It is not uncommon to hear that one needs to eat fat in order to burn fat and that this is one of the advantages of low-carb and ketogenic diets. But is this actually true? The short and simple answer is no, the longer answer is what this article is about.
Big fat truths
The concept that one needs to eat fat to burn more fat mostly came out of a misunderstanding of some of the early literature on low-carb, high-fat diets that showed an increase in the body’s use of fat for fuel.
People immediately assumed that it was the increase in dietary fat that was driving the increase in fat burning. But, the burning (or oxidation) of fat isn’t really related to fat intake per se. Rather, it’s more likely related to a lower carb intake.
Cutting carbs vs increasing fat: which drives fat burning?
The most basic science behind nutrient oxidation tells us that eating more fat does not affect fat oxidation as the body has a limited ability to oxidise fat, compared to carbs and protein. Eating less carbs does.
Eating dietary fat in and out of itself doesn’t usually have a major impact on how much fat you burn, although some studies find that it might, at very high intakes (of the order of 80 grams all at once), can have a small effect on the body’s fat oxidation.
For the most part though, how much fat the body burns during the day is related primarily to carb intake, secondarily to protein intake, and almost not at all to dietary fat intake itself.
Why a high-fat diet can work for fat loss
The reason why fasting (no food intake at all) and high-fat, low-carb diets (e.g. 30% protein, 60% fat, 10% carbs) work for fat loss is because they generate the same shift in the body’s fuel utilisation: a shift to using predominantly fat for fuel.
The commonality in both of those conditions is the lack of carbs, not the presence or absence of dietary fat. Unless following a fat fast, fasting involves little or no dietary fat consumption while low-carb type diets do.
On low-carb diets, fat is a lever not a goal. When you tend closer to your ideal bodyweight, you may want to add more in, as you’ll be burning less body fat for energy.
Based on the principles of fuel utilisation we touched on earlier, it makes sense that when eating more carbs, you burn more carbs (and less fat) and that when eating fewer carbs, you burn fewer carbs (and more fat).
This means that it’s the absence of carbs driving the increase in fat burning, not the presence of dietary fat. Therefore, driving fat burning per se isn’t a benefit of increasing fat intake.
This isn’t to say that increasing dietary fat intake can’t have benefits, such as increased fullness, food enjoyment or flexibility.
The fullness factor of higher fat diets
Scientists don’t exactly know why a high-fat diet works better. It could be partly that fat and protein are naturally more satiating, that people feel full after high-fat meals and are less likely to snack or overeat during the day. There’s little doubt that it does.
Extremely low-fat diets tend to leave people feeling really hungry all the time and research even suggests that moderate fat diets tend to lead to better long-term compliance than very low-fat diets.
While dietary fat doesn’t necessarily blunt hunger in the short-term (e.g. in the course of a single meal), it tends to keep people fuller between meals in the long term. There are two main reasons for why this occurs.
The first is that dietary fat slows how quickly meals empty from the stomach. With very low-fat meals, people are often ravenous sooner as they tend to digest quickly.
Additionally, research has shown that fat helps to keep blood sugars more stable and this generally means more stable energy levels and less pronounced hunger.
Insulin sensitivity and fat loss on high-carb vs high-fat diets
The level of insulin sensitivity can determine the magnitude of weight loss and one study suggests that, in the case of someone with low baseline insulin sensitivity, a low-carb, higher fat diet may be more appropriate.
In this study, women with either high or low insulin sensitivity followed either a high-carb (60% carb, 20% fat) or a low-carb, moderate fat (40% carb, 40% fat) diet and were divided into four groups: high-carb/insulin sensitive, high-carb/insulin resistant, low-carb/insulin sensitive, low-carb/insulin resistant.
The results showed, among other things, that the women who were insulin resistant lost twice the weight on the low-carb diet than they did on the high-carb diet.
A widely debated argument is that carbs are converted into glucose in the body, triggering the release of insulin, a culprit hormone in storing fat if constantly elevated. In that sense, leaving less room for carbs may have a positive impact on body composition.
Lowering carbs also means more stable insulin levels, which allows more fat oxidation. Elevated levels of insulin slam your fat-cell doors shut, storing fat rather than releasing it to burn.
The link between diet quality, LCHF diets and weight loss
Since the 1970s or so, people have been told to eat less fat if they want to lose body fat because of its higher caloric content – fat packs nine calories per gram whereas the other macronutrients, carbs and protein, have only four on average.
This was based on the assumption that our bodies are straightforward calorimeters processing all kinds of calories in the same way.
Of course, total calories do matter in the short term, which is why people can initially lose weight on nearly any type of diet and why so many fad diets initially seem to work.
However, increasing evidence suggests that in order to stay at a healthy weight long-term, diet quality is very important. A poor diet quality generally leads to positive energy balance.
When consumed over years, certain foods may interfere with long-term bodyweight maintenance, others have relatively neutral effects, and others promote healthy weight regulation.
For long-term weight gain, research shows that foods rich in refined grains, starches, and sugar appear to be major culprits. This is partly because those kind of foods are highly palatable and therefore easier to overeat than others.
Furthermore, independently of energy balance, rapidly digested, low fibre carbs (like potatoes, white bread, white rice, refined breakfast cereals or crackers) tend to drive many pathways related to obesity.
The mechanisms driving this association appear to include negative effects of these different types of foods on satiety, glucose-insulin responses, liver fat synthesis, fat cells’ function, visceral fat, and even brain craving and reward systems.
When foods higher in fats, such as meats, cheese, and eggs, are consumed in place of refined carbs, less weight gain or even relative weight loss is seen. The mechanistic pathways underlying these observed benefits are however still being elucidated.
Summing up high-fat diets for fat loss
Eating fat is not essential to lose fat nor is it in itself driving the increase in fat burning seen with low-carb or ketogenic diets. It is the absence of carbs which likely makes a difference, not the presence of fat.
Increasing fat intake under some conditions can be beneficial for controlling hunger, promoting fullness or provide higher diet satisfaction, all of which helps to maintain a healthy bodyweight long term.
