 | Cholesterol |
Cholesterol is a steroid that plays an essential role in the metabolism of the body. It is present in some amount in nearly all animal tissues, but it is concentrated in the spinal cord and composes about 10% of the dry matter of the brain. Cholesterol is also present in many cell membranes, especially red blood cells, liver cells and the myelin sheath around nerve cells, and there is a surprising amount of it in a human body. In a normal adult there is about a quarter of a kg of cholesterol(!), present as both the free alcohol and as the ester of fatty acids. It is also the main component of gall stones - and indeed it was first isolated from gall stones by Green in 1788. Commercially, cholesterol is obtained from the spinal cords of cattle or from wool grease.
| | Cholesterol |
 | Cholesterol forms a major part of the brain |
The liver synthesises 90% of the 3-5 grams of cholesterol that the body makes each day, from a compound called acetyl coenzyme A (acetyl CoA). If it isn't used to form biomembranes, then specialised cells in the liver, adrenal glands and ovaries convert cholesterol into many other important steroids like vitamin D, steroid hormones (such as testosterone and estradiol) and the bile acids that are necessary for efficient digestion. In fact the name cholesterol comes from the Greek words for 'bile solid'.
Most people have heard of cholesterol from the bad press it has received due to its links with heart disease. This is because cholesterol accumulates on the walls of arteries, slowly restricting the flow of blood, and eventually completely blocking them. This process is called atherosclerosis from the Greek words for 'hard' and 'porridge'. If an artery supplying the heart muscle is completely blocked, a heart attack results. The atherosclerosis process actually involves quite a delicate biochemical balance. Because cholesterol is required by many cells to form their cell membranes, it must be transported in the bloodstream from where it is made in the liver to where it is required. But since the cholesterol molecule is mostly composed of non-polar carbon rings, it is completely insoluble in water. Therefore it is carried in the bloodstream by special particles called low density lipoproteins or LDL. However in order to mop up the excess, high density lipoproteins (HDL) also circulate in the blood and serve to remove cholesterol from the bloodstream. Heart disease has been correlated with high LDL and low HDL levels in the blood. Sadly, more than half the people in the industrialised countries of the West have high LDL levels putting them in the high risk category.
 | A partially blocked artery. The build up along the inside of the artery walls is mainly composed of cholesterol. |
The reasons for this are maybe to be found in the modern Western diet - we eat far more foods rich in cholesterol and saturated fats (like butter and cream) than we require for normal metabolic processes, and so the levels in our bloodstream can become dangerously high. However, there is some evidence that unsaturated fats (such as those found in vegetable oils and margarine) are less prone to causing atherosclerosis. As a result, over recent decades, millions of people have changed their eating habits, switching from saturated to polyunsaturated fats. The results have been impressive - for example, since 1955 the death rate in the US from cardiovascular disease has decreased by 40%.