Good and Bad Cholesterol
You have probably heard doctors talk about “good cholesterol” and “bad cholesterol”. So what are these substances, and what do they mean to you? Here is a brief discussion of what we measure, and what it all means.
It is possible to just measure total cholesterol, but usually we measure a "lipid profile". A lipid profile includes total cholesterol, triglycerides, HDL, LDL, and often the ratio of total cholesterol to HDL. These are all important. The way in which these substances are produced, transported, and changed in our bodies is very complex, but here is a simple explanation.
Cholesterol is not an evil substance. It is one of the body’s vital building blocks and is part of every cell and every hormone. But the cells of the body are like little businesses running on a "just in time" delivery system. They have no storage space and can’t deal with too much cholesterol being delivered.
When you eat fat, it is absorbed across the wall of the intestine as triglycerides. Because triglycerides are the very first form in which fat is absorbed into the body, they are the most sensitive to what you have recently eaten. Measurement of total cholesterol does not even need to be fasting, but triglycerides can change a great deal in a short period of time. In the blood triglycerides are generally combined with cholesterol into larger particles called chylomicrons. Triglycerides are also made in the liver. Cholesterol is made in the liver, but is also taken into the body from animal products in the diet. The liver uses cholesterol in bile, which helps with digestion. Three quarters of the cholesterol in the intestines comes from bile, with the other quarter coming from diet. So even if you are a vegan vegetarian, taking in no cholesterol at all in your diet you will still have lots of cholesterol in your gut.
In addition to using cholesterol in bile, the liver also packages cholesterol for delivery to the cells. It is LDL (low-density lipoprotein) that carries out this job. Although it is often called "bad cholesterol", it is not bad, it is simply the delivery truck. It brings cholesterol to the cells and if the amount delivered is what the cells need, everyone is happy. On the other hand, if more is delivered than is needed, the cells refuse the delivery, and in that case the LDL delivery vehicle dumps the cholesterol at the “curb”. In this case of course, the roads are your arteries, and the curb is the lining of those arteries. If this process continues, eventually the street gets clogged and nothing gets through. This is a heart attack or stroke.
HDL (high-density lipoprotein), the “good cholesterol”, is best thought of as the recycling truck. HDL, more technically the "reverse transport system", picks up the cholesterol lying at the “curb” and brings it back to the liver for reprocessing. It is because of this function that HDL is called the "good" cholesterol. This relationship between LDL as the delivery vehicle and HDL as the recycling vehicle make it useful to look at the ratio, either between total cholesterol and HDL or between LDL and HDL. The more HDL (and thus the lower the ratio) the less likely that there will be cholesterol deposits lying around to clog your arteries. People with very high HDL levels are very resistant to the development of atherosclerotic disease.