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Dietary advice explained: Fat

Posted on Tuesday 1 October 2019

Charlotte Hawkins

For the last 70 years fat has been an emotive subject. Although fats all have the same basic construction, the word fat has come to mean two things - the fat that we eat, and the fat on our bodies. During the second half of the twentieth century it was believed that if we eat fat, we get fat. Fat contains more than twice the number of calories per gram than protein and carbohydrate (9 compared to 4), so on the surface that seems a logical conclusion. However, as we have developed a greater understanding about how the body metabolises different nutrients we now understand that the issue is not that straightforward.

What is fat anyway? We use the word "fat" to describe edible fats that are solid at room temperature, and "oil" to describe those that are liquid. In order to understand about the different fats, we first have to look at their biochemistry. Fats (also known as lipids) can have various structures, but most look a bit like the letter "E". They have a 'backbone' made of a substance called glycerol, and most fats have three chains constructed of carbon molecules that branch off the backbone, known as fatty acids. Because of the three fatty acids these fats are known as triglycerides. Most of these carbons are attached to a hydrogen atom, but in some fats, not all the carbon atoms have a hydrogen atom, and connect instead to another carbon atom.

People who take an interest in their diet and how it impacts on their health are now generally aware that there are three types of fat: saturated, monounsaturated and polyunsaturated. A saturated fat is one that is fully saturated with hydrogen atoms along the chain. This makes the chain form a straight line, and due to the way that the molecules stick together, saturated fats are solid at room temperature (e.g. butter). A monounsaturated fat is one that is saturated with hydrogen atoms except for one place, where two of the carbon atoms are stuck together instead - known as a double bond. This forms a kink in the chain, which makes them stick together less well, so monounsaturated fats tend to be quite thick, but softer than saturated fats. They are liquid at room temperature but will partially solidify when refrigerated (e.g. olive oil). A polyunsaturated fat, as the name suggests, has two or more places along the chain where two carbons are joined together. These fatty acids stick together even less well as they are very coiled due to all the double bonds, and are thin at room temperature. They will stay liquid in the fridge and will only set solid at very low temperatures (e.g. sunflower oil).

A further feature of fats is that the horizontal parts of the letter "E" - the part with the carbon atoms - varies in length from 4 to 24 carbons. These are put into fatty acid groups of short (5 or less carbons), medium (6-12 carbons), long (13-21 carbons) or very long (22+ carbons). This means that in reality there isn't just one type of saturated, monounsaturated or polyunsaturated fat, but a huge variety of combinations of degrees of saturation and length. The name of a fatty acid - perhaps the most famous are omegas 3, 6 and 9 - is given according to where the first double bond appears along the chain from the backbone. With polyunsaturated fats (e.g. omega 6 and 3), there are two or more double bonds, but the name is derived from where the first double bond occurs. The length of the carbon chain is hugely important as it means that some polyunsaturated fats can have many double bonds. Two well-known examples are found in oily fish: EPA has 20 carbon atoms and double bonds in 5 of those places, and DHA has 22 carbon atoms and 6 double bonds.

Fats in their triglyceride form cannot pass through cell membranes, so the body uses enzymes to break the "E" up and then it is reconstructed within the body as needed, and is either used for energy production, or for other metabolic processes. To complicate matters further, unsaturated fats can be in cis formation (with hydrogen and carbon atoms on the same side of the chain where the double bond is formed) or trans formation (with hydrogen and carbon atoms on opposite sides). There are some trans fatty acids found in nature, most notably in the milk of ruminant animals, but most trans fatty acids have been created artificially.

Why is all this fat biochemistry relevant?

Although it had been noted for a long time that certain foods could contribute to or cure diseases, little was understood as to why this was the case and it did not lead to a change of diet for the vast majority of people. This all changed in the 1950s when the American scientist Ancel Keys noticed a correlation between the amount of saturated fat in peoples' diets, blood cholesterol levels and heart disease. Keys' research has subsequently been heavily criticised both in its methodology and conclusions, but the message that saturated fat was a killer quickly began to spread, largely through an active publicity campaign by the American government. It did not take long for the message to cross the Atlantic, and by the 1980s the whole Western world had become fat-phobic, believing that not just saturated fat, but all dietary fat should be avoided as it led to both disease and weight gain. However, since the millennium the tide has most definitely turned, and the message that not all fat is bad and some fat is positively healthy has filtered through as it is now understood that different fatty acids have very different roles to play in the human body.

Whether saturated fat is good, bad or neutral for us is a debate that has raged for decades since Ancel Keys' research. On the good side, saturated fats are seen as stable. They do not oxidise easily when heated (and thus turn rancid), which in food makes them good for cooking at high temperatures. Saturated fats are also vital in the formation of cell membranes and are a good source of energy. The stability, function and energy provided by saturated fats is largely why the fats in human breast milk are mostly saturated as these are ideal for the growing infant. The length of the fatty acid chain makes a big difference as to how it is used within the body. For example, short-chain fatty acids, such as butyric acid commonly found in butter (hence the name) and medium-chain fatty acids, such as lauric acid found in coconut oil "burn" easily to provide a ready source of energy. Longer chain saturated fats have different roles and are largely used in the construction of cell membranes as they are solid at body temperature and help to provide structure to cell walls.

On the negative side, saturated fats have been viewed as harmful as they make cell membranes stiffer due to the way they stick together, and they impact on how blood platelets function, both of which are believed to increase the risk of heart disease. They also increase blood cholesterol levels - both "good" HDL and "bad" LDL. The link between saturated fat, cholesterol and heart disease is a hotly debated topic. Recent meta-analyses (studies of all the studies combined) have found no link between saturated fat and heart disease, and the cholesterol connection is now seen as rather simplistic. However, governmental bodies have held off changing their advice - in part because new research takes time to be assimilated as part of accepted scientific knowledge, but also because not all the experts agree with the results of more recent research. The official recommendation of the NHS, Public Health England and the World Health Organisation is still to reduce the amount of saturated fat in our diets. Continuing research may well mean that this advice is changed, so watch this space!

