

In a glycerophospholipid, there are only two rather than three fatty acids attached to the glycerol backbone. The third component of a glycerophospholipid is called the “head group” (shown below).

 

Head group

There are lots of different head groups found in nature. The head group is hydrophilic, meaning that it loves water. The fatty acid chains are hydrophobic, which means they are “water-fearing.” Lots of glycerophospholipids come together to form a biological membrane as highlighted in the image below. Note how the hydrophobic tails of the fatty acids associate with one another. The membrane shown below is technically called a lipid bilayer because it has two layers and is formed by lipids. What is a lipid? Well, lipid is simply a general word that refers to a broad group of molecules that include fats, waxes, and steroids. We will talk more about the steroids below. The lipid bilayer is by far one of the most important structures in living organisms. Without it our cells would have no shape, and we would simply be puddles of biochemical molecules! Besides glycerophospholipids, our membranes also contain important proteins that allow the insides and outsides of cells to communicate with one another. Another very important component of the lipid bilayer is cholesterol, that poor molecule that gets such a bad rap from nutritionists. Although cholesterol has been linked to heart disease, without its presence in your membranes, you wouldn’t be around reading this module.

Cholesterol

So what is the structure of cholesterol? It is shown to the left. It has four rings and is composed primarily of carbons, hydrogens, and one oxygen atom. It is insoluble in water or, in other words, it is hydrophobic. Because of that, your body has special molecules to transport it around in your blood. These molecules are called HDLs and LDLs. In the nutrition literature you will read about “good cholesterol” and “bad cholesterol.” The good cholesterol is actually cholesterol associated with HDL particles, whereas the bad cholesterol is that found associated with LDL particles. The amount of HDLs and LDLs in your blood is again something that physicians monitor.

Besides its importance in membranes, cholesterol also serves as a building block for the synthesis of various hormones such as testosterone, the male sex hormone, and estradiol and estrone, the female sex hormones (shown to the left). What is a hormone, you might ask? It is a “chemical messenger.” It is a molecule released by a cell or a gland in one part of the body that travels to and controls the function of another cell or gland somewhere else in the body.

You probably have heard about vitamin D (shown to the right). It is actually a hormone whose synthesis depends upon cholesterol. Recent studies suggest that vitamin D provides protection against high blood pressure, cancer and the thinning of bone tissue. Most foods contain little vitamin D, but it can be found in cod liver oil, salmon, tuna, sardines, eggs, and fortified milk, orange juices, and cereals.

It is also produced when sunlight strikes your skin, so exposure to the sun everyday is actually good for you as long as you don’t overdo it.

 

 

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