High cholesterol is a serious health problem that affects about fifty million Americans. It’s a major risk factor for cardiovascular disease (CVD), which half of all men and a third of all women will get at some time in their lives.

While too much cholesterol can be harmful, just the right amount of it does a lot of important work in the body. But like carbohydrates in recent years, cholesterol has gotten such a bad rap that most people don’t know the good it does. Cholesterol performs three main functions:

1. It helps make the outer coating of cells.
2. It makes up the bile acids that work to digest food in the intestine.
3. It allows the body to make Vitamin D and hormones, like estrogen in women and testosterone in men.

Without cholesterol, none of these functions would take place, and without these functions, human beings wouldn’t exist. Cholesterol is a fat, or lipid. It is also a sterol, from which steroid hormones are made. If you held cholesterol in your hand, you would see a waxy substance that resembles the very fine scrapings of a whitish-yellow candle.

Cholesterol flows through your body via your bloodstream, but this is not a simple process. Because lipids are oil-based and blood is water-based, they don’t mix. If cholesterol were simply dumped into your bloodstream, it would congeal into unusable globs.

To get around this problem, the body packages cholesterol and other fats into minuscule protein-covered particles called lipoproteins (lipid + protein) that do mix easily with blood. The proteins used are known as apolipoproteins.

The fat in these particles is made up of cholesterol and triglycerides and a third material phospholipid, which helps make the whole particle stick together. Triglycerides are a particular type of fat that have three fatty acids attached to an alcohol called glycerol—hence the name.

They compose about 90 percent of the fat in the food you eat. The body needs triglycerides for energy, but as with cholesterol, too much is bad for the arteries and the heart. The two main types of lipoproteins important in a discussion on heart disease are low-density lipoproteins (LDL) and high-density lipoproteins (HDL).

Though the names sound the same, these two particles are as different as night and day. The differences stem from their densities, which are a reflection of the ratio of protein to lipid; particles with more fat and less protein have a lower density than their high-protein, low-fat counterparts.

There are countless other lipoproteins, some of which I’ll discuss later, but in order to get a basic understanding of how cholesterol affects your body and how the food you eat affects your cholesterol levels, LDL and HDL are the ones to start with.

Low-Density Lipoproteins (LDL)

In most people, 60 to 70 percent of cholesterol is carried in LDL particles. LDL particles act as ferries, taking cholesterol to the parts of the body that need it at any given time. Unfortunately, if you have too much LDL in the bloodstream, it deposits the cholesterol into the arteries, which can cause blockages and lead heart attacks.

That’s why people refer to LDL as the “bad” cholesterol. The good news is that the amount of LDL in your bloodstream is related to the amount of saturated fat and cholesterol you eat. So, most people can decrease their LDL if they follow a reduced-fat diet. When you get a fasting cholesterol test, your doctor should test for the level of LDL cholesterol.

High-Density Lipoproteins (HDL)

HDL is basically the opposite of LDL. Instead of having a lot of fat, HDL has a lot of protein. Instead of ferrying cholesterol around the body, HDL acts as a vacuum cleaner sucking up as much excess cholesterol as it can. It picks up extra cholesterol from the cells and tissues and takes it back to the liver, which takes the cholesterol out of the particle and either uses it to make bile or recycles it.

This action is thought to explain why high levels of HDL are associated with low risk for heart disease. HDL also contains antioxidant molecules that may prevent LDL from being changed into a lipoprotein that is even more likely to cause heart disease.

Lifestyle changes affect HDL levels—exercise can increase them, while obesity and smoking lower them. As for diet, in general, the high-fat diets that raise LDL also raise HDL, while low-fat diets lower both. However, by carefully choosing the right foods, you can eat a diet that lowers LDL without lowering HDL.