No matter how much I learn about nutrition, I struggle to give up the notion that fat is somehow bad for me. The decades of “low-fat” marketing and health advice take some time to wear off, and erasing the image I have of the fat I eat somehow directly clogging my arteries is hard to shake off. Fat, as we will discover, is an essential part of our diet, but all fats are not the same and the science is still in its infancy. All the more reason to get informed, consider our options, and be open to change our beliefs!
In this post I will answer two questions: Why do we need to eat fat? And how much and what types of fat should we eat? For the second question I will roll over territory where the evidence has shifted considerably, but where much of the advice and guidance in the public domain has not been updated to recognize this, or at least acknowledge the uncertainty. I will set out the evidence as I understand it, but do so with a “health warning”. Nutrition science is riddled with questions where there is little to no consensus on the answers, and with people that are zealots for one particular point of view. Much of what you may read fails to acknowledge the uncertainty and overstates what is truly known. Therefore it is essential to read widely in order to form a balanced view, and that applies to readers of this post. I inevitably have my own biases, and I encourage you to read other points of view in order to inform your own.
Is fat essential?
Fat, along with protein and carbohydrate is a macronutrient. It’s a major source of energy, containing 9 calories per gram (compared to protein and carbohydrates, which are 4 calories per gram). It helps you absorb some vitamins and minerals. Fat is needed to build cell membranes, the vital exterior of each cell, and the sheaths surrounding nerves. It’s essential for blood clotting, muscle movement, and inflammation, and aids brain function. Finally, and importantly, fat makes food palatable, and is very satiating: it makes you feel full.
So we need fat as part of our diet, but how much? Here, the opinions start to diverge.
For a “low-fat” diet, the guidance is that fat should make up no more than 30% of your daily calories. If you’re consuming 2000 calories per day, that equates to 65g of fat. For a “Mediterranean” diet (which I will deal with in more detail below), the recommended fat content is higher at 35%-40%. And for a ketogenic diet, the fat content can be as high as 75% of daily calories.
Good fat, bad fat
At this point we need to move on to the second part of the second question – to examine the types of fat that we should eat. All fats are calorie dense when compared to carbohydrates and protein, but the health impacts of the various types of fats are very different.
Fats fall into 3 categories:
•Saturated fats, which are animal fats such as butter and lard.
•Mono-unsaturated fats, which include olive oil and canola oil.
•Poly-unsaturated fats, which include vegetable oils (which are sometimes referred to as “seed oils”.
Poly-unsaturated fats are liquid, and are sometimes converted into a solid form through an industrial process, resulting in what are called trans-fats. Trans-fats do also occur naturally in animal fats, but some of the harmful effect that we will touch on below are only present in the man-made version.
For the the last 40 years, the prevailing view on fats can crudely be summarized as follows:
•Restrict their consumption (to less than 30% of total calories), and therefore eat a “low-fat” diet.
•Saturated fats are “bad” – these should be avoided.
•Unsaturated fats are “better”.
•Industrial trans-fats became very popular in the 80’s and 90’s and have only in the last 15 years become highly regulated, or in some countries completely banned.
Very bad fat
Trans-fats are still found in products such as cakes, margarines, ice-cream, microwaveable popcorn and fast food. They became popular as an ingredient due to its low price and stability. Food manufacturers could incorporate it into products easily during manufacturing, and fast-food outlets use it as a cheap and easy to handle cooking oil. Trans-fats are believed to increase “bad” cholesterol, may increase insulin resistance, and are strongly associated with increased inflammation, which is a primary factor in chronic illnesses including heart disease, metabolic syndrome and diabetes. Whilst there is much uncertainty in many aspects of nutrition, the verdict on industrial trans-fats is clear – it should be minimized or avoided entirely.
Is saturated fat still bad?
Whilst the message on trans-fats is clear, there are two key debates raging in the nutrition world that challenge this prevailing view on the type of fats we should eat. The first is around the “badness” of saturated fats. The second concerns the safety of consuming large amounts of vegetable (seed) oils.
Taking the saturated fat question first, you may have noticed that opinion on whether to eat margarine or butter has shifted. Until recently butter was very much considered to be a “bad” food choice, and most homes had a tub of margarine in the fridge, sometimes complete with it’s cholesterol-lowering health claims on the packaging. The belief was, and to many still is. that consuming saturated fats that are high in “bad” (LDL) cholesterol will raise your blood cholesterol levels, and that having higher blood cholesterol raises your risk of cardiovascular disease. This is where things start to get more complex than they first seem.
