Question: I’ve often seen it claimed that one needs to ‘eat fat to burn fat’ and that this is one of the advantages of low-carbohydrate diets. But, like so many myths in the diet world, I’m wondering if this is actually true. Is it?
Answer: The short answer, as you might have guessed is no. Now, as always, here’s the longer answer.
I suspect that the idea that one needed to eat fat to burn fat came out of a misunderstanding of some of the early literature on low-carbohydrate/high-fat/ketogenic diets (note: I’m defining a ketogenic diet here as any diet that contains less than 100 grams of dietary carbohydrate; a topic discussed in more detail in my first book The Ketogenic Diet).
In those studies, there was clearly an increase in the body’s use of fat for fuel (indicated by a large scale decrease in something called the respiratory exchange ratio or RER) and I have a hunch that people assumed that it was the huge increase in dietary fat that was driving the increase in fat burning.
But as I discussed in Nutrient Intake, Nutrient Storage and Nutrient Oxidation as well as in How We Get Fat, the burning (oxidation) of fat isn’t really related to fat intake per se. Rather, it’s related to carbohydrate intake. That is, the act of eating dietary fat doesn’t usually have a major impact on how much fat you burn. I say ‘not usually’ as some studies find that very high fat intakes (like 80 grams all at once) have a small effect on fat oxidation by the body. But for the most part, how much fat the body burns during the day is related primarily to carbohydrate intake, secondarily to protein intake, and almost not at all to dietary fat intake itself.
Also consider that the following three conditions:
- Complete fasting (no food intake at all)
- A high-fat, low-carbohydrate ketogenic diet (e.g. 30% protein, 65% fat, 5% carbohydrate)
- A protein sparing modified fast (PSMF, such as my own Rapid Fat Loss Handbook)
All generate basically the identical shift in the body’s fuel utilization: a decrease in resting RER indicating a shift to using predominantly fat for fuel. Again I say basically since both the ketogenic diet and the PSMF will be marginally different than complete fasting due to the intake of dietary protein. But for the most part, the shift in fuel use by the body is identical in all three conditions, you see a huge drop in RER indicating a massive increase in the use of dietary fat for fuel.
And the commonality in all of those conditions is not the presence or absence of dietary fat (diets 1 and 3 have little or no dietary fat, diet 2 has quite a bit). Rather, it’s the lack of dietary carbohydrates. Which, based on what we know about how the body determines fuel usage makes sense. As I discussed in the linked articles above, when you eat more carbs, you burn more carbs (and less fat); eat fewer carbs and you burn fewer carbs (and more fat). Which means that in all three conditions above it’s the absence of dietary carbohydrates driving the increase in fat burning, not the presence of dietary fat.
Which isn’t to say that increasing dietary fat intake under some conditions can’t have benefits (such as increased fullness, food enjoyment or flexibility, limiting the daily deficit to moderate levels if that’s the goal, etc.) which are discussed in other articles on the site (I’d suggest the Comparing the Diets series for an overview of different dietary approaches). It’s simply that increasing fat burning per se simply isn’t one of them; rather, that’s accomplished by reducing carbohydrates and total caloric intake.
Hope that answers your question.