April 29, 2014

Fuel Partitioning 101: Human Body as Hybrid Car

In the first post in this series, we talked about the idea of “calories in and calories out,” and why it’s not all that helpful when we’re trying to understand human metabolism, appetite regulation, fat loss, and fuel partitioning. I rolled a pair of dice earlier, and it came up on fuel partitioning, so let’s kick things off with that. (Just kidding. It was a magic-8 ball. Never trust something as important as this to dice.)   

If you’re already pretty familiar with basic human physiology, there might not be all that much here that’s new to you. But I hope you stick around anyway. Maybe I’ll explain something in a way you’ve never thought about it before—or, more importantly, maybe you know something I don’t (very likely!), and if so, I welcome you to set me straight in the comments. (Although, like I pointed out here, this stuff necessitates a little oversimplifying. There will be a few things I’ll gloss over in the beginning, but will dive deeper into in future posts.)

And, of course, if you’re not so well-versed in this stuff, then I really encourage you to stick around. If you’ve ever scratched your head in bewilderment because you had a big breakfast (read: high-calorie) that consisted of things like orange juice, toast, a bowl of cereal, and a bran muffin, yet inexplicably found yourself hungry again not that long afterward, this series is for you. (Because your body’s reaction to that breakfast is not inexplicable. There is, in fact, a quite elegant explanation, which we’ll get to in time.)

I might have to get a little sciencey on you here and there, but like I did with the digestion series, I’ll try to make connections and create a “big picture” using layman’s terms and analogies that will help this all make sense, and more importantly—keep it fun. (If you want to seriously geek out on the science [and who doesn’t?] let Dr. Peter Attia be your guide.)

Okay, let’s jump in.

I’m going to start things off by introducing a metaphor that is going to serve us very well throughout this series (I hope). Let’s think of the human body as a hybrid car. It’s not a perfect comparison, but it’s good enough for our purposes here. Hybrid cars can run on different types of fuel, depending on the situation, right? The human body is similar. We can run on many different kinds of fuel, depending on how much of each type is available at any given time, and also depending on what kind of activity needs to be fueled, which tissue type is performing the activity, and what messages our hormones are sending throughout our body regarding which fuels to use. (And you thought all you had to do was throw a ham sandwich down the hatch. Ha! If only it were that easy.)

The main types of fuel the body can run on are:
  • Protein
  • Fat
  • Carbohydrate
  • Alcohol
  • Ketones

Let’s address the easy ones first. For obvious reasons, we don’t want to fuel ourselves primarily on alcohol. (And if these reasons aren’t obvious to you, congratulations. I guess you’ve never prayed to the porcelain god after a night of debauchery or woken up in a strange place next to a strange person with whom you may or may not have performed strange acts out of the Kama Sutra.) There’s a time and place for alcohol, of course. (Just think how many ugly people would never get laid without it. Kidding!!) A little hooch now and then is no problem, but we certainly don’t want it to be our main source of fuel.

Nothing wrong with a little adult grape juice now and then. But for the sake of your reputation (not to mention your liver!), please refrain from letting this be your primary source of calories.

Protein: Food, yes, Fuel, no. 
(Not usually, anyway.)
Next up is protein. Even though protein is a source of calories, and therefore something we can consider as “fuel,” we really don’t want to use protein as our main energy source. It’s far too valuable for the myriad other purposes it has in our bodies. Besides contributing to the physical structure of muscle tissue, here’s just a small sampling of what else is made from protein: Antibodies, peptide hormones (like insulin and glucagon), neurotransmitters (like serotonin and dopamine), organs, and blood vessels. Protein has too many other jobs to do for it to be siphoned off as an energy source instead. (Note: in the interest of accuracy, let me say here that there are circumstances under which our bodies will use protein for energy, and we’ll get to those in the future. For now, just know that under “normal conditions,” protein isn’t our optimal fuel source.)

We’ll table ketones for the time being. They’re actually a fantastic source of fuel, but they deserve a much more detailed explanation than I can give here at the outset. And even when someone’s using ketones for fuel, they’re not the main source, so regardless, ketones generally aren’t what we’d turn to to be our primary fuel source.

