May 13, 2014

Fuel Partitioning 101: Not a Binary System

Yikes. Heck of a post title, huh? Partitioning and binary. That’s two scary-ish words and we haven’t even gotten started yet. If I’m not careful, someone’s going to think I actually know something about science and/or the human body. (Some days I’m not so sure, to be honest with ya.)

Aaaaanyhow, welcome back to the series on fuel partitioning. Recall from the last couple of posts that we’re thinking about the human body as a hybrid car. It’s not a perfect comparison, but it’s suiting us fairly well so far. Two posts ago, we did a little bit of math and determined that, gram for gram, molecule for molecule, fats seem like a more efficient fuel than carbohydrates. Last time, we talked about how the body “runs on” different types of fuel, and we went right to the gas tank, to see what kind of fuel the body stores the most of—that is, which fuel the gas tank seems to prefer to hold in reserve. Here again, it seems like fat trumps carbs. (Remember that nifty chart? The one that showed the human body doesn’t keep a lot of carbohydrate on hand, but it’ll tuck away fat like a champ?)

Now that we’ve had that little refresher, we’ve got important ground to cover today, so let’s get going. (Kind of scary that I could compress two long blog posts into one paragraph. I’ve said it before: brevity was never my strong suit. In fact, my strong suit is at the cleaner’s right now. *Ba-dump-tsch!*)

Before we go any further into fats versus carbs versus ketones versus chocolate cheesecake as fuel sources, we need to set the record straight. I left off last time saying that we would answer questions like: Is fuel partitioning absolute? If I’m running on fat, does that mean I’m not using any carbohydrate at all? Can my entire body run on fat or ketones? Does my body need at least some glucose?

So here goes. Let’s spoil the surprise right here at the outset. Fueling the human body is not a binary system: one or zero; yes or no; on or off; fats or carbs. Like any good integrated system of systems, the human body has multiple redundancies, checks and balances, and failsafes, all designed to prevent single points of failure. These backups and overlaps ensure that pretty much regardless of what we put down our pieholes, our bodies can get the fuel they need. (For the most part, that is, and in the short term. This is not true for the longer term. Eventually, deficiencies will appear if we’re not sufficiently nourished, but let’s not get ahead of ourselves.)

The human body is rarely absolute about anything. There are metabolic and biochemical pathways, and for the most part, where one pathway predominates, another is limited. (Not shut off entirely, just limited.) Where one thing is stimulated, another is inhibited. (Usually because of scary things like enzymes, many of which counterbalance each other in stunningly orchestrated biochemical dances that no ballroom dance choreographer could ever hope to dream up. More on this next time.) The body isn’t wasteful. It has redundancies and overlaps, but these are intended to protect us. To keep us alive. They’re there for a reason. Beyond that, the body isn't going to waste energy simultaneously running processes that are antagonistic to each other. (For example: when you’re breaking down glycogen in order to release glucose into your bloodstream, you’re not also making glycogen for the purpose of storing glucose. ‘Cuz that would be stupid, right? Same thing with fat: if your body is actively storing fat, it’s not simultaneously burning a whole lot of it.)

Running on carbohydrates and running on fats & ketones are not mutually exclusive. To some extent, we run on all of them at the same time. This happens for a few different reasons. The three main ones are: 1) The type of activity being fueled; 2) The type of cells/tissue performing the activity; and 3) The hormonal milieu under which the activity is occurring. Let’s tackle them in order. Before we do, though, keep this in mind: there’s a reason I keep throwing in phrases like “generally speaking,” “for the most part,” and “rarely.” When it comes to fueling the body, it would be irresponsible of me to use the words “always” and “never.” (Except maybe when talking about my human body. In that case, cashews are always yummy.)

Type of activity being fueled  (slightly oversimplified)

If you’ve ever been near a piece of cardio-type equipment at a gym, you might have noticed that it had a sticker or a chart on it showing you an age and a heart rate, and whether exercising at that percentage of your (theoretical) maximum heart rate put you in the “cardio zone” or the “fat-burning zone.” If I attempt to address the more nonsensical and utterly ridiculous aspects of this, this post will be three hundred pages long and I’ll lose all four of my regular readers. (Four? Yes, I think I might have gained someone last week. Probably by accident…) So instead of trying to take down the entire concept, I’ll use what I can to my advantage. One thing those laughable charts have right is that, generally speaking, when your heart rate is lower, and your exercise is less intense, it is likely being fueled more by fat than by carbohydrate. (Another way of saying this is that anaerobic activity is fueled more by carbs, and aerobic more by fat. Note: aerobic is the stuff you can do for a long time—walking, jogging, even running at a sufficiently slow pace. Anaerobic is the stuff that makes your muscles hurt after just a short time, assuming you’re doing it right: hence sprinting, lifting, and the like.)

