The reality behind ketogenic diets for fat loss

What does the evidence say? Is fat loss enhanced by the world-famous ketogenic diet?

Writer: Sergio Garcia Busto
Editor: Isabella Boyne
Artist: Vera Liu (@veraliu_illustration)

Ketogenic diets have become, in the last few years at least, one of the most popular nutritional strategies when it comes to fat loss. Fitness magazines and nutrition gurus have been the biggest advocates of this approach and huge claims about its effectiveness are still being made today. It seems like, according to certainly questionable sources, if you eat a bowl of pasta you are not only going to be fat; you are also evil. So, is the keto diet the holy grail for fat loss?

Before tackling this question, what is keto? Food is composed of three macronutrients: carbohydrates, protein and fat. When a diet is mainly composed of fats and minimal carbohydrates, it is called ketogenic. The number that dictates how low of a carb intake is considered keto, varies but is usually limited to 50 grams of carbohydrates (ideally coming from vegetables), as a maximum. When on a high-fat diet, the amount of ketone bodies in the blood rises (this metabolic state is called ketosis); this is a metabolic response to the lack of glucose caused by extremely low carbohydrate intake. However, because of the evolution of our extraordinary metabolism, ketosis is not dangerous at all. In fact, carbohydrates are the only macronutrient that are not strictly necessary for survival (metabolically speaking), in contrast to protein and fat. Our body can survive without carbs, but it cannot survive without essential amino acids or essential fatty acids, because we cannot synthesise them. Aside from fat loss, ketogenic diets have been used from the start of the twentieth century as a treatment for epilepsy, and there is emerging evidence supporting its effectiveness for helping to treat neurological disorders (and possibly other diseases as well, such as cancer, though this is less clear).

It is important to note that claims about many different types of diets come from the common misconception that a single study can prove a certain idea. This is not the case, at least in nutrition. The sum of the evidence, findings in context and data as a whole are key to critically assess the validity of ideas. Taking this into account, let’s evaluate the efficacy of ketogenic diets for fat loss (addressing the claims for overall health deserves an article of its own).

It is true that ketogenic diets have shown incredible fat loss results, and there are a few clear reasons for this. Firstly, it has been consistently shown that high-fat meals are more satiating than low-fat meals, which prevents individuals from overeating. There are also very few ultra-processed foods (normally of a high caloric density) that do not have carbs; these foods are less filling and they are easier to overeat on. Therefore, although fat is the most calorically dense macronutrient (9kcal/g as opposed to carbs and protein, which have 4kcal/g), keto-friendly meals are more satiating and prevent the overconsumption of “not-so-healthy” foods. In addition, when consuming a very low amount of carbohydrates, muscle glycogen is consumed to maintain glucose levels. This causes weight loss (not fat loss directly) in the form of water loss, since for each gram of glycogen there are 3-4 grams of water stored with it. This seems like “free” weight being lost, but it can have negative consequences; for instance, for athletes, low glycogen stores will almost certainly cause a significant drop in performance. Protein intake also plays a role in the effectiveness of keto diets; since a whole macronutrient is taken out, protein consumption inevitably increases, and there is growing evidence that suggests that protein might have a mild positive effect on fat loss (provided that exercise is in place). This is possibly because of its high thermic effect of feeding, meaning that it takes more calories to digest and it generates a net balance of less than the free energy release associated with it.

Therefore, is the fat what is causing the fat loss? Probably not. When comparing calorie-equated diets (high-fat and low-fat) through meta-analyses and reviews, there is no noticeable difference in their effectiveness. The common denominator for building a solid fat loss diet is the following: a caloric deficit. In fact, many of the reasons (if not all) why ketogenic diets are effective are simply because they cause a smaller caloric intake or a larger caloric expenditure.

For this reason, many prefer other ways (oftentimes less extreme than keto) to achieve the same results; for example, we can increase our satiety by eating more fibre or by simply drinking more water. Other arguments against keto include the decreased performance in athletes mentioned before, the difficulty of sticking to the diet because of the narrow food options and the possible health detriments of leaving foods such as fruit (which contains a lot more carbs than vegetables) out of the equation.

The truth is that we cannot beat thermodynamics. The good old “calories-in, calories-out” model is what seems to be king. Although the “eat less, move more” advice might seem harsh, it is, as far as we know, the reality of fat loss. Therefore, if you want to try keto, go ahead – personal experimentation is always a good thing and its effectiveness has been repeatedly shown – but be aware that easier approaches exist and that there is no magic behind it; eating fat and not carbs will not give you better results.


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