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.
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.
