• Marcus Baxby

All About Fats

Updated: Feb 13, 2019

Intro - Does Fat make you Fat? Or does it do the opposite?


The idea that eating fat makes you fat was engrained in public consciousness for many years. It’s understandable – eating fat in the diet, for many people, logically corresponds with making you fat, and when most people hear ‘high fat content’, and especially ‘saturated fat’, they immediately think ‘unhealthy’.

There was some fairly dubious research done in the 70s which seemed to suggest that fat, particularly saturated fat, corresponded to poor health, and this marked the start of the anti-fat movement, and the low fat diets of the 70s and 80s. However, as nutritional science has evolved, so has our understanding of fats and the role they play in the diet. Spoiler alert – we need fats in our diet to live, and low-fat diets do not necessarily mean low-fat physiques!

However, as a back-last to the low-fat movement, there is now an diametrically opposing view that going high-fat, and low carb is the 'secret' to fat loss - this has been popularised by high-profile Instagram celebrities, and many 'bio-hacker' types pushing the ketogenic diet. This is equally as ridiculous. As we know from our Core Principles, energy balance (calories in vs calories out) is the governing principle behind weight loss. The amount of fat in a person's diet doesn't matter for fat loss - provided they are in a calorie deficit, they will lose fat. Fat is the most energy-dense macronutrient, at 9 kcals per gram, so intake must be managed appropriately, but going low-fat or high-fat are methods, not the principle.

[to skip to take-home points - scroll to the bottom]

Fat basics

So let’s start with what fat is. Get ready for some GCSE chemistry. The simplest form of fat is a fatty acid. These are chains of hydrogen and carbon atoms, bookended by other chemicals (for the chemistry geeks - a methyl group at one end, and a carbolic acid group at the other).

Saturated fats have no double bonds in their chemical structure (see below), meaning that all the carbons are joined to two hydrogens.

Unsaturated fats, on the other hand, have at least one double bond, meaning that not all of the carbons are attached to a hydrogen. (see below) Unsaturated fatty acids are either monounsaturated (one double bond), or polyunsaturated (multiple double bonds). Examples of polyunsaturated fatty acids include the often-discussed omega-3 and omega-6.

Most dietary fats (fats found in food) come in the form of triglycerides. As the name suggests, triglycerides contain three fatty acids attached to a glycerol backbone. Different fatty acids can therefore join up to form various different triglycerides. In other words, most dietary fat sources are made up of some combination of saturated, polyunsaturated and monounsaturated fats. For example, while most people consider eggs and red meat to be foods rich in saturated fat, eggs actually contain more monounsaturated fatty acids than saturated fatty acids. Indeed, 39% of the fat in eggs is saturated, while 43% comes from monounsaturated fat, and 18% from polyunsaturated fat. Beef contains 55% saturated fat, 40% monounsaturated fat, and 4% polyunsaturated fat. Humans have consumed these types of naturally-occurring combinations for their entire existence, from meat, fish and plant sources. Early human diets included all parts of the animal (which still occurs in many cultures, such as Japan), including very fatty tissues such as blubber, organs and brains. Dietary fat plays an important role in the human body, including:

  • Providing energy (as we mentioned, it is the most energy-dense of the three macronutrients, containing around 9 kcals per gram, compared to protein and carbohydrate, each of which contain around 4 kcals per gram)

  • Helping to manufacture and balance hormones (particularly sex hormones - testosterone for men, progesterone and estrogen for women - these have implications for body composition, mood, libido etc).

  • Forming our cell membranes

  • Forming our brains and nervous systems, and keeping them functioning correctly

  • Transporting vitamins A, D, E and K (these are fat-soluble vitamins)

  • Providing two essential fatty acids, which the body cannot produce itself: linoleic acid (an omega-6 fatty acid), and alpha-linolenic acid (an omega-3 fatty acid, aka ALA)

  • Providing conditionally essential fatty acids, which the body can produce itself, but often does not produce enough. These include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), both of which are omega-3 fatty acids.In order to fulfil these vital functions, the body requires a good balance of saturated, monounsaturated and polyunsaturated fats.


Since vitamins A, D, E and K are fat-soluble, we must eat some fat to be able to absorb them properly. A fat deficiency can therefore lead to deficiencies in these vitamins.

Adding butter or oil to vegetables also makes them taste delicious! Saturated Fat Initially, saturated fat was painted as the root of all evil, and was linked with cardiovascular disease, while unsaturated fat was considered the good guy. This turned out to be a gross oversimplification. The early studies that seemed to associate saturated fat with cardiovascular disease were merely observational studies, and did not take into account that the people that were eating diets high in saturated fat were also smoking, eating highly processed diets and not doing much exercise.

What has now been discovered is that not all saturated fats are created equal. Stearic acid, a saturated fat abundant in meat and milk fat, has been consistently observed to actually benefit human health by reducing blood platelet aggregation. Dairy is the poster-food for saturated fats, since it also contains protein, calcium and fat soluble vitamins. It has also been associated with lower insulin resistance, better cholesterol profiles and reduction in incidences of type 2 diabetes. (Mozaffairan et al (2010)).


