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Cholesterol and triglycerides are important fats (lipids) in the blood. Cholesterol is an essential component of cell membranes, brain and nerve cells, and bile, which helps the body absorb fats and fat-soluble vitamins. The body uses cholesterol to make vitamin D and various hormones, such as estrogen, testosterone, and cortisol. The body can produce all the cholesterol that it needs, but it also obtains cholesterol from food. Triglycerides, which are contained in fat cells, can be broken down, then used to provide energy for the body's metabolic processes, including growth. Triglycerides are produced in the intestine and liver from smaller fats called fatty acids. Some types of fatty acids are made by the body, but others must be obtained from food (see Overview of Nutrition: Fats).
Fats, such as cholesterol and triglycerides, cannot circulate freely in the blood, because blood is mostly water. To be able to circulate in blood, cholesterol and triglycerides are packaged with proteins and other substances to form particles called lipoproteins.
There are different types of lipoproteins. Each type has a different purpose and is broken down and excreted in a slightly different way. Lipoproteins include chylomicrons, very low density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Cholesterol transported by LDL is called LDL cholesterol, and cholesterol transported by HDL is called HDL cholesterol.
The body can regulate lipoprotein levels (and therefore lipid levels) by increasing or decreasing the production rate of lipoproteins. The body can also regulate how quickly lipoproteins enter and are removed from the bloodstream.
Levels of cholesterol and triglycerides vary considerably from day to day. From one measurement to the next, cholesterol levels can vary by about 10%, and triglyceride levels can vary by up to 25%.
Lipid levels may become abnormal because of changes that occur with aging, various disorders (including some hereditary ones), use of certain drugs, or lifestyle (consuming a high-fat diet, being physically inactive, or being overweight).
Abnormal levels of lipids (especially cholesterol) can lead to long-term problems, such as atherosclerosis. Generally, a high total cholesterol level (which includes LDL, HDL, and VLDL cholesterol) or a high level of LDL (the "bad") cholesterol increases the risk of atherosclerosis and thus the risk of heart attack and stroke. However, not all types of cholesterol increase this risk. A high level of HDL (the "good") cholesterol may decrease risk, and conversely, a low level of HDL cholesterol increases risk. The effect of triglyceride levels on the risk of heart attack is less clear-cut. But very high levels of triglycerides (higher than 500 milligrams per deciliter of blood, or mg/dL) can increase the risk of pancreatitis. For people older than 20, levels of total cholesterol, triglycerides, LDL cholesterol, and HDL cholesterol after fasting should be measured at least once every 5 years. Collectively, these measurements are called the fasting lipoprotein profile.
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Lipoproteins: Lipid Carriers
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Type
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Formation
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Lipid Content
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Function
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Chylomicrons
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Formed from fats in food processed by the intestine
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Mostly triglycerides
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To transport digested fats (as triglycerides) to muscle and fat cells
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Very low density lipoprotein
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Formed in the liver
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More than ½ triglycerides
About ¼ cholesterol
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To transport triglycerides from the liver to fat cells
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Low-density lipoprotein
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Formed from VLDL after it delivers triglycerides to fat cells
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More than ½ cholesterol
Less than 1/10 triglycerides
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To transport cholesterol to various cells
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High-density lipoprotein
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Formed in the liver and small intestine
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About ¼ cholesterol
About 1/20 triglycerides
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To remove cholesterol from tissues in the body and transport it to the liver
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Last full review/revision February 2003
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