So that leaves us with carbohydrates and fat. Now we’re getting somewhere.

Let’s talk about carbs. We often hear that carbs are the body’s “preferred fuel.” This isn’t exactly accurate. It’s only true in the sense that carbs (really, glucose) will always be used first. Generally speaking, as long as there’s a lot of glucose available, the body will use it first instead of turning to some other source of fuel. (Such as your stored body fat. Hmmmm...)

I’m not in the "gluten = death" camp, 
but too much carbohydrate can 
interfere with your fat loss goals.
You might have also heard somewhere along the way that the brain requires about 120 grams of glucose per day. This is true, but that doesn’t mean we have to ingest glucose (or, for that matter, other carbohydrates that get converted into glucose). The body has multiple ways of creating this glucose from other things. In fact, to be completely honest with you, the human body can survive (maybe not absolutely thrive, but certainly survive) without eating any carbohydrate at all. Think about it: you’ve heard of essential fatty acids, right? (Like omega-3s and 6s?) And you’ve probably heard of essential amino acids, too. But have you ever heard the term “essential carbohydrates?” No, you haven’t, and for good reason: there are none. In fact, according to the USDA, The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed.” (Emphasis mine. Also: please note I am not saying that no one should ever consume carbohydrates, or that they are somehow "dangerous," or not a natural part of the diet humans have evolved on. I'm saying only that they are not required in the diet to keep us alive.)

There’s really no question that carbs are an energy source. If you’ve ever fed your toddler a juice box and a couple of cookies and then found yourself having to peel him/her off the walls, that is your proof that carbohydrates are fuel.

So what about fat? Thanks to sixty years of brainwashing conditioning with low-fat hysteria dogma, we tend to think of fat in our food as something to be feared, reduced, and, as often as possible, eliminated entirely. Somewhere along the way, we’ve forgotten that fat is a wonderful fuel source. It has more “calories” per gram compared to protein and carbs, right? Well, those calories are a measure of energy. (Potential energy, anyway, for the chemistry and physics nerds among you.) If we think of calories as fuel, then being higher in calories actually makes fat a pretty darn good fuel source, no?

With this in mind, let’s compare carbohydrate and fat as fuel sources and see if we can tease out which one it makes more physiological sense for us to run our bodies on. I’ll keep it simple first and then get my geek on, for those of you who are interested in the nitty gritty.

In keeping with our car analogy, let’s say we have two fuels: one that gets us 4 miles per gallon, and one that gets us 9. (If these were actual automobile fuels, these would, of course, both be terrible. But we’re talking about the human body.) And let’s say these two fuels are the same price. What can we say right off the bat about these two fuels? If we used a fuel that only gets us 4mpg, what can we expect? We’re going to run out pretty quickly, which means we’ll have to stop to fill up more often. If we have to stop to fill up more often, it’s going to take us longer to reach our destination, and ultimately, we’ll end up spending more on gas.

So how about the fuel that gets us 9mpg? That’s over twice as many miles per gallon as the 4mpg fuel, right? Using the 9mpg fuel means we won’t run out as quickly, so we won’t have to stop as often to fill up. If we don’t have to stop as often, we’ll reach our destination sooner and ultimately spend less on gas.

So if we were planning a road trip, which fuel would we want to put in our gas tank?

4 mpg
  • Run out quickly
  • Have to fill up more often
  • Takes longer to get to your destination
  • Spend more money on gas

We would never buy this fuel. L 

9 mpg
  • Keeps your car going longer
  • Less time spent stopping to fill up
  • Get to your destination sooner
  • Spend less money on gas

J  We have a winner!!  J

If you’re a veteran calorie counter (or, dare I say, weight-watcher), you know exactly where I’m going with this. Because between carbohydrate and fat, one provides 4 calories per gram, and one provides 9. And if you’ve ever compared the calorie counts on, say, a fat-free blueberry muffin and a nice, fatty pork chop, then you know which is which. And just in case you somehow missed the conclusion we’ve arrived at, let me reiterate that the fuel that offers 9 miles per gallon (or calories per gram, if you prefer) is the more efficient one.

