There are several reasons why we should be interested in the amount of cholesterol circulating in our bloodstream.
However, to interpret our cholesterol numbers, we have to know certain things about cholesterol and its role in health and disease.
What Is Cholesterol?
Cholesterol is classified as a sterol (a combination of steroid and alcohol) and belongs to a group of substances called lipids.
It is a major structural component of cell membranes. Many essential hormones and vitamins are synthesized from cholesterol.
Dietary cholesterol is only found in foods of animal origin.
Although cholesterol is necessary for the human body, we don’t have to consume it. The liver can synthesize cholesterol. Therefore, it is not needed in the diet.
If our nutritional status is otherwise sufficient, we can live on a cholesterol-free diet because the body will produce the cholesterol needed.
How Is Cholesterol Transported in Blood?
Lipids are insoluble in water and can, therefore, not be transported in blood on its own.
To make lipids soluble in blood, the body straps lipids to specific proteins that function as transport vehicles.
These proteins carry different types of fats, such as cholesterol, triglycerides (TG), and phospholipids (1). These combinations of fats and protein are termed lipoproteins.
There are five major types of lipoproteins; chylomicrons, very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) (2,3).
LDL is called low-density lipoprotein because LDL particles tend to be less dense than other cholesterol particles.
The function of LDL is to deliver cholesterol to cells.
Excess cholesterol from cells is brought back to the liver by HDL in a process known as reverse cholesterol transport.
The liver is responsible for the production and clearance of lipoproteins.
LDL vs. HDL- Cholesterol – What’s the Difference?
LDL-cholesterol (LDL-C) is often called the “bad” cholesterol, whereas HDL-cholesterol (HDL-C) is termed the “good” cholesterol.
This is because high levels of LDL-C are associated with increased risk of heart disease, whereas high HDL-C is associated with less risk.
Of course, it is the same cholesterol. The difference is the lipoproteins.
Because high blood levels of LDL-C are associated with increased risk of heart disease, LDL-C measurements can be used to evaluate the risk of future cardiovascular disease.
Hence, we traditionally define high LDL-C as a risk factor, similar to smoking, high blood pressure, and diabetes.
What Does High LDL-Cholesterol Mean?
Many studies have shown a strong correlation between LDL-C and the risk of heart attack, stroke, and peripheral arterial disease (4). However, this association appears to become less prominent with increasing age.
Furthermore, some studies have suggested that low LDL-C may be associated with increased mortality in older adults (5).
Much available evidence suggests that lowering blood levels of LDL-C reduces the risk of heart disease (6).
However, not all people with high levels of LDL-C develop heart disease. Furthermore, many patients who develop heart disease do not have high LDL-C.
LDL particle size may also be important when assessing risk. Thus, having small LDL particles is associated with higher risk (9).
There are specialized receptors on cell surfaces that bind LDL-C. These are called LDL-receptors.
A lack of LDL-receptors may reduce the uptake of cholesterol by the cells, forcing it to remain in the circulation, thereby raising blood levels.
Familial hypercholesterolemia is a genetic disorder where the LDL-receptors don’t function correctly (10). Consequently, LDL cannot be removed from the blood, leading to high LDL-C levels in the blood, which may severely increase the risk of heart disease, even at a young age.
How Is LDL-Cholesterol Calculated?
Cholesterol levels are measured in milligrams (mg) of cholesterol per deciliter (dL) of blood in the US. Canada and most European countries measure cholesterol in millimoles (mmol) per liter (L) of blood.
Although blood tests typically report LDL-C, these numbers are usually not based on measurements but a calculation using the so-called Friedewald equation (11).
Direct LDL-C measurements are available but are less often done due to higher costs.
The Friedewald equation includes total cholesterol, HDL-C, and triglycerides. This equation assumes that the ratio of triglyceride to cholesterol is constant, which is not always the case (12).
Thus, LDL-C calculations may have limitations when blood triglyceride levels are either high or low.
If mg/dl is your unit, like in the US, the formula looks like this:
LDL cholesterol = [Total cholesterol] – [HDL cholesterol] – [TG]:5
If mmol/l is your unit like in Australia, Canada, and Europe, the formula looks like this:
LDL cholesterol = [Total cholesterol] – [HDL cholesterol] – [TG]:2.2
It is considered essential to keep cholesterol levels, especially LDL-C, within certain limits.
If other risk factors are present, such as high blood pressure, diabetes, or smoking, keeping LDL-C low becomes even more critical. Hence, when considering the “normal” range for LDL-C, other risk factors have to be considered.
Here is how LDL-C levels are interpreted:
– above 190 mg/dL (4.9 mmol/L) is considered very high
– 160 – 189 mg/dL (4.1 – 4.9 mmol/L) is considered high
– 130 – 159 mg/dL (3.4 – 4.1 mmol/L) is considered borderline high
– 100 – 129 mg/dL (2.6 – 3.3 mmol/L) is considered near ideal
– below 100 mg/dL (below 2.6 mmol/L) is considered ideal for people at risk of heart disease
– below 70 mg/dL (below 1.8 mmol/L) is considered ideal for people at very high risk of heart disease
How To Lower LDL-Cholesterol
For many individuals, reducing LDL-C may lower the risk of heart disease.
If LDL-C is high, most doctors will probably suggest lifestyle changes.
Quitting smoking will be helpful, and so may eating whole grain, oatmeal, olive oil, beans, fruit, and vegetables.
Most doctors will recommend eating less saturated fat, found primarily in meat and dairy products.
Trans fats should be avoided. These fats are found in fried foods and many commercial products, such as cookies, crackers, and snack cakes.
However, keep in mind that food products labeled “trans-fat-free” may still contain some trans fat. In the US, if a food is labeled “trans-fat-free” if it contains less than 0.5 grams of trans fat in a serving.
Soluble fiber may help lower LDL-C. It is found in oats and oat bran, fruits, beans, lentils, and vegetables.
Some studies show that low-carbohydrate diets may positively affect LDL particle size and number (13).
Weight loss in overweight or obese people is usually recommended.
If lifestyle changes don’t help, your doctor may suggest medications that lower cholesterol. So-called statins are the most commonly used drugs for lowering cholesterol.
Studies have shown that statins improve prognosis among patients with established cardiovascular disease.
Their role for treatment of raised LDL-C in healthy people (primary prevention) is less clear (14).
The decision to give statins in primary prevention is usually based on other risk factors as well as the LDL-C value itself (15).