Cholesterol: Significance and Regulation

The liver synthesizes cholesterol from acetyl CoA and transports it via lipoproteins, which are lipid-protein complexes that solubilize water-insoluble cholesterol. Three lipoprotein types facilitate this transport: VLDLs carry triglycerides to adipose tissue, LDLs deliver cholesterol to peripheral tissues, and HDLs collect excess cholesterol for return to the liver.

LDLs transport cholesterol to peripheral tissues for hormone and membrane synthesis but deposit excess cholesterol in artery walls, increasing cardiovascular risk. HDLs collect excess cholesterol from tissues and artery walls, transporting it to the liver for degradation. LDL levels above 160 mg/dL are unhealthy, while HDL levels above 60 mg/dL help reduce atherosclerosis.

Saturated fatty acids trigger the liver to produce more cholesterol, raising blood levels. Conversely, unsaturated fatty acids facilitate cholesterol elimination by promoting its breakdown into bile salts, which are excreted in feces. Dietary sources account for approximately 15% of blood cholesterol, with the remainder synthesized by the liver and intestines.

Although not an energy source, cholesterol serves as a foundational structure for bile salts, steroid hormones, and vitamin D. It is also a crucial component of plasma membranes. Cholesterol is eliminated through conversion into bile salts, which are eventually discarded in feces, maintaining proper cholesterol balance.

Cigarette smoking and stress lower HDL levels, while regular aerobic exercise and estrogen decrease LDL levels and increase HDL levels. Body shape also correlates with cholesterol distribution; individuals with upper body and abdominal fat tend to have higher cholesterol and LDL levels compared to those with fat concentrated in hips and thighs.

Adults should maintain total blood cholesterol under 200 mg/dL, though monitoring individual LDL and HDL levels is more important for assessing cardiovascular risk. LDL levels should remain below 160 mg/dL, while HDL levels above 60 mg/dL are beneficial. Elevated LDL is a stronger predictor of atherosclerosis, which can lead to heart attacks and strokes.

Excess cholesterol is eliminated through conversion into bile salts by the liver. HDL particles collect cholesterol from peripheral tissues and artery walls, transporting it to the liver where it is degraded and converted into bile salts. These bile salts are then excreted in feces, completing the cholesterol elimination pathway.