Different fats

Fat is an important nutrient that is essential to normal body function.   Fat supplies us with energy, it is our means of storing energy and it allows other nutrients such as fat soluble vitamins  to function efficiently.  Every cell of the body is surrounded by a fatty layer (called a ‘bi-layer’), otherwise known as a fatty acid membrane.  The type of fat we eat in our diet becomes part of this membrane and has major influences on the function of that cell (imagine, for example, if you ate only pies, doughnuts and chips, your cell membranes would be made up of sticky, rigid hydrogenated fats!)

Different fats also have different functions.  Some are easily used as fuel, some are mainly structural and others have a more specialised role, influencing how our brains and bodies function.

Chemically, fats are usually known as fatty acids and the majority of edible fat is in the form of triglycerides (three fatty acids on a glycerol backbone in chemistry terms).  There are two major sources of fat, however: animal and vegetable.  When fat is at room temperature it can be in liquid or solid form: those which are liquid at room temperature are usually referred to as oils and, those which are solid at room temperature, are referred to as fats.  Oils are usually derived from plants and fat is most often derived from animals.

Fatty acids are composed of carbon and hydrogen molecules in a chain-like formation.  The numbers of carbon atoms which form the chain define the fatty acid. Those fatty acids containing  carbons 18 carbons are known as short-chain fatty acids and those with 18 or more known as long-chain fatty acids. Short-chain fatty acids can actually be converted to long-chain fatty acids in the body, albeit an inefficient process, via enzyme elongation reactions. Fatty acids can be divided into the following categories:

Saturated fat

Saturated fatty acids are so called because all the carbon atoms molecule of fat are covered in hydrogen atoms.  This means that each fatty acid is very rigid, so when several saturated fatty acids are put together they fit very efficiently, packing together tightly.  Saturated fat is therefore quite solid at room temperature and is most common in animal products such as butter, cheese and the white fat found on meat. Many processed foods, such as cakes, biscuits, pastries and crisps, also contain high levels of saturated fat because of their butter or margarine content.  Eating lots of saturated fats over a long period of time can be associated with increased health problems such as cardiovascular disease and stroke.  This is because the fat we eat becomes incoporated into our cell membranes, making them very rigid, slowly affecting cell communication, as well as the passing of nutrients and waste products in and out of the cell.  Saturated fats can be obtained from the diet or made by the body from carbohydrate and protein.

Monounsaturated fat

Monounsaturated fat differs from saturated fat in that one of the carbon atoms is missing a hydrogen atom, causing what is known as a double bond.  The word mono, meaning ‘one’, tells us that monounsaturated fats have one double bond.  The significance of this is that these fats have a bend caused by the double bond and therefore can’t pack together very efficiently, which also makes them flexible, unlike the rigid saturated fats.  Because of this, most monounsaturated fats are liquid oils at room temperature. If we have lots of these types of oils in our diets, our cell membranes are more fluid, cells can communicate with other cells more efficiently and also promote the efficient flow of nutrients and waste products in and out of the cells.

Monounsaturated fats can be obtained from the diet or made by the body from carbohydrate and protein. Found in foods such as olive oil and avocados, these fats are considered to be very beneficial to our health. They are central to the Mediterranean diet, which is considered to have many health benefits, including reduced risk of major chronic disease. such as heart disease and cancer.

Polyunsaturated fat

Polyunsaturated fats have many double bonds (‘poly’ meaning  ‘many’), making them just about the most healthy of all the fats because they are so flexible, which is particularly beneficial for our cell membranes and cells to function optimally. Unlike saturated and monounsaturated fat, there are certain polyunsaturated fats which cannot be made by the body and therefore have to be consumed in the diet to avoid a deficiency – hence the term  ‘essential’. The description ‘essential’ doesn’t relate to the physiological benefits they bestow, although they do independently have important roles in our health. Indeed, it is their longer-chain relatives which are most important for health, but these are not termed ‘essential’ because they can in theory be manufactured by the body (in reality this process is not terribly efficient).

Essential fatty acids are plant-based ‘short-chain’ fatty acids , divided into two families known as omega-3 and omega-6.  Once these fats are consumed we have the chemical machinery, in the form of enzymes, to modify and convert these fats into more useful fats, known as ‘long-chain’ fatty acids.  Whilst short-chain fatty acids have a major role in energy supply, the long-chain fatty acids play a significant role in regulating inflammation, the immune response and the cardiovascular system, through their conversion to hormone-like substances called eicosanoids.

Enzymes are needed to convert one fatty acid to another

Desaturase enzymes  remove two hydrogen atoms from the fatty acid chain, creating a carbon/carbon double bond.   Elongase enzymes add carbon atoms to the fatty acid chain, making it longer.

Cyclooxygenases (COX) are enzymes that modify AA, DGLA or EPA to produce prostaglandins. There are two main types: COX-1 is responsible for the baseline levels of prostaglandins, whereas COX-2 produces prostaglandins through stimulation.