It is also worth noting that the early humans who consumed diets high in naturally occurring fats (including lots of saturated fat) had none of the cardiovascular and heart disease issues that continue to plague modern populations.


Dietary guidelines which say that saturated fat should be avoided or limited to 30g per day or to no more than 10% total energy (UK Guidelines), are overstated. In general, we know that variety is key, and that most naturally occuring fats contain a balance of saturated, mono- and poly- unsaturated fats. A diet that is massively skewed towards, say, butter as the sole source of fat is probably not healthy, but this doesn't mean that butter should be restricted, or is inherantly 'bad' - just that a variety of fats is beneficial. Ultimately, the impact of a person's saturated fat intake is very minor in the context of an overall diet, so for the vast majority of people, it shouldn't be something that is worried about provided you are putting the more important 'big rocks' in place (eating a variety of mostly real food, in energy balance, exercise, sleep etc) [Core Principles]

Trans Fats

The turning point for the increase in cardio-vascular diseases in humans seems to be when processed ‘trans fats’ are introduced to the diet.


Trans fats are created in factories by taking an unsaturated fat and bubbling hydrogen ions through it (the fats are now partially hydrogenated). This changes the chemical structure of the fatty acid, making it behave more like a saturated fat, and food companies do this because it is cheaper than using naturally occurring fats in large-scale production of food products (such as baked goods and other processed foods).


The process also increases shelf life, makes the fat more stable during deep frying, and changes the texture of the food product. All good for the companies’ bottom line, but not for our health.

Trans fat has been shown to have a negative impact on cardiovascular health, since it raises LDL and decreases HDL. LDL is sometimes known as ‘bad cholesterol’, and HDL as ‘good cholesterol’ (for more on cholesterol, see ‘Do Eggs Raise Cholesterol’).

Margarine is an example of a trans fat, so you won’t it on supermarket shelves these days. Trans fats are also found in ultra-processed foods made in factories, such as salty snacks, sugary cereals, ready-meals and industrially-made bread and desserts, which unfortunately now make up over half of UK family food purchases, the highest proportion in Europe.

Also worth mentioning here are Interesterified Fats (IE), which were introduced as a replacement for trans fats in food production. IE fats undergo a process whereby fatty acids within triglycerides are rearranged. A great analogy comes from Marie Spano, Nutritionist for the Atlanta Falcons, who says to ‘think of it like a sports team that moves players on the team to different positions, or swaps players from different teams to change how the team plays. Swapping fatty acids within or between fats changes how the fat acts.’

Again, these may be found in margarines and industrially-produced baked goods (though since there is no current legislation around IE fats, estimations of current intake levels are nigh on impossible). While some studies appear to show that IE fats are not bad for health, those studies used IE fats that are not actually found in the food supply. One other study suggested that a commonly consumed IE fat was more harmful than the fat of the same structure that had not been created through IE.

Further research is needed, and the impact of IE fats is not yet fully understood. Trans fats, which have such a negative impact on human health, and IE fats, whose impact is unclear, are man-made, and are therefore not found in unprocessed ‘whole food’ sources.


The evidence points to choosing mainly whole foods, which should make up the majority of our diets, particularly where fat is concerned. [Eat Real Food Core Principle]. Omega-3 fatty acids We previously mentioned alpha-linolenic acid (ALA), which is an omega 3 fatty acid, and one of the two essential fatty acids that the body cannot produce itself. Essential fatty acids are precursors to conditionally essential fatty acids, which the body can produce itself, but often does not produce enough of. These include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), both of which are omega-3 fatty acids. Consumption of EPA and DHA has an appreciable number of positive health effects, including decreases in blood platelet aggregation, lowered blood pressure, enhancement of smooth muscle function, decreased inflammation, alleviation of dyslipidema, and treatment of mood disorders. There’s even emerging evidence pointing to the benefits of omega-3 fatty acids on bone health.


Linoleic acid, an omega-6, is also an essential fatty acid. However, highly processed oils contain a disproportionately high amount of omega-6 fatty acids versus omega-3 fatty acids.

Today, consumption of omega-6 to omega-3 fatty acids is estimated at roughly 25:1. This is due in part to a predominance of processed omega-6 oils available commercially in our food supply (corn oil, sunflower oil, safflower oil, refined packaged grain products and pastries). Industrial production of omega-6-rich animal feeds, particularly in the US, has also resulted in animal tissues (livestock, eggs, and cultured fish) rich in omega-6 and poor in omega-3 fatty acids. This disproportionately high intake of omega 6’s increases the risk of thrombosis, hyperlipidemia, and vasoconstriction. These are long words - suffice to say that those conditions are bad!


Although an optimal ratio of omega-6 to omega-3 fatty acids is not known, the reverse of the negative effects listed above, as well as the other positive benefits of omega-3s listed above, occurs simply by increasing the proportion of omega-3’s, whose sources include fatty marine fish such as salmon, mackerel, herring, walnuts, algae oil and flaxseed oil.


Getting enough omega-3 is therefore hugely beneficial for human health.