Another way to think of this is like building a fire. Carbs are your kindling. They’re flimsy and short-lived. Kindling helps get things started, but it burns out pretty quickly, and then it’s gone, right? (Like the old newspapers you throw on the logs in your fireplace.) If all you had was kindling, you’d have to be putting more of it on the fire constantly just to keep it going. (Keep this inefficiency in mind when you hear armchair nutrition experts saying that you should eat every couple of hours to “keep your blood sugar up.” More on this down the line.)

Fat is like big, thick, long-burning logs. These are your go-to fuels. They burn long, steady, and hot, and while you do occasionally have to throw another log on to feed the fire, you have to do so far less often than you would if all you had to burn was newspaper. Put a nice, heavy log on and the fire pretty much takes care of itself for a while.

And now, for the nerds, here’s a more scientific explanation of this. But even if you’re not a nerd (or just won’t admit to being one, even though you totally are and you soak this stuff up like a sponge), at least stick around for the next paragraph.

The thing about the human body is that we don’t just burn fuel. We don’t burn “calories,” and we don’t even burn carbohydrate, fat, protein, alcohol, or ketones. See, the body can’t actually do anything with those macronutrients, per se. It can’t even do anything with the products of those macronutrients, like glucose or fatty acids. Think of it like a carnival or amusement park. When you get inside, what do you have to do? You have to exchange some cash for tickets or tokens, because the vendors and ride operators don’t take cash, right? The “currency” of the amusement park is not dollars and cents, but tickets and tokens.

In the same way, the human body can’t use carbohydrates or fats (or any of the other fuels) as they are. They have to be converted into ATP, the body’s equivalent of tickets and tokens—the “energy currency” of human physiology. So we can think of donuts, steak, ice cream, chicken, broccoli, eggplant, and everything else we might toss down the hatch not as calories, or fuel, or energy, but as potential sources of ATP. (Why only “potential?” Well, the likelihood of these foods being converted into usable energy is dependent on a few different things, which we’ll cover in the next couple of posts. For now, I’m using that word to stress that just because we ingest a food doesn’t mean that food is going to be converted into usable energy.)

And since I promised you some nerdery, here goes. Let’s compare the ATP generated from carbohydrates (glucose, specifically) to that generated from fats. If you’re not well-versed in biochemistry and the chart below is scary and/or makes your head spin, don’t worry. You do not have to understand any of it to grasp the takeaway here. Just skip down to the next paragraph.


  • Glycolysis = 2 ATP (net)
  • Pyruvate --> Acetyl CoA = 6 ATP
  • Krebs cycle = 24 ATP (2 molecules Acetyl CoA x 12 ATP per cycle)
  • Glycerol-3-phosphate shuttle = 4 ATP



(Note: The longer the carbon chain of a fatty acid, the more ATP generated upon its oxidation.)

(Assumes an even-chain fatty acid)

  • # of carbons/2 (β-oxidation cleaves off 2 carbons at a time) = # of Acetyl CoA generated
  • # of Acetyl CoA x 12 (Krebs cycle)
  • # of cleavages x 3 (3 ATP per NADH entering electron transport system)
  • # of cleavages x 2 (2 ATP per FADH2 entering electron transport system)
  • Subtract 2 ATP (consumed to activate fatty acyl CoA)
Stearic acid (C18:0 – a saturated fat with 18 carbons) = TOTAL: 146 ATP

Lauric acid (C12:0) – a saturated fat with 12 carbons = TOTAL: 95 ATP

Take a gander at that chart. Molecule for molecule, fatty acids provide way more ATP than carbohydrates do. In the example of stearic acid (common in such delicious things as porterhouse steak and dark chocolate), we generate 146 ATP. And glucose only brings 36 ATP to the party? He should be ashamed of himself! No wonder you have to eat carbohydrates constantly just to keep fueled.

