The Other Cholesterol Source

For most people—especially those with high cholesterol—the liver and other cells aren’t the body’s only sources of cholesterol. Our society’s typical high-fat diet also packs a powerful cholesterol punch. How can cholesterol from a hamburger and French fries eventually make its way to your heart’s arteries?

As you eat food with cholesterol, your intestines go through a complex process of breaking down fat molecules and building them into new molecules that the body can use. Intestinal enzymes rapidly dismantle the long, complex fat molecules into their component fatty acids.

Reassemble them into new triglyceride molecules, and package these new triglycerides— along with a small amount of cholesterol—into chylomicrons, a lipoprotein that has a very, very low density. The amount of triglyceride-rich particles in the blood increases for several hours after a meal as the intestines release a barrage of chylomicrons filled with triglycerides.

At the same time, dietary carbohydrates and proteins that are absorbed from the intestines pass to the liver, which converts them to triglyceride molecules, packages them with apolipoproteins and cholesterol, and releases the resulting very low-density lipoproteins (VLDL) into the bloodstream.

As chylomicrons and VLDL course around the body, they temporarily stick to the walls of blood vessels in muscles that need energy or in fatty tissue that stores energy. Enzymes come along and remove most of their load of triglyceride molecules, which are then transported inside the muscle or fat cells.

As triglyceride is drained from the chylomicron or VLDL particles, their protective protein coats are rearranged and reconfigured, essentially giving them a new address label that can be read by the liver or other tissues that take up lipoproteins. Both chylomicrons and VLDL become more and more dense as they give up their low-density fatty cargo.

Eventually, all that remains is the packaging material—the protein and cholesterol— and a fraction of the original triglyceride. Chylomicron remnants don’t linger in the circulation—the liver filters them from the system and recycles their components.

Many of the triglyceride-depleted VLDL remnants, though, keep circulating and undergo further modification of their lipid and protein content. Eventually these particles are converted to LDL. Virtually all cells in the body can take up and use LDL for their individual needs.

But because there are usually more LDL particles in circulation at any one time than your body can use, it’s your liver’s job to clear the excess from the blood and use it to make more bile acids or new lipoproteins. If the liver can’t keep up with the supply of LDL, these particles can come to rest in the wrong places. typically in the lining of blood vessels.

In extreme cases, they may settle in the skin and tendons, where they form yellow deposits. HDL is made by the liver and intestines and has two main jobs. HDL particles give chylomicrons and VLDL the proteins that signal the liver to trap them and extract their fat.

They also sponge up excess cholesterol from the linings of blood vessels and elsewhere and carry it off to the liver for disposal. People who can’t package lipoproteins effectively in the liver because of a genetic mutation still carry out the majority of the body’s functions quite well.

Although they do tend to have problems absorbing vitamins A, D, E, and K and often have blood cell and neurological problems as a result. Those vitamins are fatsoluble, meaning they are carried in the fat particles that make up lipoproteins, so if the body can’t package these molecules, it can’t absorb the vitamins.

Knowing how cholesterol is made in the body and how cholesterol is absorbed from food is the foundation for understanding how the right eating plan and, when necessary, cholesterol-lowering drugs, are effective.