Fat is an important nutrient in our diets, but there’s a lot of talk of different types of fats, and whether these types are beneficial or harmful to our health. These different fat classifications have their roots in chemistry – and chemistry can also help explain their effects. This graphic takes a look at the different classifications, their sources, and briefly about how they act in our body.
Firstly, it’s worth emphasising that fat has an important role to play in a balanced diet. The fat in our diets is a source of some ‘fatty acids’ that are termed ‘essential’: that is, the body is unable to make them itself, and relies on our diets as their source. They’re required because they help the body absorb vitamins that are fat-soluble. We’ve looked at vitamins before in a previous post – the vitamins that fats help to absorb are vitamins A, D, E and K. Additionally, fats help provide us with energy, and they are also important for a variety of functions in the body.
Fats also play an important role in the body with respect to cholesterol. Cholesterol is a substance we produce in our livers, and it is also present in some foods. Animal cells require cholesterol to maintain their cell membranes, and it’s transported in the blood by lipoproteins. There are a number of types of these proteins, but the carriers we’re mainly interested in when discussing fats are low density lipoproteins (LDL) and high density lipoproteins (HDL).
Discussion of LDL and HDL is often simplified to referring to them as ‘bad’ and ‘good’ cholesterol respectively. However, despite this perhaps over-simplified nomenclature, both are required; it’s the levels of each that are important. Cholesterol is still the same molecule in both cases, it’s merely the carrier that differs. LDL are the main cholesterol carriers in the bloodstream, but if their levels become too high, they can collect in the walls of blood vessels. As such, it’s generally thought that higher LDL levels are associated with detrimental health effects. HDL, on the other hand, helps transport cholesterol back to the liver, and as such higher HDL levels (as a percentage of total blood cholesterol) have been correlated with better health. Different fats can affect LDL and HDL in different ways.
So how do we distinguish between the different types of fat? Dietary fat comes primarily in the form of triglycerides. These are large molecules, consisting of a backbone based on the molecule glycerol, to which three different fatty acid chains are attached. Those with some chemistry knowledge will know that, in chemistry, we also refer to fatty acids as carboxylic acids. The chemical identities of these fatty acids is variable – in a particular triglyceride, it’s common for the three fatty acids to all be different.
It’s the chemical identities of the fatty acids that make up a triglyceride that we’re discussing when we talk about types of fat. There are four types we’ll discuss here: saturated fat, monounsaturated fat, polyunsaturated fat, and trans fat. Though the focus of this graphic is primarily on the differences in chemical structure and sources, I’ll also briefly discuss what we know about how they act in the body.
Fatty acids which contain no carbon-carbon double bonds are referred to as saturated fats. This type of fat is found in foods including meats, cheeses, dairy products, cakes, and chocolate. For a long time, saturated fats were the bad guys of the fat world – government guidelines recommended keeping their intake to a minimum, and high dietary saturated fat intake was associated with increased risk of coronary heart disease. Saturated fats do increase the levels of LDL in the bloodstream, so this seemed like a sensible suggestion.
However, in recent years, the assumption that saturated fat is unquestionably bad for you has been challenged. The original study that prompted the guidance of many of the world’s governments has been criticised for scientific flaws. Today, it remains unclear how big a factor saturated fat is with respect to heart disease.
Fatty acids which contain, at some point in their chemical structure, one carbon-carbon double bond are known as monounsaturated fats. They are found in high levels in red meat, nuts, high fat fruits such as avocados, and olive oil. Unlike saturated fats, they lower the levels of LDL in the blood, whilst maintaining HDL levels. As such, they improve the HDL to total blood cholesterol ratio in favour of HDL, and they’re thought to be beneficial to health.
Fatty acids which contain a number of carbon-carbon double bonds within their chemical structure are known as polyunsaturated fats. Two commonly mentioned subtypes are omega-6 and omega-3 polyunsaturated fats. The number in these subtype names refers to the position of the final carbon-carbon double bond in the fatty acid’s structure. This number is counted from the final carbon at the end of the hydrocarbon chain.
Polyunsaturated fats are found in significant levels in fish, walnuts, seeds including sunflower seeds and sesame seeds, and are also present to a degree in some meats. Much like monounsaturated fats, they lower blood levels of LDL, and as such are similarly considered to be beneficial to health.
The final category of fats, trans fat, is one that’s primarily introduced artificially into foods. Though trans fats are found at low levels in a small number of foods completely naturally, more often they’re introduced to oils via a process known as hydrogenation.
Hydrogenation involves the reaction of oils with hydrogen. This removes double bonds from polyunsaturated fats, converting them into single bonds. The process can convert liquid oils into solid, saturated, ‘hydrogenated’ fats if the hydrogenation is run to completion. However, trans fats can be produced when the hydrogenation is only partial.
Trans fats are so called because they still contain double bonds, but in a different orientation to that usually seen in fats found in nature. Those usually have their double bonds arranged in a cis configuration; in trans fats, they are in the trans configuration. This is a little difficult to explain in words, but there’s an old post on isomerism here which succinctly explains the difference.
Trans fats occur naturally in the milk and body fat of cattle and sheep. However, the partially hydrogenated oils used in the production of fast food, cakes, and shortenings have increased our intake of trans fat. These trans fats tend to raise levels of LDL in the blood, and have been associated with a cardiovascular health risk, to the point where their introduction into foods has been banned in a number of countries. Denmark has already eliminated commercial sources of trans fat, and in the US the FDA has recently followed suit, giving manufacturers 3 years to remove added trans fats from their products.
So which fat is best?
This is really a question that’s tough to answer. However, at the very least, we can say which type of fat is worst. Trans fat is clearly a type of fat that should be avoided where possible, due to its positive associations with the development of heart disease. The evidence on saturated fats is currently still conflicted, though it seems that they may not be as big a contributor to heart disease as was previously thought. Monounsaturated and polyunsaturated fats, on the other hand, have been associated with some health benefits as part of a balanced diet. I suppose the big take-away is that the different types of fat aren’t created equal; they all induce slightly different responses in the body, in ways we still don’t completely understand.
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References & Further Reading
Reappraisal of SFA & cardiovascular risk – T A Sanders
Comparison of effects of dietary fatty acids on lipoproteins in man – F H Mattson & S M Grundy
Trans fats in America: use, consumption, health implications & regulation – V Remig & others
Fats & fatty acids – Chemistry Explained
Trans fats & effects on coronary heart disease in Iran – D Mozaffarian
The truth about fats: the good, the bad, and the inbetween – Harvard Health Publications
Cholesterol and lipoproteins – Heart UK
Lifting the ban on total dietary fat – D Mozaffarian & D S Ludwig