“Are Carbs the Body’s Preferred Fuel Source?” by James Barnum

“Are Carbs the Body’s Preferred Fuel Source?” by James Barnum

The subject of this article is one that divides coaches, training partners, and even families.  OK…Maybe families haven’t been driven apart discussing whether the body prefers either carbs or fat to produce energy, but this is a topic that a lot of people take very personally.  The rise in popularity of Paleo and low carb dieting within the fitness community made believers out of many of us.

Lately though, it seems like a more balanced approach – like the one Eat To Perform espouses – has made a comeback.  Carbohydrates can super-charge high intensity athletic performance!  This is because glycogen, the intramuscular fuel source, is the human equivalent to plant starches and is thus replenished faster when we eat a moderate amount of starchy carbs – potatoes, rice, oats, quinoa, etc. – around our workouts!  How can we then, as a group, have gotten things wrong?  Well, it’s happened before…

Recall that at one point a high fat diet was attributed to all kinds of diseases and disorders related to the heart and circulatory system.  We know now that fat isn’t bad for us – in fact, we know that a diet rich in essential fatty acids from both plant and animal sources has very little to do with obesity and heart disease; the right balance of dietary fats can actually lower the risk and severity of many conditions!

So if fat was bad but it wasn’t, and carbs were bad but they aren’t…What gives!?  Which one of these energy substrates is actually the body’s preferred fuel source?  I hear you screaming “TELL ME NOW SO I KNOW WHAT TO EAT!”

Personal Preference vs. Physiological Fact

Settle down and consider this, dear reader.  When we talk about what the body prefers, it’s an issue of semantics.  Bodies are a collection of cells.  Cells don’t have “preferences” per se…They don’t care whether they watch Game of Thrones or The Walking Dead.  Green and blue are both A-OK.  Yes, there are circumstances where our cells function either better or worse due to what we eat, but I’m getting ahead of myself.  Individual cells just don’t care, because they can’t.

People, however, are very picky.  From a macroscopic perspective, when we talk about the body – this collection of tiny pieces – we’re really talking about the PERSON.  YOU are your body.  It’s more a question then of what YOU prefer if we’re examining preferences.  Preferences are almost entirely subjective – i.e. they’re based upon experiential bias.  Believe it or not, a lot of the people handing out nutrition information on the internet are VERY biased, to the point where they may even reject facts in favor of anecdotal evidence.

While it’s always healthy and fun to consider anecdote, as objective, rational people who want to make the most progress they can in the shortest amount of time, we aren’t really concerned with what someone THINKS works best (especially when they’re ignoring basic nutrition science).  We don’t care if Guru #1 says dietary carbs are unnecessary since the body can make them from protein, nor if Guru #2 swears by a 90% carbohydrate diet provided by 30 bananas a day – we’re SCIENTISTS and we want to go based upon data and observable, repeatable, factual information!

What really confounds the issue then is that when you trust someone’s word, unbeknownst that it’s actually just a matter of personal preference, you might end up making some big mistakes and deterring your own progress or the progress of others.  Experience is great, but we need a baseline and that baseline is grounded in facts, not belief.

If you still follow me, you understand then that it’s not a matter of “preference” at all when we’re talking about fuel sources – whether it’s carbs or fat.  It’s a matter of cold, hard FUNCTION.  While we cannot venture to guess what our bodies PREFER, we know pretty much exactly how our bodies FUNCTION.

Let’s Change the Question

When we ask instead, “Are carbs the most EFFICIENT fuel source for the body?” we get closer to the answer…But not quite all the way.  Indeed, the fastest way to make cellular energy within almost all of your bodily tissues is through glycolysis – the process of breaking down glucose to make ATP.  The only major exception is your heart, which runs almost exclusively on fat.  Your brain however cannot efficiently use fat for energy – it needs ketone bodies (derived from fat and protein) and glucose.  In the end, the only time fat is the most efficient fuel source is when there is no glucose available.

So glucose is the most efficient fuel source for your body…”But is efficiency always important?”  No, not necessarily.  There are scenarios where relying upon fat metabolism has distinct benefits.  To take advantage of your body’s natural Metabolic Flexibility, you can eat more fat when you’re not exercising intensely – mainly when you’re not lifting weights.  Everything from watching TV to jogging at a comfortable pace can be adequately fueled by fat.  In fact, by flexing your metabolism to fat burning mode by eating mostly meat and veggies when you’re not lifting weights, you can improve body composition and health, as well as your body’s sensitivity to carbohydrates.  When it’s time to kill it and improve your WOD time or hit a new PR, you can go for some sweet potatoes and flex to carbohydrate metabolism to make energy available quickly and efficiently.


The Answer

In summary, both fat and carbs are technically the “preferred” fuel source of the body.  It just depends upon what the body is doing!  The only reason this question ever comes up is because internet gurus, be it with good-intention or not, have pushed their agendas so far that they’re neglecting basic nutrition science.  This is a debate very swiftly ended by a Wikipedia search and it really doesn’t have much to do with preference – it’s a matter of efficiency and like most things, a balanced approach is the right approach.  With that in mind, don’t take this to mean that you should shovel pizza and donuts into your belly before and after every workout – the quality of the food you eat, as well as the quantity matters.  You’re just cleared now to have rice and potatoes again without fear.