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Dyslipidaemia means undesirable changes in your blood fats. This is a complicated field and what follows is a simplification.

Cholesterol

A raised blood cholesterol is the best known form of dyslipidaemia.. Cholesterol is an important component of cell walls – we all need it if we are to function properly. The cholesterol in the blood is carried by proteins called lipoproteins which can very roughly be divided into two forms – low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C).

LDL-C – “bad cholesterol”

LDL-C is available for laying down in the walls of the arteries in the plaques of atheroma (“hardening of the arteries”) – the pinch-points that narrow the artery. This is ‘bad’ cholesterol. A high level of LDL-C is a risk factor for atheroma, leading to heart attacks and strokes.

There are various causes of high LDL-C, including genetic factors, being overweight, having an underactive thyroid gland, some forms of kidney disease and eating a high-fat diet, particularly a diet with a lot of ‘saturated’ fat.

Saturated/unsaturated/trans fats

There are three main categories of fats: saturated fats, unsaturated fats, and trans fats. All are made up of carbon, hydrogen, and oxygen molecules. Saturated fats are saturated with hydrogen molecules and contain only single bonds between carbon molecules. They are largely derived from animals – cream, butter, fatty joints of meat and, particularly, prepared meat products such as meat pies, patés and sausages. Fat is cheaper than lean meat, so food manufacturers use it liberally. Saturated fats tend to be solid at room temperature. Eating too much saturated fat raises blood LDL-C.

Unsaturated fats have at least one double bond between carbon molecules and tend to be liquid at room temperature.Unsaturated fat is usually of plant origin and includes peanut oil, vegetable oils, such as sunflower and corn oil, fatty fish, such as salmon and mackerel, nuts and seeds, such as almonds, peanuts, cashews, and sesame seeds. Eating unsaturated fat does not raise LDL-C and may even bring it down.

Trans fats are vegetable oils which have been artificially hydrogenated to extend their shelf life. Examples include margarine and frying oils.Consumption of trans fats increases LDL-C and lowers HDL-C and is associated with an increase in all forms of cardiovascular disease.

HDL-C – “good cholesterol”

HDL-C is the form which transports cholesterol to the liver where it is broken down and excreted in the bile. This is ‘good’ cholesterol. A high level of HDL-C is protective for arterial disease while a low level is a risk factor for atheroma. The main causes of a low HDL-C are cigarette smoking, obesity, diabetes and physical inactivity

Normal levels

The ‘normal’ levels of these fat fractions is somewhat arbitrary and there is a tendency for each new generation of cholesterol police to set the limits for total cholesterol and LDL-C lower and lower. Currently the recommended upper limit for total cholesterol is 5.0 mmol/litre and for LDL-C it is 3.0. The bell-shaped curve of normal distribution would put most of the population in the too-high category.

HDL-C makes up around one quarter of the total cholesterol – the current recommendation is that it should be above 1.0 mmol/l. A better measure of risk for atheroma and cardiovascular disease is the ratio between HDL-C and total cholesterol. This should be below 4.0 and preferably much lower. This is the measure of cholesterol level used in algorithms for calculating risk of CVD (‘Q-Risk3’; see www.qrisk.org).

Triglycerides

These are esters derived from glycerol and three fatty acids. They are the main constituents of body fat in humans and other vertebrates and are carried in the blood stream. Raised levels are associated with genetic factors, unhealthy diet, obesity, diabetes, sedentary living and cigarette smoking. Raised triglyceride levels carry an increased risk of atheroma – heart attacks, strokes and peripheral vascular disease.

Next time

I will tell you about the effect of exercise on blood fat levels.

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