Cholesterol Absorption

Why This Matters

Not all cholesterol entering the intestine is absorbed. The body carefully regulates how much cholesterol is allowed to enter the circulation and how much is returned to the intestinal lumen for elimination. Two transport systems — NPC1L1 and ABCG5/ABCG8 — work in opposite directions to control this balance. Their coordinated activity plays a major role in cholesterol homeostasis and forms the basis for one of the most effective lipid-lowering therapies.

Cholesterol in the Intestinal Lumen

The cholesterol present within the intestine comes from two sources: dietary cholesterol, and biliary cholesterol secreted by the liver — often the larger of the two. Both sources mix within intestinal micelles before reaching the brush border of enterocytes.

Only a portion of this cholesterol is ultimately absorbed. The remainder is excreted in the feces.

NPC1L1: The Cholesterol Importer

Niemann-Pick C1-Like 1 (NPC1L1) is a transporter located on the brush border of intestinal enterocytes.

Its function is to transport cholesterol from the intestinal lumen into the enterocyte. After entering the cell, cholesterol is incorporated into chylomicrons and eventually reaches the systemic circulation through the lymphatic system.

NPC1L1 therefore serves as the body's principal mechanism for intestinal cholesterol absorption.

ABCG5 and ABCG8: The Cholesterol Exporters

Working in opposition to NPC1L1 are two ATP-binding cassette transporters, ABCG5 and ABCG8.

These proteins function as a heterodimer that transports cholesterol and plant sterols out of enterocytes and back into the intestinal lumen. Similar transporters are also expressed on the canalicular surface of hepatocytes, where they promote secretion of cholesterol into bile.

Together, ABCG5 and ABCG8 protect the body from excessive accumulation of dietary sterols.

A Delicate Balance

The amount of cholesterol ultimately absorbed depends on the balance between NPC1L1-mediated uptake and ABCG5/ABCG8-mediated export.

When absorption predominates, more cholesterol reaches the liver. When export predominates, more cholesterol is eliminated through the intestine.

This dynamic equilibrium allows the body to adapt to changing dietary intake while maintaining relatively stable intracellular cholesterol concentrations.

Clinical Disorders

Rare inherited mutations illustrate the importance of these transporters.

Loss-of-function mutations in ABCG5 or ABCG8 cause sitosterolemia, a disorder characterized by excessive absorption and impaired excretion of plant sterols and cholesterol. Affected individuals develop markedly elevated plasma plant sterol concentrations and may experience premature atherosclerotic cardiovascular disease.

Genetic variation in NPC1L1 and ABCG5/ABCG8 also contributes to the wide differences in cholesterol absorption observed among otherwise healthy individuals.

Clinical Relevance

Ezetimibe, covered in Chapter 27, selectively inhibits NPC1L1.

By reducing intestinal cholesterol absorption, less cholesterol is delivered to the liver. In response, hepatic intracellular cholesterol falls, LDL receptor expression increases, and the liver removes more ApoB-containing lipoproteins from the circulation. The clinical benefit of ezetimibe results not simply from blocking cholesterol absorption but from enhancing hepatic clearance of atherogenic particles.

Clinical Pearls

• Both dietary and biliary cholesterol are presented to intestinal enterocytes.
• NPC1L1 mediates intestinal cholesterol absorption.
• ABCG5 and ABCG8 transport cholesterol and plant sterols back into the intestinal lumen and into bile.
• Cholesterol absorption varies substantially among individuals.
• Ezetimibe lowers LDL cholesterol by inhibiting NPC1L1 and increasing hepatic LDL receptor activity.

Common Misconceptions

Misconception: Dietary cholesterol is the only cholesterol absorbed by the intestine.

Reality: Much of the cholesterol available for absorption originates from bile rather than directly from food.

Misconception: Ezetimibe lowers LDL cholesterol simply by blocking cholesterol absorption.

Reality: Its greatest effect results from lowering hepatic cholesterol stores, thereby increasing LDL receptor expression and ApoB-containing lipoprotein clearance.

Misconception: Everyone absorbs cholesterol equally.

Reality: Cholesterol absorption varies considerably because of genetic and physiological differences in transport proteins such as NPC1L1 and ABCG5/ABCG8.

Clinical Bottom Line

NPC1L1 and ABCG5/ABCG8 work together to regulate intestinal cholesterol absorption and excretion. Their coordinated activity is a critical component of cholesterol homeostasis, and inhibition of NPC1L1 with ezetimibe lowers cardiovascular risk by increasing hepatic LDL receptor-mediated clearance of ApoB-containing lipoproteins.