Reducing fat intake in the diet has been a key message of the government and health professionals for the last few decades but, in fact, some fats are essential for the proper functioning of the body. Fats provide vital energy for cells. They make up adipose tissue, which stores energy, cushions and protects vital organs, and provides insulation. Cholesterol which, technically, is not a fat, is needed to make cell membranes and the critically important sheaths around nerve cells and is the raw material from which the body makes many important hormones, such as oestrogen, progesterone and cortisol.
To enable these things to happen, fat must somehow get from your digestive system to your cells. This is not as easy as it sounds. Like oil and water, fats and blood do not mix. If digested fats were just dumped by the liver and intestines straight into the blood, they would congeal in oily blobs and be unusable. So the body has devised a way of packaging fats into particles coated with proteins, which are able to dissolve in the blood and flow with it. These tiny particles are called lipoproteins (lipid plus protein) and contain some cholesterol to help stabilise the particles.
Like a busy motorway, your bloodstream carries many sizes and shapes of fat-transporting particles. Lipoproteins are generally classified by the balance of fat and protein they contain. Those with a little fat and a lot of protein are heavier and denser than the lighter, fluffier and less dense particles that are more fat than protein. In addition to coating the particles and helping them to move in the bloodstream, the proteins also act like address labels that help the body direct fat-filled particles to particular destinations.
Some of these lipoproteins influence the risk of developing heart disease. The most important lipoproteins in this regard are high-density lipoprotein (HDL), low-density lipoprotein (LDL), and very-low-density lipoprotein, which is composed of triglycerides.
LDL is often referred to as ‘bad cholesterol’. When your bloodstream carries too many of these particles, they end up inside the cells that line the blood vessels. Once there, LDL is attacked by highly reactive free radicals and transformed into oxidised LDL. Oxidised LDL can damage the artery lining and set off a cascade of reactions that result in clogging of the artery and increase the risk of artery-blocking blood clots.
In contrast, HDL particles sponge up excess cholesterol from the lining of blood vessels and elsewhere and carry it off to the liver for disposal. They also help the liver to recycle other lipoprotein particles.
Triglycerides make up most of the fat you eat and most of the fat that circulates in your bloodstream. Triglycerides are essential for good health as your tissues rely on them for energy. Like cholesterol, though, too much triglycerides may be bad for the arteries and the heart.
If you have your blood tested for cholesterol levels, results will often be shown as total cholesterol and as the ratio between total cholesterol and HDL.
The levels of cholesterol fall into the following categories:
- Ideal level: cholesterol in the blood <5mmol/l
- Mildly high cholesterol level: 5 to 6.4 mmol/l
- Moderately high cholesterol level: 6.5 to 7.8 mmol/l
- Very high cholesterol level: above 7.8 mmol/l
In the UK the average total blood cholesterol level is 5.7 mmol/l compared with 3.3 mmol/l in rural China.
The ideal ratio of total cholesterol/HDL is below 3.5:1.
Willett, W.C. (2005). Eat, Drink and Be Healthy – the Harvard Medical School Guide to Healthy Eating, Free Press, New York.
Posted by: Jane Philpott MA (Oxon), MSc, PhD