Sources of Cholesterol The liver manufactures cholesterol, which is then transported through the blood to tissues. Humans also consume cholesterol in meat, eggs, dairy products, and fried foods, especially if those foods are high in saturated or trans fats.

Cholesterol and Atherosclerosis Years ago researchers compared cholesterol levels and heart attack rates in different groups of people. In certain villages in Japan and Yugoslavia, the average cholesterol level was 160. In those populations, the heart attack rate was very low—fewer than five attacks for every 1000 men over a ten-year period. In parts of Finland, researchers found mean cholesterol levels of 265. In that population, the heart attack rate was 14 times higher! Research indicates that high cholesterol levels, along with other risk factors, lead to atherosclerosis and higher risk of heart attack.

What controls the level of cholesterol in blood? Is there any medical treatment that can lower cholesterol and reduce the risk of atherosclerosis? These questions led researchers Michael Brown and Joseph Goldstein to studies that earned them a Nobel Prize in 1985.

Identifying the LDL Receptor Brown and Goldstein discovered LDL receptors on the cell membrane of liver cells, as shown in Figure 33–13. LDL binds to these receptors and then is taken into the cells. Once inside, cholesterol is broken down and then stored or used for making bile or more cholesterol. When blood cholesterol levels are high, liver cells take cholesterol from the blood and do not make it. When blood cholesterol levels are low, the liver produces it.

In Your Notebook Make a feedback loop to demonstrate the relationship between blood cholesterol levels and healthy liver cells.

FIGURE 33–13 LDL Receptors When blood LDL levels are high, liver cells with normal LDL receptors take up LDL and use it or store it. However, the liver cells of some people have defective LDL receptors. Those cells cannot remove cholesterol from the blood and do not stop producing cholesterol.

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