May 6, 2014

Fuel Partitioning 101: The Hybrid Car's Gas Tank

Hey kids!

Recall from the previous post in this series that we’re attempting to unravel the mysteries of fuel partitioning, appetite regulation, fat loss, and metabolism, and we kicked things off by thinking of the human body as a hybrid car. Using this as a guiding idea, we established that, just as a hybrid car can run on gasoline or electricity, our bodies can run on different types of fuel.

Last time, we looked at the major sources of calories we eat or drink, in order to determine which one(s) make the most sense to use as fuel: fat, carbohydrate, protein, and alcohol. (I haven’t forgotten about my beloved ketones, but we’ve got to put those on hold for now.) We said that protein is too valuable for other purposes to let it be used as our primary fuel source. And unless you can stay gainfully employed and safely operate your motor vehicle under the influence of copious amounts of alcohol, alcohol isn’t the fuel we want as our go-to either. So that leaves fat and carbohydrate.

We compared carbohydrate and fat to quick-burning kindling and long, steady-burning logs, respectively, and determined that fat is a more efficient fuel. (Remember the 4mpg and 9mpg gasoline analogy?)

But let’s not stop there. In order to answer this question and really convince ourselves which type of fuel it’s most sensible to have our bodies run on, let’s take a look at the gas tank. In terms of fuel efficiency, so far, it looks like fat is a better choice than carbohydrate. But which of these is the gas tank, itself, designed to hold?

Leaving aside the car as a whole for now, what kind of fuel does the gas tank want to hold? What does it seem designed to hold? And, just so we’re clear, it is designed to hold something. Just like your car’s gas tank is designed to run on petroleum-based fuel. You wouldn’t fill your car’s gas tank with orange juice and expect it to run as well as it does on gasoline. It’s the same with your body’s fuel tank. It is designed to store and use some fuels better than others.

Check this out:

*Based on an individual weighing 155 pounds (approx. 70.5kg)
Source: Devlin, T., ed. Textbook of Biochemistry with Clinical Correlations. Wiley & Sons, Inc. 2011

Let’s start at the top and work our way down. Regarding stored potential sources of fuel, we have about 6000 grams of body protein, for 24,000 calories of stored fuel (going with 4 calories per gram). Holy mama, that is a lot of stored energy! But where is that protein stored? In our muscles, right? And our organs, glands, bones, and other precious tissue that we do not want to break down (catabolize) for fuel. (More on this down the line when we get to weight loss—specifically, why we want to aim for fat loss rather than “weight loss.” See, we don’t want to lose these physiologically and metabolically valuable body parts. We don’t just want to lose weight; we want to lose fat and hang onto as much muscle mass and other lean tissue as we can.) And remember from last time that protein is not what we want to use as our primary fuel. Bottom line: those 24,000 calories are awfully tempting, but as a fuel source, they're out.

We work way too hard to put muscle on our bodies to have it siphoned off and used for fuel.

Let’s move on to carbohydrate. (Abbreviated as CHO in biochem shorthand in the chart above.) Our bodies have three compartments to hold carbohydrate—specifically, glucose. The first is in body fluids—mostly the blood. This is the 20 grams (or 80 calories) of fuel you see above. It’s not much. (And if you ask Dr. Mike Eades, of Protein Power fame, the amount of glucose in the blood of someone with a healthy blood glucose level is actually much less than that—more like 5 grams or less.) 80 calories isn’t anything to write home about, so let’s move on to the other form of stored carbohydrate in our bodies: glycogen.

Glycogen is to human beings what starch is to plants: it’s the form in which we store carbohydrate. (We store it as glycogen, and a potato, for example, stores it as starch.) Since our blood can only hold so much glucose at any given time (even for a type 2 diabetic with sky-high blood sugar), our bodies have to find somewhere else to stick it. This “somewhere else” is our liver and our muscles. Looking at the chart above, the liver can only hold about 70 grams of carbohydrate as glycogen, for about 280 calories’ worth. That’s still not much. As a native New Yorker, I can tell you that just one of those classic New York bagels nearly the size of your head can probably pack a 70-gram wallop of carbohydrate all by itself. So this liver glycogen, like the glucose in the blood, and like the protein in the muscles, doesn’t seem like such a great fuel for the body to rely on.

