The term lipid describes a naturally occurring organic molecule that can't dissolve in water. Although many people use the terms fat and lipid interchangeably, a fat is a lipid but a lipid is not necessarily a fat. Your body uses lipids as a source of energy, to produce hormones, to absorb fat-soluble vitamins and to provide structure to cell membranes, and stores these lipids in a variety of ways.
Your body breaks down the food and drinks you consume into usable energy. Any energy not used right away gets converted into a type of lipid known as triglyceride that stores the energy for use at a later time. You should try to avoid consuming excess calories because a high level of triglycerides in your blood increases your risk for heart disease, the leading cause of death in the United States. For this reason, the American Heart Association recommends keeping your triglyceride levels at less than 150 milligrams per deciliter. In addition, your body stores excess triglycerides in fat cells.
Lipids classified as fatty acids are stored in fat cells, also known as adipose tissue. Fat cells consist of up to 90 percent fat globules and triglycerides. Although the molecule known as glycogen stores glucose for use at a later time, glycogen only provides enough energy to maintain your body functions for approximately one day. In contrast, fat cells contain enough energy to keep your body functioning for 30 to 40 days, according to information provided by Elmhurst College.
All cell membranes store a small amount of lipids in the form of phospholipids. Phospholipids give cell membranes their structure. Phospholipids consist of a water-soluble head with a tail that repels water, making it hydrophobic. The phospholipids form a bilayer with the tails facing each other and the heads facing outward. This unique structure serves as a selective barrier that regulates the flow of molecules in and out of the cell.
Liver cells produce a specialized type of protein known as a lipoprotein. Because lipids cannot dissolve in blood, since blood consists mainly of water, lipoproteins bind to the lipids to carry them through the blood vessels. Lipoproteins serve as a temporary storage for triglycerides and cholesterol, both classified as lipids. Low-density lipoproteins, called LDL, keep excess cholesterol in the blood and contribute to an increased risk for heart disease. High-density lipoproteins, HDL, pick up excess cholesterol and carry it back to the liver, which converts the cholesterol into bile acids. To maintain a healthy cardiovascular system the National Heart Lung and Blood Institute recommends you keep your total lipoprotein level to less than 200 milligrams per deciliter.