How is cholesterol excreted

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Is a common pain reliever safe during pregnancy? Cholesterol is broken down and eliminated only by the liver. The intestine plays a key role in regulating the net balance of cholesterol by serving as the site of both absorption of dietary cholesterol and reabsorption of biliary cholesterol. What is not absorbed is excreted in the feces. David E. Cohen, MD, PhD, described the mechanisms involved in this balance of cholesterol synthesis, absorption, and excretion. He went on to show the necessity of inhibiting both synthesis and absorption to achieve optimal control of serum cholesterol levels.

Together they make a substantial contribution to serum cholesterol. The intestine delivers cholesterol to the liver in the form of chylomicron remnants. The liver then incorporates a portion of this cholesterol into very low-density lipoprotein VLDL , which gives rise to atherogenic LDL particles in the plasma.

The body's tissues make all the cholesterol they need eg, for membrane homeostasis , but only the liver can break it down in large amounts.

Because of this, transporting cholesterol to the liver is important. High-density lipoproteins HDLs are primarily responsible for "reverse cholesterol transport" to the liver, where some cholesterol molecules are converted to bile salts that allow packaging and secretion of cholesterol from the body. Cholesterol is insoluble in water, and lipoproteins carry it in the serum.

In the intestine, bile salts are highly soluble. They are similar in molecular structure to cholesterol but have detergent-like properties, which keep insoluble molecules in solution.

Search for:. Metabolism and the Liver. Learning Objectives Describe cholesterol metabolism. Key Takeaways Key Points Cholesterol is also synthesized in the adrenal glands and reproductive organs.

Cholesterol is kept in balance by homeostatic mechanisms: higher dietary intake leads to reduced synthesis in the body. Dietary cholesterol is poorly absorbed; therefore, intake of cholesterol has little effect on blood cholesterol levels.

Cholesterol forms part of the bile acids excreted from the liver. Crystallized cholesterol is the primary component of gallstones. Cholesterol in the colon is metabolized by colonic bacteria which convert it to a nonabsorbable sterol that is excreted in the feces. Key Terms gallbladder : In vertebrates, a small organ that aids mainly in fat digestion and concentrates bile produced by the liver. Examples Eggs are an excellent, low-calorie source of protein, but they also are fairly high in cholesterol, so, in the past, it was thought that eggs should be limited in the diet.

Regulation of Blood Cholesterol Levels Cholesterol is transported through the blood by lipoproteins which direct cholesterol to where it is needed.

Learning Objectives Differentiate among the cholesterol transport lipoproteins. Key Takeaways Key Points There are several types of lipoproteins that are classified by their density or ratio of lipid to protein.

The lower the protein, the less dense it is. LDL lipoproteins contain the highest percentage of cholesterol and are the major carriers through the bloodstream. LDL and its receptor form a vesicle which fuses with a lysosome that releases the cholesterol for use in membrane biosynthesis or cellular storage.

HDL lipoproteins transport cholesterol back to the liver for excretion or reprocessing. High levels of HDL lipoproteins are associated with better health outcomes. LDL and its receptor form a vesicle which fuses with a lysosome that releases the cholesterol to be used in membrane biosynthesis or to be stored in the cell. When a cell has sufficient cholesterol LDL receptors are blocked, but when the cell is deficient, more LDL receptors are created.

Rodwell, et al. Harper's Illustrated Biochemistry, 30e. McGraw Hill; Accessed November 12, McGraw Hill. Download citation file: RIS Zotero.

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