What is cholesterol?
Cholesterol travels around the body in the blood using a special “transport system” made of proteins. This combination of cholesterol and proteins are called lipoproteins, of which there are two types:

• Low-density lipoproteins (LDL), which carry cholesterol from the liver to the body’s cells. LDL is often called “bad” cholesterol.
  
• High-density lipoproteins (HDL), which carry surplus cholesterol back to the liver. HDL is sometimes known as “good” cholesterol.   

The term “blood lipids” refers to all the fatty substances carried around the body in the blood, including HDL cholesterol and LDL cholesterol.

Why are high cholesterol levels dangerous?
High cholesterol levels are dangerous because they significantly increase the risk of heart disease, which, in turn, increases the risk of heart attacks and strokes. In people who have high cholesterol levels, particularly those with low HDL cholesterol and high LDL cholesterol, fatty materials build up on the inside of artery walls causing the blood vessels to narrow – a condition called atherosclerosis.

This narrowing of the arteries makes it more difficult to get adequate supplies of blood and oxygen to the body’s vital organs. It also increases the risk of blood clot formation, which can block arteries that supply the heart muscle and brain.  

What causes high cholesterol?
High cholesterol levels can be caused by a number of factors, including a family history of the condition (familial hypercholesterolaemia) and certain medical conditions, but in most people they arise from a diet that is high in saturated fats and a lack of physical exercise, which tends to raise LDL cholesterol levels and lower HDL cholesterol.

In summary, high cholesterol levels result from one, or a number of the following:

• A poor diet that is high in saturated fats
• A lack of physical exercise
• Being overweight or obese
• Smoking
• Alcohol abuse
• Familial hypercholesterolaemia
• Certain medical conditions, including diabetes, kidney disease and an underactive thyroid.

What are healthy cholesterol levels?
Cholesterol levels are measured using a simple blood test. However, because cholesterol levels can vary from day to day, and tend to increase as we get older, the doctor will usually need to take a number of measurements on separate days after the patient has fasted for around 12 hours to ascertain “true” cholesterol levels.

Because HDL removes cholesterol from the artery walls and returns it to the liver it is important to measure levels of both HDL and LDL, as well as total cholesterol levels, to find out whether or not a person has a healthy ratio of these blood fats. Recommended healthy cholesterol levels differ between countries and are measured in either millimoles per litre (mmol/L) in Europe or milligrams per decilitre (mg/dL) in the US.

Based on a number of other measures, such as age and gender as well as presence of other risk factors, results of a cholesterol test will be used to estimate a person’s risk of heart disease, and to indicate whether or not lifestyle changes and/or treatment is needed to reduce cholesterol levels:

As a guide, the latest US classifications of blood cholesterol levels are as follows:

Total cholesterol (mg/dL)
Under 200    -   desirable 
200-239       -   borderline high
Over 240      -   high

LDL cholesterol (mg/dL)
Below 100    -   optimal
100-129        -   near optimal
130-159        -   borderline high
160-189        -   high
Above 190    -   very high

HDL cholesterol (mg/dL)
Below 40      -    low
60 or above   -    high 

In the UK, recommended levels of HDL cholesterol should be no higher than 5 mmol/l and LDL cholesterol should be below 3 mmol/l.
 

Achieving healthy cholesterol levels
For people with borderline/high cholesterol levels, lifestyle changes may be an effective way to reduce these levels and improve the ratio of HDL to LDL cholesterol.

• Diet – Research has shown that it is possible to reduce the level of cholesterol in blood by between 5% and 10% just by eating healthily. Cholesterol is not found in large amounts in many foods, except in eggs and in offal such as liver and kidneys. However, limiting the amount of saturated fats is particularly effective in reducing cholesterol. This should also be combined with the recommended five portions of fruit and vegetables each day. Limiting salt intake is also recommended as a high intake can lead to high blood pressure, which is also a risk factor for atherosclerosis.
  
• Exercise – Engaging in regular physical activity increases the amount of HDL cholesterol in the blood, while reducing the amount of LDL cholesterol. It also promotes weight loss and weight control while increasing the effectiveness of the body’s blood circulation system. 

However, in some people, such as those with very high cholesterol levels and those with familial hypercholesterolaemia, simple lifestyle changes may not be enough to reduce cholesterol levels and doctors may also recommend certain medications, such as cholesterol-lowering drugs called statins, to achieve the desired effect.  

Further information:

American Heart Association
http://www.americanheart.org/presenter.jhtml?identifier=1200000

British heart foundation
http://www.bhf.org.uk/

National Heart, Lung, and Blood Institute
http://www.nhlbi.nih.gov/  

Harvard School of Public Health: Fats & Cholesterol
http://www.hsph.harvard.edu/nutritionsource/fats.html 

Heart UK
http://www.heartuk.org.uk/