The omega-3 family

Omega-3 fatty acids are so called because the very first double bond occurs at the 3rd carbon from the omega end.  The significant omega-3 fatty acids are:
α-linolenic acid (alpha-linolenic acid or ALA) –  the ‘parent’ short-chain omega-3 fatty acid found mainly in flaxseed, canola and soybean oils.
Stearidonic acid (SDA) – a short-chain omega-3 fatty acid primarily found in echium seed oil.
Eicosapentaenoic acid (EPA) is a long-chain omega-3 fatty acid found in fish, fish oils and eggs.  EPA is the precursor to the series-3 prostaglandins, series-3 thromboxanes and series-5 leukotrienes.  Vegetarians have to synthesise their own EPA from other fatty acids such as ALA and SDA.
Docosahexaenoic acid (DHA) is a long-chain omega-3 fatty acid found in fish, fish oils and eggs and gives rise to resolvins and protectins.

The omega-6 family

Omega-6 fatty acids are so called because the very first double bond occurs at the 6th carbon from the omega end. The significant omega-6 fatty acids are:
Linoleic acid (LA) –  the ‘parent’ short-chain omega-6 fatty acid found mainly in flaxseed, canola and soybean oils.
-linolenic acid (gamma-linolenic acid or GLA) is a short-chain omega-6 fatty acid found in borage, evening primrose and black current seed oils.
Di-homo-linolenic acid (DGLA) is a long-chain omega-6 fatty acid found in meat, poultry and eggs.  DGLA is the precursor to the series-1 prostaglandins and series-1 thromboxanes.

Arachidonic acid (AA) is a long-chain omega-6 fatty acid found in meat, poultry and eggs.  AA is the precursor to the series-2 prostaglandins, series-2 thromboxanes and series-4 leukotrienes.

Fatty acids functions

Eicosanoids are hormone-like substances derived from long-chain fatty acids, which regulate immunity, inflammation and blood clotting, comprising  prostaglandins, thromboxanes and leukotrienes.

Prostaglandins are produced from AA, DGLA or EPA by an enzyme called cyclooxygenase.  Prostaglandins can be inflammatory (derived from AA) or anti-inflammatory (derived from DGLA and EPA).  Prostaglandins are involved in the activation of the inflammatory response, production of pain,  muscle contraction and fever.

Thromboxanes are produced from AA, DGLA and EPA. Thromboxanes are involved in blood clotting and the contraction or dilation of arteries.

Leukotrienes are produced from AA and EPA. Leukotrienes are involved in immune response and lung function.
Cyclooxygenases (COX) are enzymes that modify AA, DGLA or EPA to produce prostaglandins. There are two main types: COX-1 is responsible for the baseline levels of prostaglandins, whereas COX-2 produces prostaglandins through stimulation.

Fats to avoid

Trans fat

Trans fats or hydrogenated fats are not real fats but are synthetically made by an industrial process. This process literally fills in the gaps found in liquid unsaturated fats by adding hydrogen atoms to make them saturated and therefore more solid. Trans fats are popular in the food industry and are added to food to increase the shelf life and improve on taste. Trans fats have no nutritional benefit, however, and are considered to be health negative by increasing LDL cholesterol (bad cholesterol) and lowering HDL cholesterol (good cholesterol), which in turn raises the risk of developing coronary heart disease and stroke.

Cholesterol is a fat-like substance which cannot travel in the blood on its own and  must group with special proteins to form a lipoprotein.  The main groups of lipoproteins, based mainly on their different sizes and density are: high-density lipoproteins (HDL), low-density lipoproteins (LDL), very low-density lipoproteins (VLDL).  Each group has a different function in the body.  The majority of cholesterol is actually produced in the body by the liver, but  is also consumed via the diet in animal products such as eggs, meat and dairy.  The body uses cholesterol usefully to manufacture hormones, make vitamin D, build cell walls, and create bile salts that are needed to digest fat.  Cholesterol is therefore an important molecule, though too much cholesterol may cause fat to build up in the walls of the larger arteries, leading to plaque formation. Otherwise known as atherosclerosis, this can cause narrowing of the arteries, leading to heart attacks and strokes.

LDLs are produced by the liver and carry cholesterol and other fats from the liver to the different areas of the body, like muscles, tissues, organs, and the heart. If levels of LDL are high then this is an indication that there is an excess which then has the ability to stick to blood vessels, causing plaque build up.  LDL is therefore known as “bad cholesterol”. VLDLs are another type of lipoprotein that carry cholesterol and triglycerides from the liver to organs and tissues in the body.  VLDLs can become LDLs, and are also associated with atherosclerosis and heart disease.

In contrast, HDL is known as “good cholesterol”. Rich in protein, it also carries cholesterol away from the cells and back to the liver, where it is broken down in readiness to be excreted from the body.  Most importantly, HDL can actually remove cholesterol from plaques that have built up on artery walls, thereby helping to lower the risk of heart disease. High levels of HDL cholesterol are therefore beneficial; ideally the amount of HDL should be more than 20% of total cholesterol, which should be less than 5mm/L.

Total cholesterol levels and risk of heart disease.