Question: I’ve seen it claimed (by members of the paleo diet movement) that fat is the preferred fuel source of the body and, for that reason, dietary fat should make up a large part of the diet (i.e. 40-70% of total calories). Is this true?
Answer: Ok, before addressing this question in terms of the physiology involved, I want to get a little bit pedantic (or at least semantic) and look at what it means for something to be ‘preferred’. According to the dictionary definition preferred means “more desirable than another” but what does that really mean?
Let’s say that you’re thirsting for a caffeinated beverage so I offer you two drinks, coffee and tea. Since you like coffee more, you choose to drink coffee. In this case, coffee is preferred. Let’s contrast that to a situation where I offer you tea or nothing. You choose tea because it’s the only option available. In that second situation, tea wouldn’t really be your preferred choice (a true pedant would argue that you still preferred it to nothing but bear with me), it’s a choice by exclusion: you’re thirsty and with only one option you drink what you’re given. Don’t worry I’m getting to the point.
So let’s turn to human physiology and talk about what fuel the body ‘prefers’ to use, with the above definition in mind. Now, for the most part, all tissues in the body can generate energy (ATP for the biochemically minded) from either glucose or fatty acids. There are a few exceptions, mind you; the brain uses almost exclusively glucose but shifts to mostly ketones under certain conditions. What the brain can’t use is fatty acids directly for energy. There are a couple of other weird ones, a handful of tissues in the body that only use glucose: the retina is one, part of the kidney, there’s a third I’m forgetting. There’s one other exception to this that I’ll come back to at the end.
But ignoring those exceptions most tissues can use either fatty acids or glucose for fuel (there is a separate issue of metabolic flexibility, the body’s ability to shift back and forth but that’s getting into a different topic). And although both are stored in the body to be sure, this has to do with dietary intake, carbohydrates versus fat intake.
So what happens when you provide the body with both carbs and fats in the diet? Which fuel source is preferred? Well the answer is clear: carbs. That is, when you give the body both carbs and fats (or more generally when carbs are available), the body will use the carbs for fuel and store the fat. Carbs are clearly, by the definition I bored you with above, the preferred choice. Reiterating: if the body is given a choice of carbs or fats, it will prefer carbs for fuel. No question and no debates.
Now, if you remove all of your dietary carbohydrates, as in a low-carbohydrate/ketogenic diet, the body does make a huge shift towards relying almost exclusively on fatty acids (and ketones) for fuel. But this is akin to my second example above, when I only offered you tea; it’s only a choice by exclusion where the body switches to using predominantly fats for fuel because that’s all that is available. But that’s not the definition of preferred; it’s only a choice relative to nothing.
So, you ask, where did this idea that fat is the preferred source of fuel by the body come from? Mind you, it’s not new and the paleo diet people aren’t the first to make this claim. Well remember the other exception I mentioned above to the general idea that most tissues in the body can and (and in fact) will use glucose or fat depending on what’s available?
That exception is heart (cardiac) tissue. For fairly logical reasons (i.e. the heart can’t ever be in a situation where energy isn’t available) cardiac muscle tissue prefers fatty acids to glucose for fuel. But it’s the lone exception in the body and certainly (and fairly obviously) is not representative of the rest of the body.
So while it’s clear that the body can and will shift fuel source choice depending on what’s available, the idea that ‘fat is the preferred fuel source in humans’ is incorrect.