Linda Crampton is a writer and former teacher with a first-class honors degree in biology. She writes about the scientific basis of disease.
An Essential Chemical
Cholesterol is an essential chemical in the human body and has many vital 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. Taking steps to keep the substance at a healthy level is therefore very important.
Cholesterol exists in several 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 the LDL form under control.
LDL cholesterol and HDL cholesterol are often referred to as different chemicals. The only difference is the specific type of molecule attached to the cholesterol, however. The attached substance acts as a carrier.
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 chemical maintains the correct fluidity of the membrane at different temperatures. It increases the fluidity at low temperatures and decreases 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 waxy substance. It’s not inherently “bad.” In fact, your body needs it to build cells. But too much cholesterol can pose a problem.
— American Heart Association
LDL Cholesterol Facts
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. The lipoproteins transport their cargo from the liver to the rest of the cells in the body, which is a necessary function. When there is too much LDL cholesterol in the blood, however, 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, impeding blood flow.
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Atherosclerosis and Arteriosclerosis
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 may 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 may 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. When the artery walls become stiff and inflexible, a person is said to have arteriosclerosis.
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 their cargo 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 is still considered valid by many nutritionists. 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 that decrease the level of the substance 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. It's probable that there is more to learn about the other versions of the substance as well.
Researchers have 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.
Blood Test Results
Several types of blood tests 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 the chemical.
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 the LDL form of the substance, the VLDL type stimulates cholesterol to build up in the arteries. The normal blood level of the chemical is often said to be between 5 and 40 mg/dL.
Sometimes the ratio of total cholesterol to HDL cholesterol is reported. The lower this ratio, the better (within certain limits). A desirable ratio is said to be 4.0. A ratio of 5.0 is considered to be a borderline value and 6.0 a high one.
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 level at less than 50mg/dL (1.3 mmol/L).
|Total Cholesterol Level (mg/dL)||Total Cholesterol Level (mmol/L)||Possible Significance|
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)||Possible Significance|
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)||Possible Significance|
Less than 40 mg/dL
Below 1 mmol/L
High 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 (but see note below)
Healthy Fats in the Diet
According to the majority of health experts, 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 the HDL type.
Our liver helps control the amount of cholesterol in our body. For example, if we obtain the substance from food, the liver reduces the amount that it makes. However, there is a limit to the liver's ability to help us.
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 Facts and Effects
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.
Foods containing significant amounts of soluble fiber include the following:
- grains such as oatmeal and barley
- vegetables such as peas, beans, beets, parsnips, carrots, potatoes, and sweet potatoes
- fruits such as bananas, apples, pears, strawberries, plums, prunes, and citrus fruits
Other Foods That Affect Cholesterol Level
While there is strong evidence that certain foods lower LDL cholesterol, the evidence that specific foods raise the HDL type is less strong. Alcohol appears to increase the substance, 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 has been found to lower LDL cholesterol and significantly raise 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 this is necessary).
A Diagnosis of High Cholesterol
If you are diagnosed with high cholesterol, there is a lot that you can do to improve the situation. You may not even need to take cholesterol-lowering medications, although of course a doctor's recommendations should be sought and followed. You can reduce or remove harmful foods from your diet, add foods known to lower LDL cholesterol, add healthy foods that might increase the HDL type, stop smoking, and get regular exercise.
If you are very out of shape, it's important to begin exercising slowly. It's also important to get a health check-up and advice from your doctor before you start an exercise program. These steps are worth doing. Suitable exercise and diet will help to ensure that cholesterol stays a friend instead of becoming a foe.