Firstly, the link between dietary cholesterol and blood cholesterol is contested – there are studies that both suggest the causal link is there and those that don’t, The body creates it’s own cholesterol, and it appears that it adapts to the dietary cholesterol levels that we consume.
Secondly, the causal link between high (LDL) cholesterol levels and cardiovascular disease are also contested, again with studies being published on both sides of the argument. To illustrate the complexity of cause and effect, there are many drugs that target reducing CVD through reducing LDL cholesterol that were successful in reducing cholesterol, but in fact raised the risk of CVD!
However, in the absence of unambiguous evidence that it is safe to consume at higher levels, the official guidance to reduce or moderate saturated fat consumption still exist, with recommendations ranging from 5%-12% of total calorie intake.
What about ‘seed’ oils?
The other “fat’ debate that’s raging in nutrition circles is the potential harm of increased vegetable (seed) oil consumption. There is a hypothesis that increased consumption of this type of fat, which is commonly used in ultra-processed foods, is changing the ratio of our consumed Omega 6 fatty acids and Omega 3 fatty acids (which are present in oily fish and nuts), and this is driving inflammation, which is in turn increasing the risk of CVD. The evidence is weak at the moment, but this debate highlights an important issue around applying nutrition research: that is it’s all very well using research to define what not to eat, but for public health, consideration has to be given to what will be eaten instead. Vegetable oils are the fastest growing segment of the food industry and have grown partially as a result of the drive to reduce saturated fat consumption. The policy to reduce saturated fat has caused us to eat a category of fat in unprecedented quantities, and the long term effects of this are unknown.
Enter the Mediterranean diet
By now you are probably wondering “well, what CAN I eat?”. Fat is essential, but saturated fat is still considered to be bad, but perhaps not as bad as we previously thought. Vegetable (seed) oils are perhaps not as good as we thought. As we’ve already discovered, industrial trans-fats are a definite no-no. The category of fats that continues to carry the “good” tag are mono-unsaturated oils, such as olive oil. These are at the heart of the Mediterranean diet, which also features vegetables, legumes, nuts, some meat and fish, and even some red wine. The Mediterranean diet was the subject of a study in 2013 where it was compared to a typical American “low-fat” diet. The results of the study in terms of lowering the risk of cardiovascular disease (CVD) were so dramatic in favour of the Mediterranean diet that the study was ended early for ethical reasons. True to the controversy around nutrition science, the results of the study have since been contested, but it remains a very powerful piece of evidence for those arguing for the health benefits of mono-unsaturated fat.
So, lots of controversy, lots of uncertainty, and overall a complicated picture. But what do I recommend in terms of eating the fat that you need?
As with most things health related, my starting principle is moderation. We have to accept that currently knowledge is limited, and that changing one variable, even if it’s impact is fairly well known, will likely have further uncertainly because of what we eat more or less of to compensate for it. Eat less saturated fat? It may well be the thing to do, but if you eat more highly processed carbohydrates instead, then you may in fact be worsening your diet rather than improving it. Moderation mitigates our risks.
My second principle is to avoid processed foods wherever possible. There is an growing body of evidence that points towards processed food as driving obesity, mainly due to its hyper-palatability and low-satiating properties: it tastes very good, and doesn’t fill you up. Moreover, eating it means you lose track of how much fat you are consuming, and in the worse case means you are eating trans-fats without realizing it.
My third principle is to enjoy food. If you really like the taste of a fatty cut of steak, then have that over a leaner cut. If you like butter on your toast – slather it on! But just be mindful that fat is highly calorific, and so you might need to eat more green vegetables or beans that day to balance out your intake.
So, to sum up, I would recommend switching to cooking with and eating mono-unsaturated fats such as olive oil where you can and consume them liberally, within the constraints of your target calorie consumption – remember they’re still highly calorific at 9 calories per gram. Keep your saturated fat and vegetable oil consumption moderate. Avoid trans-fats (often labelled as hydrogenated vegetable oil or fat) where at all possible. Having said all that, I refer you back to the caution I recommend about any nutrition advice: it is all subject to biases and to how the author interprets the current incomplete (and flawed) evidence base. I hope that this post has helped you to make more informed decisions about what you eat and prompts you to read more widely. Just remember that if what you read is presented as certainty, chances are it’s wrong!