For some strange reason, there has been a pendulum swing from low fat to high fat diets over the last decade. As high fat diets have become popular, there have been many claims made about the use of high fat diets and why they might be the best tool for fat loss. Let me enumerate them here in a short list:
- Eating fat makes you burn fat
- High fat diets make you burn more calories
- A ketogenic state makes you burn the most fat and offers a metabolic advantage
- Eating more fat makes you lose more fat since you are using fat for fuel
- Carbohydrates are stored more easily than fat
- You eat less when you eat high fat meals
- Fat is more satiating than carbohydrates
- Eating fat before a meal makes you eat less
Now as science is a methodology to test hypotheses, and all of these are hypotheses and they have been tested, the data is out there. Let us see what we can learn from the data.
What is a High Fat Diet?
When you follow a high-fat diet, like keto, you’re filling up on fats, which could make up about 60% to 80% of what you eat every day. Instead of getting your energy from carbs, your body starts using fat for fuel—a switch called ketosis.
Some people say this helps you lose weight and gives you more energy. But the truth is, our bodies are pretty complicated. The latest research shows that just because you eat a lot of fat doesn’t mean you’ll lose more weight or feel fuller longer, especially when you compare it to meals with more protein or carbs.
What we thought we knew about high-fat diets is getting a second look, and it turns out there’s more to the story than we first believed.
Does Eating Fat Make You Burn More Fat
The short answer is yes, your body adapts quite quickly and robustly to the types of foods you consume. If you eat a low-carb, high-fat diet your body will start using more fat for fuel (1, 2, 3, 4). If you eat a low-fat, high-carb diet, your body will start using more carbohydrates for fuel. This is incredibly well documented in the literature. Here are just a few data points and studies showing this is true (1).
Here is the other interesting thing, it appears that it isn’t the “high fat” that makes you burn more fat, it is the lack of carbohydrates. When you consume high fat and high carbs you don’t push the RER as much as if you just restrict carbohydrates.
You also see the same effect if you eat low carb and low fat or if you are fasting (5, 6, 7). Carbohydrates really appear to be the main “fuel selector” (more on these details in a later article). This really should fundamentally change how you view high fat diets and their mechanisms.
As you consume less carbohydrates and more fat your body WILL start oxidizing more fat for fuel. That is really incontrovertible. However, we need to be very smart with how we interpret this. Using more fat for fuel does not equal more weight loss. This has also been demonstrated in several studies, including the following.
Clearly, more fat oxidation does not equal more weight/fat loss. More on this topic to follow.
Does a High Fat Diet Make You Burn More Calories
It has been claimed that eating fat makes your body expend more calories due to something about “efficiency” or some weird perturbation of biochemistry.
Honestly, these claims never made any sense to me from a basic physics and chemistry stand point and the fact if you wanted to lose weight you would want to be less “efficient”, but I will entertain the idea nonetheless and see what the data says.
To test this we simply have to look at studies that compare energy expenditure between high-fat and low-fat diets (1, 8, 9). These have been done dozens of times and there is plenty of data to pour through.
The data are pretty clear at this point that consuming a higher percentage of your overall food intake from dietary fat does not convey any magical metabolic effects that increase overall energy expenditure. In short, no, a high fat diet does not make you burn more calories.
Here are some data to demonstrate the effect of increasing fat intake as a percentage of diet and showing the effect on energy expenditure: nada (10).
Does Ketosis Offer a Metabolic Advantage?
Once we see the data that a high fat diet does not offer any metabolic advantage the next logical question is, does pushing that to the extreme and entering a state of ketosis offer any advantage? If there were any dietary perturbations that one would think would offer a metabolic advantage it would be a ketogenic state as it essentially requires an additional “step” in human metabolism.
The data exists for this question and the answer is quite compellingly no (8, 11). In several different populations studied in very rigorous study designs, there is no metabolic advantage one can observe. Here is some data from Kevin Hall’s study showing a ketogenic state does not offer a metabolic advantage nor a benefit to fat loss in a calorie-controlled, protein clamped state, even when ketones are elevated.
Now it IS possible that this effect is so tiny we can’t see it, but then what would be the point, pragmatically speaking.
Does a High-Fat Diet Help You Lose Weight?
The data above show that eating fat does make you oxidize more fat, but does that actually translate into greater overall fat loss since it doesn’t change total energy expenditure?
