Is Fat the Preferred Fuel Source in the Body – Q&A

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.

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12 thoughts on “Is Fat the Preferred Fuel Source in the Body – Q&A

  1. Thank you for your helpful perspective.
    I have been following a fairly primal lifestyle for some time now and am familiar with the idea that fat is the preferred fuel because the human body stores fat so well. And under survival cicumstances in our prehistoric past having reasonable amounts of energy stored on the body would have been helpful. But that doesn’t necessarily mean fat is preferred, it only means that it can be used. But since there was very little carbohydrates to come by the alternatives were very limited. Your argument following your definition of “preferred”(which I consider correct) makes far more sense to me. To call something preferred if there is no other choice is not very convincing. I wonder why I didn’t think of this. Unfortunately many people find it difficult to change their points of view once they have identified themself with certain ideas. Thanks for the input. Great site.

  2. Lyle has made it look so simple and easy to understand. Great work Lyle!!
    Just my two cents –
    Now, you may wonder why body prefers fat for storage compared to carbs (actually body also stores carbs in form of Glycogen , a polymer of glucose, but the storage is limited to liver and muscles ) . There are two things why fat is preferred for storage
    1. It has more calorific value i.e. when you burn fat it gives you more energy than the carbs almost twice the amount
    2. The storage space required – if you want to store one molecule of glycogen , you need two molecules of water to hold it. It means it takes more space and also has less calories.

  3. You wrote, “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.”

    By the same logic, shouldn’t the brain prefer fatty acids? It seems to me that the brain is an equally critical organ. Maybe it’s a good idea that our two most vital organs aren’t fighting for the same (potentially scarce) energy source??

    Everything else you said made sense to me. . . but that phrase stuck out as questionable.

  4. Take a biochemistry course. A variety of organs prefer
    FAs for energy.

    [WORDPRESS HASHCASH] The poster sent us ‘1762401312 which is not a hashcash value.

  5. Thanks for the post, Lyle.

    What are your thoughts on some folks arguing that an eating strategy that supplies more glucose than fatty acids will generate more reactive oxygen species. One individual I had a discussion with stated that preferred should be defined differently- being the fuel source that results in the least potential damage.

  6. A better question would be “Is Fat the _Best_ Fuel Source in the Body” because I think thats the claim that is being made by most who say it is preferred.

  7. And it’s still not. No matter how loudly they assert it. Also, words mean things. If they mean ‘best’ say ‘best’. If they mean ‘preferred’ say preferred.

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