Once again, the evidence points to intake of whole foods (such as oily fish), as being the best approach, and consuming 2-3 portions of oily fish (salmon, sardines, anchovies, mackerel etc) per week would exempt most people from having to take supplements. For those who really dislike fish, and are at risk of being deficient in omega-3s, supplements may act as a back-up option. Fat as fuel

Fat, as we mentioned before, is the most energy-dense macronutrient, at around 9kcals per gram, compared with carbohydrates and proteins at around 4kcals per gram. Carbohydrates have long been cited as the body’s preferred source of fuel, but once carbohydrate stores are depleted, the body looks for alternative sources of fuel, and this is where dietary fat can be useful.


When blood sugar (glucose) is in short supply (i.e. when there are no carbohydrates available), the liver produces fuel-efficient molecules called ‘ketones’ from fat, as an alternative fuel source, which the body and brain can then use for the activities ahead. This transition to using fat as the main source of fuel is known as ‘ketosis’. The more fat, and fewer carbohydrates, there are in the diet, the quicker a state of ketosis can be achieved, and this is the aim of the ketogenic diet, which is a low carb, high fat diet that has risen in popularity in recent years. This trend has lead to a number of high-profile athletes adopting a low carb, high fat approach, but often this has been misguided, and blanket recommendations for all athletes to follow such an approach are overstated.

The type of fuel that you function best on as an individual can be fats, carbohydrates or a mix of both. This is the Law of Biological Individuality, and can never be ignored. It’s down to you to find which works best for you. We have seen in the research that some very rare individuals appear to react remarkably well to adaptation to a ketogenic or low-carb, high-fat diet, but there are limitations to the conclusions that we can draw from this.

For most people, the ketogenic diet is probably unnecessary, and retaining some carbohydrates in the diet will work best for physiological (fuelling activity) and psychological (not going insane from restricting carbs!) reasons. However, there will be rare cases where people cope best on the ketogenic diet, perform well, and are able to maintain this without feeling sluggish, or deprived from foods they love, as well as keeping calories in line with their goals. In certain situations, a ketogenic diet may be the most appropriate diet for an individual. For example, as treatment for individuals (particularly children) with epilepsy.

In other situations, being able to utilise fat as an energy source may be beneficial to performance. This may be the case in ultra-endurance sports, for example elite cycling. Some studies used a period of carbohydrate restriction, in order to create the requisite conditions for the body to adapt to the use of fat as a fuel source, and found ‘an enhanced resistance to fatigue and a significant sparing of carbohydrate during low-to-moderate intensity exercise’ – the inference being that cyclists who rack up hundreds of miles a day could utilise fat for the long, slower part of the ride, and save their carbohydrates for the sprint finish. However, other studies appeared to show that this inference was overstated, and in fact, while providing some benefit for low-intensity exercise, being fat-adapted was not beneficial to exercise capacity or performance per se. In fact, being fat-adapted may actually impair the body's ability to utilise carbohydrate for high-intensity exercise.

As this suggests, fat is an effective fuel source for low intensity, long duration exercise. However, higher intensity exercise (such as football, rugby, CrossFit, weight lifting, HIIT, etc) will require carbohydrates for optimal performance.


The idea of 'train low, compete high' in terms of carbohydrate availability may provide an application for higher fat intakes as a percentage of energy intake (though as the name suggests, any benefit is likely to come from the reduction of carbohydrate, rather than the increase of fat).


There is still further research needed into this fascinating area, but what is clear is that the ketogenic diet does not provide optimal performance, and that at best, there may be a benefit to certain, very rare individuals (though evidence is equivocal and anecdotal).

Unfortunately, when it comes to the ketogenic diet, this is another area where charlatans operate in large numbers, and there are many who try to extol the supposed benefits of the ketogenic diet for everyone in every situation, which is misleading at best, dangerous at worst.


There’s even a Netflix documentary called the Magic Pill (the introduction to which says that all evidence is ‘anecdotal’, which should be an immediate red flag). What’s worse is that some of these keto evangelists are doctors, people in positions of trust and responsibility. Dig a little deeper, however, and all of these supposed ‘experts’ stand to profit in some way from being pro-keto, and where there is money involved, people will go to great lengths to say whatever they need to in order to maximise profits.

Bottom line – if you are one of those rare people that cope best on a low-carb, high-fat diet, then crack on. Otherwise, please, please do not listen to anyone who says that low-carb or keto is the optimal human diet, or the best way to achieve your fat-loss, muscle-gain, or any other goal. Remember the principles [Core Principles], and find what you can stick to.

Take Home Points

  • Fat is essential for many bodily functions, and humans thrive on a good variety and balance of fats.

  • The best approach is to eat fats from a variety of whole food sources (see my Top 10 High Fat Foods here), especially those rich in omega-3s, such as oily fish.

  • The ketogenic diet is not optimal for performance, although some very rare individuals may experience performance benefits.

  • As with any diet, overall calorie intake must be considered. Fat contains 9kcals per gram, so foods high in fat may be higher in calories. The degree to which this is important will depend on what your goal is.

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