Cholesterol — Functions, Dangers, and How to Control It
An Essential Chemical
Cholesterol is a very important chemical in the human body and has many essential functions. Our bodies make all of the cholesterol that we need. If we eat certain foods or follow certain lifestyles, the cholesterol level in our body may increase and cause health problems, such as heart disease and strokes.
Cholesterol exists in two common forms. Excess LDL cholesterol stimulates the buildup of fatty deposits in the linings of our arteries. HDL cholesterol helps to remove these deposits. While our bodies need both of these substances, it's important to keep the amount of LDL cholesterol under control.
LDL cholesterol and HDL cholesterol are often referred to as different chemicals. However, the only difference is the molecule attached to the cholesterol that carries it around the body.
Functions of Cholesterol
Essential Functions in the Cell Membrane
Cells are surrounded by a membrane that determines which substances get in and out of the cell. Cell membranes require cholesterol in order to function properly. The cholesterol maintains the correct fluidity of the membrane at different temperatures, increasing the fluidity at low temperatures and decreasing it at high temperatures. It also reduces the permeability of the membrane to certain substances.
Improving Nervous System Function
A neuron (nerve cell) has an extension called an axon. The axon transmits nerve impulses to the next neuron. The myelin sheath is a covering that surrounds and electrically insulates axons. This insulation speeds up the transmission of nerve impulses. Myelin contains a large concentration of lipids, including cholesterol.
Cholesterol is a steroid molecule and is converted to steroid hormones in the body. These hormones include the reproductive hormones estrogen, progesterone and testosterone. Other steroid hormones made from cholesterol are cortisol and aldosterone. Cortisol has many functions in the body, including helping to regulate the blood sugar level. Aldosterone affects the amount of sodium ions and water in the body.
Vitamin D Production
Our skin makes a vitamin D precursor from 7-dehydrocholesterol when it absorbs ultraviolet light. This precursor is then converted to active vitamin D inside the body. Researchers are discovering that vitamin D has many very important functions in the body. The vitamin is needed for the absorption of calcium in the small intestine and also plays a role in immunity and cancer prevention.
Bile Acids and Salts
Bile is a yellow-green liquid made by the liver. Bile emulsifies fats in the small intestine, which prepares them for digestion by enzymes. The emulsification is performed by bile acids, which may exist in the form of bile salts. Bile acids are made from cholesterol.
Cholesterol is a type of lipid. Since lipids can't dissolve in the watery blood plasma, cholesterol molecules are attached to lipoprotein molecules. These transport the chemical around the body in the blood. A lipoprotein contains both lipid and protein.
LDL cholesterol contains lipoproteins that have low density and is often called the “bad” cholesterol. Low-density lipoproteins transport cholesterol from the liver to the rest of the cells in the body. When there is too much LDL cholesterol in the blood, cholesterol molecules are deposited in the lining of arteries. Here they combine with other substances such as fat and calcium to form a material called plaque. The plaque may protrude into the channel of the artery.
Heart Attack, Stroke and Peripheral Artery Disease
The buildup of plaque in arteries is called atherosclerosis. Plaque can decrease the available space for blood flow. It also increases the probability of blood clots. Bits of plaque may break off, leaving a rough surface which can cause a blood clot to develop. The clot and broken bits of plaque may move to other areas, blocking the flow of blood. These processes can cause a heart attack if they happen in a coronary artery, since the coronary arteries supply oxygen and nutrients to the heart muscle.
Blockage of the carotid arteries going to the brain can cause a stroke. Blocked arteries in the arms and legs can result in peripheral artery disease (PAD), also called peripheral vascular disease. Peripheral artery disease affects the legs more commonly than the arms and can cause leg numbness and weakness. Plaque can also cause artery walls to become less flexible.
HDL cholesterol consists of cholesterol attached to high-density lipoproteins. It's known as the “good” cholesterol because it reduces the risk of heart disease in most people. High-density lipoproteins transport cholesterol from the arteries to the liver, which processes the chemical or eliminates it from the body.
For some time, the standard medical advice has been "LDL cholesterol bad, HDL cholesterol good". The recommendation still stands today. There is plenty of evidence that an excessive level of LDL cholesterol increases the risk of cardiovascular problems. In addition, multiple research projects have shown that drugs which lower LDL cholesterol reduce the risk of health problems such as heart disease. New research suggests that we don't completely understand the role of HDL cholesterol, however.
Researchers have recently found that people with a rare gene mutation have a very high level of HDL cholesterol in their blood. They also have an increased risk of heart disease. This observation doesn't necessarily mean that the increased risk is due to the extra HDL cholesterol. "Correlation doesn't imply causation" is a common statement in scientific investigation. The observation does indicate that we need to do more research, however.
In the United States, cholesterol levels are measured in milligrams of cholesterol per deciliter of blood. In Canada and many countries in Europe, they are measured in miilimoles of cholesterol per liter of blood.
Blood Test Results
There are several types of blood tests that can be used to measure the cholesterol level. One test determines the total level of the chemical in the blood. If this test shows that the level is higher than it should be, more specific tests can be performed to discover the levels of the different forms of cholesterol.
A lipid profile is a blood test that measures the level of total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides (neutral fats) and sometimes VLDL cholesterol (very low density lipoprotein cholesterol) as well. Like LDL cholesterol, VLDL cholesterol stimulates cholesterol to build up in the arteries. The normal blood level of VLDL cholesterol is between 5 and 40 mg/dL.
Sometimes the ratio of total cholesterol to HDL cholesterol is reported. The lower this ratio the better. A desirable ration is 4.0. A ratio of 5.0 is borderline and 6.0 is a high ratio.
