In biochemistry, lipids are molecular organic compounds, composed largely of carbon and hydrogen, that are essential for cell growth. Lipids are non-soluble in water and combine with carbohydrates and proteins to form the majority of all plant and animal cells. They are more commonly synonymous with the word "fats" when speaking in terms of personal health. Although all fats are lipids, not all lipids are fats.
Purposes
The three major purposes of lipids in the body are storing energy, aiding the development of cell membranes and serving as components of hormones and vitamins. In healthcare, physicians order lipid tests or lipid profiles to measure cholesterol and triglycerides in a person's blood. "Lipoprotein" is the medical term used to refer to a combination of fat and protein.
Cholesterol
Cholesterol is a naturally occurring substance in the body and is comprised of lipids. It is separated into two types: high-density lipoprotein (HDL) and low-density lipoprotein (LDL). HDL is often referred to as "good cholesterol" because it is beneficial to a person's health. LDL is often called "bad cholesterol" because too much of it can be harmful.
Testing
In a lipid test, the lipoproteins are separated so that the level of each one in the patient's blood can be measured. Lipid tests are often part of preventative routine care because they help determine whether there is significant risk for artherosclerosis, a hardening of the arteries that interferes with or interrupts blood flow. Lipoprotein levels in the blood are measured, and dietary changes usually are in order when total cholesterol levels approach certain levels or when there are unsatisfactory ratios of total cholesterol to HDL or of HDL to LDL.
Fatty Acids
Fatty acids, which also are comprised of lipids, are an important dietary concern. Some fatty acids are essential, and others are harmful. Fatty acids are categorized as mono-saturated, mono-unsaturated or poly-unsaturated. Some essential fatty acids cannot be created by the body and must be consumed in the diet through foods such as fish and beans. Other essential fatty acids include linoleic acid and alpha-linolenic acid, which are available from sources such as many common vegetable oils and seed oils.
Saturated and Unsaturated Fats
Saturated fats come mostly from animal sources such as milk, butter and meats, as well as certain plant sources, such as coconut milk and cocoa butter. Excessive consumption of foods containing high levels of saturated fat may raise cholesterol levels in the blood. Unsaturated fats are mostly of vegetable origin, although some types of seafood also contain unsaturated fats. These fats help decrease a person's blood cholesterol levels.