Current scientific consensus views monounsaturated fats as either beneficial for us or neutral, but certainly not  harmful. These fatty acids are the predominant fat in foods such as olive and rapeseed oil, many nuts (especially hazelnuts, almonds and pistachios), and also fruits such as avocados and olives. The high consumption of olive oil in the Mediterranean region has long been touted as the reason for the longevity and robust health of the inhabitants of those areas, although it is impossible to decipher whether it is due to the monounsaturated fat, special antioxidant compounds called polyphenols within the oil itself, or completely unrelated dietary and lifestyle habits. Within the world of fat research, monounsaturated fats are not seen as controversial.

Although there are many different types of polyunsaturated fat, the two types that are of most interest to researchers are those known as omega 6 and omega 3 fats. The human body can manufacture most fatty acids itself, but these two fats are known as "essential" as the body can't make these and we have to get them from our diet. Omega 6 fats are abundant in the foods we eat. They are found in large amounts in seeds and grains, and with such a large proportion of our calories now coming from "vegetable" (i.e. seed) oils and wheat, they are consumed in far greater proportions than they were in previous generations. Omega 3 fats are harder to obtain. They are found in some nuts and seeds (notably walnuts and flax), leafy green vegetables, and oily fish - none of which are consumed frequently in the average Western diet. The crucial issue around how much polyunsaturated fat we consume is actually about the balance between them rather than the overall amount. The optimum ratio of omega 6 to omega 3 fat for our health is an area of much scientific debate. It is believed that in Neolithic times the ratio was around 1:1, although it is impossible to back this up with hard evidence. Some researchers believe that this is our optimum level, although others put an acceptable ratio as up to 4:1 in favour of omega 6 fats. What is generally recognised, however, is that our current Western diet has a ratio of around 20:1 or even higher.

Why is this a problem? These fats are metabolised in a similar way. This means essentially they are in competition with each other, but omega 6 fats and omega 3 fats have very different functions within the body. Amongst other roles, most omega 6 fats tend to promote inflammation and omega 3 fats dampen it down.  Both processes are vital to us, but excess inflammation plays a role in many human degenerative diseases, and many researchers believe that the imbalance of omega 6 to omega 3 fats in the Western diet is the root cause of this.

The only fats that everybody agrees are bad for us are artificially created trans fats. These are made by a process called hydrogenation - where hydrogen and a metal catalyst (usually nickel) are mixed at a high temperature with a vegetable oil such as sunflower oil. This has the ability to change the structure of unsaturated fats by moving the hydrogen atoms in a double bond to the opposite (trans) sides of the fatty acid chain. This makes them behave more like saturated fatty acids in terms of their aggregative qualities as moving the hydrogen atom straightens the fatty acid out and makes the molecules stick together more easily. It makes them very useful in the manufacture of processed foods such as biscuits and margarines. They are cheap, vegetarian, easy to create and have a long shelf-life, so it is easy to see why the food industry has embraced them. They are, however, not useable by the human body other than for energy production, and evidence points to their link to many diseases when consumed at a high level. There has been pressure from many Western governments on food manufacturers to phase out their use of trans fats, but this has been difficult to do without substantially changing the processed foods which contain them. That said, there has been a move away from them in food manufacture in the last couple of decades, so trans fat consumption has dropped since the 1980s and 1990s.

So, in summary, these are the four types of fat in our diet. Further issues to consider are the fact that no foods contain just one type of fatty acid. All foods have differing combinations of fatty acids, but we identify a fat in food as being  saturated, monounsaturated or polyunsaturated based on the predominance of each type of fatty acid and whether they are solid or liquid at room temperature. For example, coconut oil is known as a saturated fat, as around 82% of it is saturated. Lard is thought of as saturated as it is solid at room temperature despite being comprised of only 40% saturated fatty acids, with 45% being monounsaturated. Olive oil is recognised as being a monounsaturated fat as it is composed of around 73% of this type of fatty acid. Corn oil is regarded as an omega 6 polyunsaturated acid as it is around 54% omega 6. Flax oil is regarded as an omega 3 polyunsaturated fat as it has around 53% of this type of fatty acid.

Also, while the debate usually centres around what type of fat we should be eating, it is now being questioned by some researchers whether the type of fat is actually as relevant as the degree of processing. The reductionist way of looking at our diets has meant that this point has for a long time been missed (and to a large degree, still is). Fats found in nature - be they saturated, monounsaturated or polyunsaturated - are found in real foods which also contain thousands of other compounds, which in themselves may or may not be good for us. But the crucial point is how they interact with other compounds within the same food - the issues outlined above surrounding olive oil is a classic example. Omega 3 fish oil supplements have been shown to be not as good for us as the consumption of oily fish, and there are numerous other similar cases.

We too have been led into a reductionist way of looking at things, and this has created a lot of worry and confusion about which types of fat are good or bad for us and which fats we should or shouldn't be eating. We have always survived - and thrived -  as a species by eating real food, but ultra-processed foods which contain a concoction of both isolated natural and artificial compounds have been at least partly, if not solely, responsible for the huge burden of suffering from diet-related  degenerative conditions such as obesity and type II diabetes since they became such a large part of our diet. In my view, if we stop worrying about the balance of fat that we eat and just eat real food - preferably that grown or from animals reared in as natural an environment as possible - we will probably be OK.

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