The tables below show the generally accepted meanings of the blood test numbers. Women usually have a higher HDL cholesterol level than men. While men are said to have a low HDL cholesterol level at less than 40 mg/dL (1 mmol/L) , women have a low HDL cholesterol level at less than 50mg/dL (1.3 mmol/L).
Total Cholesterol Level (mg/dL)
Total Cholesterol Level (mmol/L)
Less than 200 mg/dL
Less than 5.2 mmol/L
200 to 239 mg/dL
5.2 to 6.2 mmol/L
240 mg/dL and above
More than 6.2 mmol/L
LDL Cholesterol Level (mg/dL)
LDL Cholesterol Level (mmole/L)
Less than 100 mg/dL
Below 2.6 mmol/L
100 to 129 mg/dL
2.6 to 3.3 mmol/L
130 to 159 mg/dL
3.4 to 4.1 mmol/L
160 to 189 mg/dL
4.1 to 4.9 mmol/L
190 mg/dL and above
Above 4.9 mmol/L
HDL Cholesterol Level
HDL Cholesterol Level (mmol/L)
Less than 40 mg/dL
Below 1 mmol/L
Major risk of heart disease
40 to 59 mg/dL
1 to 1.5 mmol/L
Less risk of heart disease
60 mg/dL and above
Above 1.5 mmol/L
Protective against heart disease
Our liver helps control the amount of cholesterol in our body. For example, if we obtain cholesterol from food, the liver reduces the amount that it makes. However, there is a limit to the liver's ability to help us.
Fats in the diet should be mainly monounsaturated. Monounsaturated fats have been found to have a neutral or even beneficial effect on blood cholesterol. Olive oil is rich in monounsaturated fatty acids and lowers LDL cholesterol. The same is true for almonds and avocados. Walnuts, which contain mainly polyunsaturated fats, also lower LDL cholesterol.
Nutritionists generally recommend that saturated fats be restricted in the diet (but not eliminated), since they have been found to increase the cholesterol level in the body. Saturated fats are usually found in foods that come from animals, such as fatty meats and full-fat dairy foods. Some plant foods contain saturated fats too, such as coconut oil.
Eggs are low in saturated fat but high in cholesterol. For most people, eating foods containing cholesterol doesn’t increase the blood cholesterol level significantly. Eggs are packed full of nutrients and are a great addition to the diet, except for people who have been diagnosed with an inherited form of hypercholesterolemia. In this disorder, the blood cholesterol is abnormally high. Doctors generally advise people with hypercholesterolemia to avoid or limit foods containing cholesterol.
Artificial trans fats have been partially hydrogenated to change their properties. Nutritionists say that they should be completely removed from the diet, since they increase LDL cholesterol and decrease HDL cholesterol.
Healthy fats are an essential component of the diet. They are high in calories, however, and should be eaten in moderation. It's a good idea to avoid unhealthy additions to good fats, such as the salt and roasting oil added to some nuts.
Soluble fiber has been found to lower LDL cholesterol. This type of fiber forms a gel when it mixes with water in the small intestine. There are several theories concerning how this gel lowers cholesterol. One theory is that the gel prevents the reabsorption of bile acids from the small intestine. Bile acids are usually reabsorbed once they have done their job. If they aren’t reabsorbed, they pass out of the body in the feces. The liver then has to convert more cholesterol into bile acids, thereby reducing the blood cholesterol level.
Grains containing high amounts of soluble fiber include oatmeal and barley. Vegetables containing significant amounts of soluble fiber include peas, beans, beets, parsnips, carrots, sweet potatoes and potatoes. Fruits can also be a good source of soluble fiber. Bananas, apples, pears, strawberries, plums, prunes and citrus fruits are all rich in healthy soluble fiber.
Other Foods Which Affect Cholesterol Level
While there is strong evidence that certain foods lower LDL cholesterol, the evidence that specific foods raise HDL cholesterol is less strong. Alcohol does increase HDL cholesterol, but excess alcohol consumption can cause other health problems. Some evidence suggests that cranberry juice, raw onions and omega-3 fats found in fish such as salmon and sardines can increase HDL cholesterol. Salmon is a heart-healthy food even without affecting HDL cholesterol, since it lowers the level of trigylcerides in the blood and is low in saturated fat.
Niacin, or nicotinic acid, is a type of B vitamin. Niacin lowers LDL cholesterol and significantly raises HDL cholesterol when taken at high doses. These doses can cause unpleasant and possibly dangerous side effects, so high-dose niacin should never be taken without a doctor’s supervision.
Some types of processed foods contain cholesterol-lowering additives. For example, plant sterols and stanols have been shown to decrease LDL cholesterol level and are added to certain types of orange juice and margarine.
Smoking should be avoided, since it decreases the level of HDL cholesterol. On the other hand, regular exercise increases the level of HDL cholesterol, and so does weight loss (if it’s necessary).
A Diagnosis of High Cholesterol
If you are diagnosed with high cholesterol, there is a lot that you can do to improve your cholesterol profile. You may not even need to take cholesterol-lowering medications, although of course a doctor's recommendations must be sought. You can reduce or remove harmful foods from your diet, add foods known to lower LDL cholesterol, add healthy foods that might raise HDL cholesterol, stop smoking and get regular exercise. Begin exercising slowly if you haven’t exercised for a long time. These techniques will help to ensure that cholesterol stays a friend and never becomes a foe.
References and Further Reading
Questions & Answers
© 2011 Linda Crampton