But the muscles—now we’re getting somewhere. Even a relatively non-muscular person still has a fair bit of muscle mass. (It might be hiding underneath a thick layer of body fat, but it’s still there. In fact, heavier people usually have more muscle than we might think by looking at them. After all, if they weigh more--even if that weight is fat, rather than muscle--the muscles in their legs, back, and all the rest of them have to work harder just to keep them moving around and performing the same physical tasks anyone else does. But I digress.)

You will be hungry again in 90 minutes. 
Mark my words.
The hypothetical 155-pound person represented in this chart stores about 120 grams of carbohydrate in their muscle glycogen, for around 480 calories. Not too shabby, but nothing to brag about, either. If you’ve ever been to the Olive Garden (in your pre-Paleo/Primal/low-carb days, of course), you probably crammed more than 120 grams of carbohydrate down the ol’ piehole in just one meal. (Breadsticks, pasta, wine, dessert. Heck, you might have been at 120 from dessert alone.) So again, not quite a reliable fuel source. (Plus, another mark against relying on muscle glycogen as a fuel for the whole body is that it can only be used to power activity in the muscles in which it's stored. It does the rest of the body no good. Glycogen stored in, say, your biceps, can't be released into the bloodstream when your blood sugar is getting a little low. Only liver glycogen can do that. In fact, that's pretty much liver glycogen's raison d'etre (or purpose, if you will) -- to keep you from getting woozy if you find yourself needing to go a few hours without eating any carbohydrate, or any anything, for that matter.) 

Remember from last time: carbohydrates are kindling. They burn quickly and then they’re gone, and we have to keep fueling the fire (refilling the tank) constantly. I’d rather use a fuel I don’t have to top off quite so often, wouldn’t you? You wouldn’t have to stop and have a snack every two or three hours, and you wouldn’t be on a first-name basis with the guy who refills your office’s vending machine. And you wouldn’t get those hypoglycemic symptoms when your tank starts getting low, because your body would be running on a fuel that you didn’t have to constantly put more of in the tank.

Is there a fuel that fits this bill? One that our gas tank can hold a lot of; one that burns slowly and steadily; and one that we don’t have to ingest more of almost as soon as we finished the last refill?

Don't look now, but I think we just found 
a pretty great fuel source.
Let’s see if we can find a fuel like this. Let’s look at fat. Do you see what I see? Holy cowza WOWZA! The person in the chart above stores about 15,000 grams of fat in his adipose (fat) tissue, for a whopping 135,000 calories!! (9 cals per gram.) NOW WE’RE TALKING, PEOPLE. THAT is some serious fuel storage.

The human body has an almost unlimited capacity to store fat and accumulate adipose tissue. Please pardon my terrible, terrible political incorrectness here, but if you don’t believe me, go people watching at an airport or a Wal-Mart sometime. Trust me. The human body has an almost unlimited capacity to store fat.

Taking this into account, it almost seems as if nature (or evolution, or the big voice in the sky, or whatever you happen to believe in) evolved/created/designed our bodies to run on fat, because that is the type of fuel our gas tanks are designed to hold the most of. Why would we want our bodies to run on carbohydrate—a fuel that we’ve already established is not that efficient, and we’ve just shown the gas tank isn’t capable of holding all that much of?

Answer: we wouldn’t. Remember this next time you hear some poor, misguided soul say that carbohydrates are the body’s “preferred fuel source.”

(Also: see how inclusive I just was, with that whole “evolution/creation/design” thing? That was to make up for the Wal-Mart comment. I’m a nice person, I assure you. I’m not judgmental at all. [Except when evaluating clients’ food journals. Then I judge like crazy. You ate WHAT?! Are you joking?! No wonder you still feel awful and haven’t lost any weight, clown. Little Debbie called; the date you had with her Friday night is off.” No, just kidding here. Definitely kidding.)