If you’re a fan of Robb Wolf, you’ve probably heard him use the phrase “glycolytically demanding” when referring to certain forms of exercise. That means they require a lot of glycolysis and/or glycogen breakdown – i.e., a lot of glucose. He is referring to intense activity—CrossFit, intense lifting, sprinting, for example—that is fueled more by carbohydrate than by fat. Again, this isn’t a binary all-or-nothing deal, just a balancing act where, in this case, the balance leans toward carbs. (Before anyone gets up in arms because I’m saying intense activity is usually glycogen dependent, I fully acknowledge right here and now that there are elite athletes who are “fat-adapted” and largely fuel darn near any exercise on fat, but I’ll spare you yet another potential 200-page post and table that subject for another time.)  

Okay, so the intense stuff is fueled (mostly) by carbs. What about the non-intense stuff? What about a nice, comfortable stroll through the park? What about gardening, or vacuuming, or mowing the lawn? (All of which can be intense if you want them to be, but can also be fairly slow.) What about all the stuff I’ve said before we don’t think of as “exercise” or “burning calories,” but which does use energy? Think about it: pretty much anything we do requires at least some energy, even just sitting in your chair reading this blog. (Think about all the postural muscles in your back and neck working hard just to keep you upright the entire time you’re there on your rear end. And they're working even harder if you have a stand-up desk.) All that kind of sittin' around doin' nothin' type activity is mostly fueled by fat. (Remember the heart rate chart: when your heart rate is in the lower range, that’s the “fat burning zone.” [Though I still hesitate to use that to help make the point.])

According to the smarties, “Fatty acids are the main source of energy in skeletal muscle during rest and mild-intensity exercise. As exercise intensity increases, glucose oxidation surpasses fatty acid oxidation.”

So yeah: intense activity, lots of heavy lifting, stuff that makes you completely exhausted at the end? Probably more glycogen (carb) dependent. Not so intense? Fueled more by fat. This is why people who do intense activity generally fare better with a little bit more carbohydrate in their diet (and glycogen storage in their muscles) than folks who do not regularly pound themselves into the ground. If you've spent any time around various Paleo, Primal, or low-carb forums, you'll recognize this as a reason some people who participate frequently in intense athletics feel like dog excrement and decide that those dietary approaches "don't work." The simple truth is, they just need a bit more carbs in their diet than someone who's trying to lose weight or reverse metabolic derangement.)

Okay. I’m pretty sure I’ve made my point with the “generally” and “mostly” stuff, so I’m gonna cut it out going forward. Just remember that none of this black and white.

Type of cells/tissue performing the activity

Did you know that some types of cells can’t use fats for fuel? And that some don’t do so well on glucose? This is why, regardless of what type of food is coming in, and even regardless of the hormonal situation, the body is fueled by multiple kinds of fuel at all times.


Red blood cells: RBCs have no mitochondria. For those of you who’ve been out of biology class for a few decades, mitochondria are the “powerhouses” of most cells. The energy factories, if you will. And when fats are used as fuel, they are “burned” in the mitochondria. So if a cell has no mitochondria, it can’t use fat, right? (If your stove isn’t a gas stove, then it can’t use gas. It has to use electricity or some other form of energy. Simple enough.) And since we’ve already said fat is our primo, go-to fuel, why would there be cells in our bodies that can’t use it? Ya gotta love nature: In producing energy, the mitochondria use up a lot of oxygen. But what is the job of RBCs? They transport oxygen (via the bloodstream) to the rest of the body, right? Well, what good would it do if RBCs used up all the oxygen they’re supposed to deliver to the rest of the body? It would be like the UPS man keeping all the packages for himself so that none of the cargo gets to its intended recipients. If you think it’d be a bummer to have your shipment of books from Amazon stolen off the truck, imagine how bummed you’d be if your heart (or gonads) stopped getting oxygen.

Enterocytes: These are the cells that line the small intestine. And what is one of their big jobs? To move nutrients (such as glucose) from the lumen of the intestine into the bloodstream, yes? (Yes.) So what good would it do if these cells used that glucose to fuel themselves? The rest of the body wouldn’t get its requisite share. (Greedy lil’ sum’bitches!) So instead of glucose, the main fuels for these intestinal cells are the amino acids L-glutamine and L-glutamate.