Well, there is a plethora of data to tease this out. Mechanistic studies show us that high fat diets do not result in more fat loss.
No mechanistic evidence exists to support this idea (See the two studies shown above). The case is pretty much closed on that front.
Now that doesn’t mean in the real world high fat diets might have some other magical property that results in more weight loss and fat loss. This has also been tested, ad nauseam in trials that look at the diets in more real-world settings among diverse populations. I mean seriously, here are just a few studies, along with their data (12, 13, 14, 15).
Here is a meta-analysis that we discussed in an earlier post (15).
A roughly 2-pound benefit over 12 months, that is almost meaningless when we are talking about the goal of diets for weight loss, especially when you hold the data juxtapose to data showing adherence effects weight loss at much more drastic scales (pun intended).
Below you can see how high adherence results in 10-20 pounds more lost over 12 months, whereas diet composition had zero effect on weight loss.
The data is just crystal clear, there is no meaningful benefit to high fat diets for fat loss. We can probably stop spending our dwindling science dollars funding them**.
Are Carbohydrates Stored More Easily Than Fat?
The claim is often made that when consumed in excess carbohydrates convert to fat and are stored through De Novo Lipogenesis. Yes, that is a big, cool, fancy sounding word, but it doesn’t really mean all that much in humans.
A lot of people cite rodent studies showing that excessive carbohydrate intake results in the creation of lots of fat. Rodents have VERY different liver metabolism than humans and the human capacity for De Novo Lipogenesis is a fraction of rodents.
In fact, one of my favorite papers on the subject is titled, “No common energy currency: de novo lipogenesis as the road less traveled”. In this seminal editorial, Hellerstein opens with the lines, “Bees make wax (lipid) from honey (carbohydrate). Pigs fatten on a grain diet. Indeed, all organisms, from bacteria to mammals, have the enzymes of de novo lipogenesis.
The physiologic function of de novo lipogenesis has therefore seemed obvious to biochemists: the de novo lipogenesis pathway links carbohydrates and fats, the 2 most important forms of chemical energy for most organisms.
Because storage of energy as lipid is much more efficient than storage as carbohydrate, the biochemical logic of using this pathway has always been compelling.”
But here is the problem, the fat (stored lipid) created from excess carbohydrates is not even on the same order of magnitude as fat intake leading to fat storage. The overall effect of high-carb diets on fat storage is at least an order of magnitude less than dietary fat.
If you want to read a paper that discusses this in depth, check out this review (16).
Does a High-Fat Diet Make You Eat Less?
A major idea behind the high-fat diet phenomenon is that eating fat is more satiating than carbohydrates, and therefore you eat less overall and lose weight as a result.
The evidence here is also quite clear. The hypothesis was reviewed by Johnstone and colleagues in a seminal paper titled “Effects of High-Protein vs High-Fat Diet on Energy Expenditure and Weight Loss”. To summarize the conclusion, “Overall, these data indicate that it is unlikely that a high-fat diet results in greater weight loss than a high-protein diet, and that the high-fat diet does not show any particular advantage for either energy expenditure or fat loss”.
Similarly, another recent meta-analysis concluded that there is no significant difference in weight loss between high-fat and low-fat diets.
The results of many studies have shown that adherence is the primary determinant of weight loss success. Thus, choosing a diet that one can adhere to is more important than its macronutrient composition.
Fat vs Carbs: Which is More Satiating?
High-fat diets often claim they are more satiating than high-carb diets. The evidence for this is also not as clear-cut as one might think. Several studies show that while fats might be more satiating in the short term, the overall impact on weight loss is negligible when compared to protein.
When protein is factored into the comparison, protein often outperforms both fat and carbs in terms of satiety. A comprehensive review concluded that higher protein intake is more satiating than high-fat or high-carb diets.
The Wrap Up
So let us sum up those initial statements and evaluate their “trueness” and “falseness”
- Eating fat makes you burn fat: True
- High fat diets make you burn more calories: False
- A ketogenic state makes you burn the most fat and offers a metabolic advantage: False
- Eating more fat makes you lose more fat since you are using fat for fuel: False
- Carbohydrates are stored more easily than fat: False
- You eat less when you eat high fat meals: False
- Fat is more satiating than carbohydrates: False
- Eating fat before a meal makes you eat less: False