Nutrition Basics: Lipids
Fat is the final macronutrient that we are going to discuss in our Nutrition Basics series. Like protein and carbohydrate, fat has calories. Unlike protein and carbohydrate, however, which have 4 kcal per gram, fat is more energy dense and contains 9 kcal per gram on average.
In nutrition, fats are more generally called lipids, and they include a very wide variety of molecules with almost as many functions in the body. The one constant is that they do not dissolve in water. They can be structured in chains of carbon atoms (called fatty acids), links of chains (triacylglycerols and phospholipids), and in rings (like cholesterol).
Triacylglycerols (TAG) make up most of the lipids that we consume each day (triacylglyerols used to be called triglycerides). Commonly, we call TAG that are solid at room temperature fats and TAG that are liquid at room temperature as oils. Different TAG may be solid or liquid largely based on how saturated they are.
There are saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids. In this context, the degree of saturation refers to how many carbons in the fatty acid are full of hydrogen. A saturated fatty acid is full of hydrogen and there are no double bonds between carbon atoms. In a monounsaturated fat, there is one double bond between carbon atoms, and polyunsaturated fats have multiple carbon-carbon double bonds.
How does this determine whether the TAG is solid or liquid at room temperature? I am glad you asked.
Saturated fats without any double bonds are very uniform molecules. This means that when they are stacked on top of each other their carbon backbones can line up and the hydrogen atoms weakly bind to each other from fatty acid to fatty acid.
When a fatty acid has a carbon double bond, it adds a bend to the otherwise uniform chain. These bends mean that the carbon back bones cannot lie as flat and the hydrogen atoms cannot interact as strongly. So, the more bends (the less saturation), the more fluid a fat will be.
Plant oils tend to be polyunsaturated while animal fats are more saturated.
Trans fats got a lot of news time a few years ago and have largely been ignored since. Trans fats are a type of unsaturated fat, but the ‘trans’ does not refer to how saturated it is. Rather, it refers to the shape of the carbon chain. In a cis fat, the hydrogen bonds on carbons sharing the double bond are on the same side; in a trans fat, they are on opposite sides.
Most naturally occurring unsaturated fats are cis, while artificially saturating a fat results in the trans configuration. These are the hydrogenated oils and fats that are seen in some processed foods. Manufacturers do this as a way to cheaply improve the texture and/or shelf life of their products.
There are some naturally occurring trans fats that do not seem to have the same deleterious effect on health as the industrially created ones.
There are two fatty acids that are recognized as essential for humans: linoleic acid (LA) and alpha-linolenic acid (ALA). Remember, essential nutrients are not just ones that the body needs, but ones that it also cannot synthesize for itself.
Lipids have many important roles in the body. Lipids are stored as what we commonly call body fat. Though some of us have too much body fat, having some is important. Fat provides a good store of energy and helps to insulate us. For these purposes it is stored as triacylglycerol.
It also insulates our nerves, helping them to function properly and efficiently. In fact, the brain is nearly 60% fat.
Sterols are the class of lipids that are characterized by a ring structure. Cholesterol is the most common sterol in animals, including humans, and it has many important roles despite its bad press. Cholesterol is integral to cell membranes, it serves as a precursor to bile acids, sex hormones, and vitamin D.
Phospholipids are another class of lipids and they are characterized by the addition of a phosphate group. These make up the phospholipid bilayer that makes up the cell in your body.
In future posts we will dive a little deeper in the particulars of each class of lipid. But first, in our next post we will look at how these macronutrients can contribute to our energy balance.