Okay, back to being serious.

I’ve been using phrases like “the fuel we want our bodies to run on” deliberately. Because when I say this, I don’t mean just when we’re literally running (or doing any other type of workout, for that matter). See, every single thing our bodies do requires energy. Yes, intense exercise uses fuel, but so does sitting upright, and breathing, blinking, digesting, the heart beating, the hand holding a pencil to write something—every single process that goes on inside us, whether we’re aware of it or not. Since this is the case, which do we think is more important: the calories we might burn during one measly hour at the gym every day, or how our bodies are using fuel the other 23 hours of the day? Even if we were to work out three hours a day (and if you have time for that, my condolences on your unemployment), which would be more important: what we burn during those three hours, or what our bodies are doing metabolically and biochemically the other 21 hours of the day?

Seriously, you could do this for four hours a day, and it would still matter more 
what happens during the other twenty.

Can we agree that what matters more is what’s going on with how our bodies use fuel during all the hours we’re not specifically dedicating to physical exercise and exertion? (‘Cuz, y’know, you actually use fuel while you sleep. Yes! I knew there was a reason naps are so awesome. They’re practically a workout! [No, not really. My point is, even when we’re not doing anything we normally think of as “burning calories,” our bodies are using energy all the time, whether we’re walking, sleeping, cooking, folding laundry, reading a blog, or watching Barefoot Contessa reruns on Food Network reading the latest nutrition journals. And it is the energy we use during all these other activities that packs a bigger punch when it comes to body composition, fat loss, and blood sugar regulation.])

Coming up in the next few posts: Continuing down the hybrid car path. How do our bodies know which fuel to use? Is there a way to prime our bodies so that they’ll run more on fat than on carbohydrate? (Yes.) Is this fat/carbohydrate debate like the Mac/PC thing? (No.) Is fuel partitioning absolute? If I’m running on fat, does that mean I’m not using any carbohydrate at all? (No.) Can my entire body run on fat or ketones? (No.) Does my body need some glucose? (Yes.) Does any of this relate to willpower? (Yes and no.) Are all overweight people lazy gluttons with no discipline? (NO!!) Is Amy one of those über-militant low-carb whackjobs who thinks that a slice of bread is a one-way ticket to the nefarious underworld? (Not even close. In fact, the reason Little Debbie had to cancel on you is because she’s meeting me on Friday instead. [Just kidding. It’s Ben and Jerry. Debbie is all yours.])  

*Continue on to the next post: Fuel Partitioning 101: Not a Binary System

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 wouldn't say that carbs are the body's preferred fuel source for daily activities and standing around, but I've seen evidence that carbs are the _muscle's_ preferred energy source (in the form of glycogen) during bouts of intense exercise. Anecdotally I've heard of weight-lifters on very-low-carb diets who either need longer rest times between sets, need to tone down the intensity, or both.

    Supposedly, lower-intensity exercise burns a higher percentage of body fat while higher-intensity exercise burns a higher percentage of stored glycogen. I don't think I've ever depleted glycogen stores before finishing a weight lifting session, but this did seem to manifest on long cycling trips where after a few hours I needed to slow down because I found myself pushing against the pain threshold at my previous pace--perhaps because my leg glycogen was running low and my body was telling me to maintain a pace at which fat could be metabolised quickly enough to cover the energy requirement.

    Maybe you planned to cover this in the next couple posts but I wanted to bring up something your readers might be considering with respect to fuel partitioning and exercise intensity.

    1. Thanks for the great comment. And yes, I agree with you. Low-carb isn't for everyone, and there have been a lot of stories in "the community" during the last couple of years of people doing intense athletics (especially *frequent* intense athletics) on too few carbs and really messing themselves up. There's a time and place for higher carb intakes, and no one diet works for everyone.

      I'm planning to cover this in a future post (probably the very next one, in fact, or maybe the one after that). It's hard to do it all at once. I feel like my posts are already about three times longer than most people care to read. ;-) (Sometime between when I was little and now, the population's collective attention span shrunk dramatically...)

