What are lipids?
Cholesterol is a type of lipid (fat). Lipids are substances that do not dissolve in water but are soluble in organic solvents such as chloroform or dry cleaning fluids. Other important types of lipid include triglycerides and fatty acids.
Lipids have several important roles in the body, providing:
- a source and store of energy
- an important part of the membrane surrounding every body cell
- the basic building blocks from which several hormones (chemical messengers) and bile acids (digestive juices) are made
- components of the nervous system.
All about cholesterol
The importance of cholesterol
Some cholesterol comes from your food, but most is made in your body, mainly in your liver, from the saturated fats (animal and dairy fat) that you eat.
Cholesterol occurs in every cell of your body and forms a vital part of the membrane that surrounds each cell, preventing cells from being too leaky.
Cholesterol is also the basis for many hormones, essential for the regulation of growth and the way your body works on a day-to-day basis. Hormones made using cholesterol include the following.
Oestrogens and progestogens
These hormones are produced by the ovaries and are responsible for female sexual characteristics and the menstrual cycle.
This hormone is produced by the testes and is responsible for male sexual characteristics and sperm production.
Cortisol is produced by the adrenal glands (there is one on the top of each kidney). Cortisol regulates your body’s response to stress.
Like cortisol, aldosterone is produced by the adrenal glands. Its main function is to ensure that the levels of salt and potassium in the body are normal.
1,25-Dihydroxycholecalciferol, the active form of vitamin D
Vitamin D is present in a well-balanced diet and is also produced in your skin on exposure to sunlight, particularly in summer months. Vitamin D is modified by your liver and kidneys to produce a hormone, 1,25dihydroxycholecalciferol. This controls calcium absorption from your gut, as well as normal bone development and health. Vitamin D deficiency in childhood causes rickets, whereas deficiency in adults leads to softening of the bones (osteomalacia).
Bile acids are also made from cholesterol in the liver and act like detergents in your gut, dissolving fat from your food. This is necessary for the normal digestion and absorption of lipids and fat-soluble vitamins (A, D, E and K). If bile acids don’t reach your gut from the gallbladder, where they are stored, fat is not digested properly – the result is diarrhoea, and the fat is lost in pale, very smelly stools.
Although it is clear that too much cholesterol can cause heart disease, it would be impossible to survive without it.
Chemical structure of cholesterol
Cholesterol has a very different structure to triglycerides (see page 22), although both are virtually insoluble in water. Cholesterol consists of carbon molecules linked in a series of rings; substances with this type of structure are termed ‘sterols’.
All about triglycerides
The importance of triglycerides
Your body’s fat stores contain triglycerides and these act as an important source of energy. Some are made in your body, whereas others come from your food.
After a meal, small amounts of sugar (glucose) are stored in your liver and muscle as larger molecules called glycogen (molecules are the smallest identifiable units of a substance).
Glycogen can supply short-term energy needs between meals or during short bursts of physical activity, such as sprinting. This is because it can make energy available very rapidly by breaking down to glucose. However, glycogen provides less than half the calories per gram that can come from fat stored as triglycerides.
In addition, glycogen is stored surrounded by water whereas triglycerides are not. You would therefore be considerably heavier if all your energy stores were in the form of glycogen (a 70-kilogram [kg] man would be 210 kg!).
Triglycerides, through the fatty acids that they contain, provide most of your body’s energy needs when you have not eaten for several hours, and can also supply your energy needs directly during prolonged exercise, such as running any distance greater than 400 metres.
If the process of using fatty acids to provide energy needs is disturbed in any way, you get hypoglycaemia (low blood sugar levels), particularly between meals.
As well as providing a vital source of energy, triglycerides help to make food palatable. It is extremely difficult to tolerate for very long a diet that includes less than 25 grams of fat per day.
Chemical structure of triglycerides
Triglycerides are composed of three fatty acids linked together by a small molecule called glycerol.
All about fatty acids
Fatty acids consist of long chains of carbon atoms linked together. There are two main types of fatty acids: saturated and unsaturated. Unsaturated fats can be either monounsaturated or polyunsaturated.
Saturated fatty acids
All the carbon atoms in the chains are linked by single chemical bonds; this type of fatty acid is found in large amounts in the triglycerides present in animal fat.
Monounsaturated fatty acids
All the links between carbon atoms in the chains are saturated bonds except for one that is unsaturated (a double bond). These occur in vegetable oils, such as olive oil and rapeseed oil.
Polyunsaturated fatty acids
More of the links between carbon atoms in the chain are unsaturated – as many as five or more. Fish oils are a particularly rich source of polyunsaturated fatty acids of a type sometimes called omega-3 fatty acids. Polyunsaturated fatty acids are also found in plant oils, such as sunflower oil.
How do fatty acids affect health?
The type of fatty acids in your diet affects your health. People with a relatively high intake of olive oil or fish have a lower incidence of heart disease than those who have a relatively high intake of saturated fat.
Saturated fatty acids raise blood cholesterol levels, whereas monounsaturated fatty acids do not affect blood cholesterol levels. The main effect of polyunsaturated fatty acids on blood lipids is to lower serum triglyceride concentrations, but they do not affect cholesterol.
Several other lipids are also found in your body. Some are vital components of your brain. An important group includes phospholipids, which resemble triglycerides in structure – the main difference is that one of the fatty acid groups is replaced by one that contains phosphorus. This alters the properties of the molecule because half of its structure is soluble in fat whereas the other dissolves in water. These dual solubility properties allow phospholipids to act as detergents and, together with special proteins, phospholipids are able to interact with cholesterol and triglycerides to form packages of lipids that are stable in water.
Triglycerides (found in animal and dairy fats) are the main form of energy store in your body
Cholesterol is a lipid (fat) and does not dissolve in water
Cholesterol is essential for normal cell structure
Several vital hormones are made from cholesterol