What is LDL and how do they function?
As mentioned earlier, not only will the liver receive cholesterol and some fat from chylomicrons, but it is also a primary cholesterol- and triglyceride-producing organ in the body. Fat and cholesterol in excess of the liver’s needs are packaged up into VLDL and released into our circulation. As VLDL circulates throughout our body, they unload a lot of their fat, mostly in fat cells. As a result their lipid to protein ratio decreases, which renders them denser, and they classify as LDL (see Figure 5.10). Therefore, LDL is derived from circulating VLDL.
LDL has two fates. One fate is to continue to circulate throughout the body and deposit cholesterol in various tissues. The second fate is to be recognized by tissue, removed from the blood, and broken down. Many tissues throughout our body can do this, but the liver handles more than half of the task. The longer LDL circulates, the more opportunity there is for cholesterol to be deposited throughout our body.
Figure 5.10. VLDL releases their fat to tissue (mostly adipose tissue) yielding LDL, which continue to circulate and deliver cholesterol to tissue. LDL are then removed from blood mostly by the liver.
Where does HDL come from and what do they do?
The last type of lipoprotein is HDL. HDL is made in our liver and to a lesser extent in our intestines. It is HDL’s job to circulate and pick up excess cholesterol from tissues throughout our body and return it to the liver. The whole process is very interesting because in order for circulating HDL to return the cholesterol to our liver, some of the cholesterol is first passed to circulating LDL. The LDL is then subject to removal from our circulation by the liver and broken down. HDL delivers the rest of its cholesterol directly to the liver. In regard to heart disease, if LDL wears the villin’s black hat as higher levels are linked to increased risk of a heart attack and stroke and HDL wears the hero’s white hat as higher levels are linked to lower risk. We spend more time talking about blood lipids and cardiovascular disease in Disease/Prevention.
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