    2. Also: when you post an intelligent comment like this, you should come out of anonymity. Might as well let the world know who you are when you say something useful. :)

  2. Great topic and I really like your writing style. I am a little confused about the muscle glycogen stores though. I have learnt that muscle glycogen is the sole source of energy for the first half hour of running and that fat burning doesn't kick in until after that. Now running for half an hour would consume something like 350 to 400 kcal. This would almost deplete the muscle storage. Sounds a little dangerous to me. I have learnt the wrong thing?

    1. Hey Stefan, to be honest, I don't know precisely how the math would work out. It makes sense that it could depend on how full one's glycogen stores are to begin with, right? Did someone carb load the night before, or have they been eating very low-carb and *didn't* put a whole lot into the tank prior? If someone doesn't have enough glycogen stored to fuel 30 minutes of running, I suppose the body would have to turn to something else. (Otherwise maybe hit the wall and collapse in a heap, but that seems unlikely after 30 minutes, no matter how much glycogen someone started out with.) It might also have to do with how intense the running is. One man's warmup is another's all-out effort.

      I'm probably scaring off the few readers I had by giving the honest answer of "I'm not sure." :-/ You could check out Volek & Phinney's book, since they are probably the go-to experts on this:

  3. But if you are working at a constant rate, not an extreme rate, is the body relying more on fat or carbohydrates? Think of humping 60 lbs of kit on a forced march. J.

    Anonymous of the internet unite!

    1. Good question...probably depends on the intensity. To some extent, it's a mix of both, since there are some types of tissue (RBCs, for example, which have no mitochondria) that *can't* use fats. More on this in an upcoming post.

      As for being anonymous, I wish you weren't, J. I have so few "regular readers," and almost no commenters (except for the vitamin J post). But you've chimed in a few times, and I appreciate that. It would be nice to know who you are, but I'll respect your privacy.

  4. I'm sure you have a lot more regulars who are just not posting. You're giving real write-ups based on real experience with an entertaining side of sarcasm, which stands by itself amongst the paleo/primal/got to do something about my health blogs. Now that Seth Roberts (RIP) has moved on the final self-experiment, there should be more readership to attract.

    Anonymous by accident, as this was the easy profile to select when posting a comment. Just a Paleo Luddite when it comes to this high tech sometimes. J.

    1. :) Thanks! Nice to know someone out there appreciates the sarcasm.

  5. Amy, very interesting posts. I have only just found you, but really like your writing style, and the length of your posts are fine, no need to dumb down jst because everybody else does :-) I am looking forward to learning more, thanks a lot

    1. Thanks so much! Always happy to have satisfied readers.

  6. so to use the analogy correctly, humans don't have just one gas tank, we have several. The fuel goes into the same [pie]hole but then gets segregated out in different directions once past that. Also different parts of the human engine burn different fuel wait, prefer different feedstock for their ATP source, so for example the heart prefers to get its ATP from fatty acids. Makes sense. The worst thing for the heart muscle is to develop a bunch of lactic acid and go through what other muscle tissue goes through when it just can't take up any glucose after reaching a certain metabolic limit (described on one of the two books by Volek and Phinney) (And my theory as to why so many marathon runners keel over and die in races is because they haven't been eating a high enough fat diet to keep their hearts happy and strong...but I digress). So actually you could say that the human body is made up of a number of different engines - not one engine -- and just as a diesel or gasoline or electric engine have a certain mechanical profile, the different engines in the human body have different metabolic profiles, and some of them are hybrid engines (to carry forward with your analogy...)

  7. oh just thought of one other thing. The human body is not just made up of fuel tanks and engines. We forgot that it also contains the refinery or rather multiple refineries!! that's what the feedstock is going into, something akin to those great big oil refineries like the one on the edge of Denver, CO or Anywhere, Texas.

  8. One probably "burns more calories" in their kidneys making pee during the day than could possibly burn on an elliptical machine in a day.