Cardiac/heart cells: Your heart is one of the most aerobically active of all your cherished parts and pieces. Think about it: your heart is using energy 24/7/365. IT. NEVER. STOPS. (Well, that is, until it stops for good. But other than that, it is a muscle that is contracting and relaxing every minute, every day of your life.) It never gets a rest. It needs to be fueled all the time, no matter what. In order to make sure this happens, your heart is loaded with mitochondria. Oxygen users like crazy! So since the heart has all these mitochondrial fuel generators available, it might as well use ‘em to generate some fuel—and what did we see two posts ago? The nutrient that gives us the most fuel per gram is fat. So the heart uses fat like champ. But don’t just take my word for it. According to people way smarter than I am, “Between meals, cardiac muscle cells meet 90% of their ATP demands by oxidizing fatty acids. Although these proportions may fall to about 60% depending on the nutritional status and the intensity of contractions, fatty acids may be considered the major fuel consumed by cardiac muscle.”

And some more people smarter than me: “The heart has virtually no glycogen reserves. Fatty acids are the heart’s main source of fuel, although ketone bodies as well as lactate can serve as fuel for heart muscle. In fact, heart muscle consumes acetoacetate in preference to glucose.” (Emphasis mine. If you have no idea what acetoacetate is, don’t worry. The point is, the heart runs on at least two different types of fuel better than it runs on carbohydrate.)

Hepatocytes/Liver cells: The liver is the main site of ketone production, but hepatocytes don’t use ketones all that much. They’re just like the RBCs and enterocytes: if they consumed the bulk of the products they’re supposed to be exporting to the rest of the body, that would be some pretty bad metabolic juju. Again, according to the experts: “α-Ketoacids derived from the degradation of amino acids are the liver’s own fuel. Furthermore, the liver cannot use acetoacetate as a fuel, because it has little of the transferase needed for acetoacetate’s activation to acetyl CoA. Thus, the liver eschews the fuels that it exports to muscle and the brain.” (English translation: the liver doesn’t use all that much glucose or ketones. It spares them for other parts of the body that need them.)

Brain cells: The brain requires glucose. No two ways about it. Even the most ardent low-carber can’t deny that the brain needs glucose. However, it doesn’t need as much glucose as we tend to think it does, as long as the glucose debt is made up for by fuel coming from an alternative source, like ketones. Sadly, fatty acids are generally not used as fuel for the brain. Glucose and ketones are the main players up in your noggin. “Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation [OR CARBOHYDRATE RESTRICTION, says Amy] ketone bodies generated by the liver partly replace glucose as fuel for the brain.

The hormonal milieu under which the activity is occurring

This post is already super-long, but let’s see if I can sum up the effects of hormones quickly.



HA! Right. Sum up quickly? As they say across the pond, “Not bloody likely!” I can do it more quickly than you’d think, but I’ll end here for now and get that last bit posted sometime later in the week. I may have gone a little too deep into the various cells above, but what can I say; I'm fascinated by this stuff and like sharing nifty information with people. If you were bored, no worries. At least now we're scientifically accurate and are all on the same page regarding what we mean when we talk about the body using different fuels and this not being black and white. The next post will be a little more relevant in terms of what governs fuel partitioning. 

*Continue to the next post: Fuel Partitioning 101: Hormones

Remember: Amy Berger, M.S., NTP, is not a physician and Tuit Nutrition, LLC, is not a medical practice. The information contained on this site is not intended to diagnose, treat, cure, or prevent any medical condition.


  1. I commented on one of your posts a few weeks ago as anonymous, but a different anonymous commented last week. Hey, You might have two more readers;-)

  2. Third reader here !! - great stuff Amy. I'm really enjoying the science bits in your posts.

  3. This comment has been removed by the author.

  4. Luckily I keep my dictionary and nutrition books close by, along with the recipe books. Think I may have to find a recipe for chocolate cashew cheesecake. Some words seem to stick in the brain after reading....


    1. Can't say blame you! Those tree sound like a good combination. ;-)

  5. I am learning so much reading this!

    1. Thanks! Now all I need is a couple thousand more people to feel the same way, and then HIRE ME. ;D (BTW: Is this the "TM" I think it is?)

  6. This is so good! You answered so many of my questions, thank you for that. And I love your writing style, really.