The takeaway: Gram for gram, molecule for molecule, fats give us more energy than carbohydrates. And not just “more,” but longer lasting, slower burning energy—the kind that can keep us feeling fine—physically, emotionally, and cognitively—for several hours without consuming more food, and without experiencing mood swings, irritability, immediate and urgent hunger, lightheadedness, dizziness, or any of the other unpleasant signs and symptoms of hypoglycemia, even if it’s been many hours since we last ate. If you do experience these things regularly, that’s a pretty sure sign that you’re a “sugar-burner” and not a “fat-burner.” That is, your body is running on pathetic, inefficient kindling rather than the nice, heavy, long-lasting logs. More on this next time.

In the meantime, go cook yourself a steak. And put a pat of butter on it for good measure. (And if you do, invite me over for dinner!)

*Continue to the next post: Fuel Partitioning 101: The Hybrid Car's Gas Tank

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. This is a great discussion of the relative efficiency of the two "fuel sources" (and I really mean it; this post is probably going to feature as one of my resources for my older students for teaching them to communicate science well). What might make it even more powerful for the more nerdy amongst us is to add to the chart the relative molecular masses of the molecules. You've asserted that "molecule for molecule" (so far so good, it's demonstrated by the ATP/molecule breakdown in the table), "gram for gram, fats give us more energy than carbohydrates" - well, you could justify the latter by showing that lauric acid, C12H24O2 (rfm 200), has a remarkably similar relative molecular mass to glucose, C6H12O6 (rfm 180). This means that the molecules are roughly comparable, and after doing a simple crunch-of-the-numbers, you'd find that they give (respectively) 0.2 and 0.475 moles of ATP per gram of fuel. This translates to about two and a half times the ATP per gram from lauric acid versus glucose.

    Too nerdy? Ah well, I'm a chemistry teacher. Can't see a biochem problem without trying to make it more maths-y.

  2. It's never too nerdy! I might not understand it all, but that doesn't mean it's too advanced...just too advanced for *me.* I think I follow you, though. If the bottom line is that we get far more ATP from fats than from carbs, that much I can stand behind. ;-) Thanks so much for the compliment, too. Really flattering that you'd share this with your students.

  3. Personally, I view fats like diesel fuel and carbs like nitrous. But I'm a boy, so it resonates with me. Interestingly, IIRC, the Krebs cycle shuffles the carbons around, and glucose yields about 6 ATP per carbon, and fatty acids yield 8 ATP per carbon. I don't entirely remember, because fatty acid metabolism was barely mentioned in my A&P class. The only mention of Krebs in pretty much any personal trainer certification is within the context of glucose metabolism, and entry level physiology courses seem to only reference glucose metabolism as well. This is a huge problem, as only people that are super geeked on physiology and understand the differences between the two main fuel sources "get it" that fat is the better fuel source, especially saturated.

    1. I'm not exactly sure about the ATP "per carbon," but as this post shows, the ATP per molecule of a fat is *way* higher than per molecule of glucose. :) (Kinda makes you wonder why we've been told for so long to eat carbs for energy, when fat provides so much more of it...)

    2. Well actually, it doesn’t make me wonder so much, as it seems to all go back to the Ancel Keyes flawed study of “saturated fat is bad for you” where he didn’t differentiate between naturally occurring saturated fats (like tallow, lard, goose fat, coconut oil, palm oil) and the man-made hydrogenated garbage fats (margarine, shortning) that he actually used. He also cherry-picked the data, so a good scientist he is not. THEN he convinced our government, who made incorrect low-fat & high carb recommendations, and the rest is (bad) history following bad science, which led to the upside-down food pyramid and the low-fat/no-fat fad that is still with us today.

      Mary G. Enig, PhD lipid biochemist, presented numerous times before Congress but they preferred the lies and flawed science proposed by Keyes. I think that the bottom line there was ultimately money in probably more than one fashion. Who paid whom? Who lobbied? Who benefitted financially? Who remained in power? The lies of health and nutrition run deep indeed, but the truth will always out, eventually.
      Great post, great info, great site! Thank you!

  4. Kinda makes you wonder why we've been told for so long to eat carbs for energy, when fat provides so much more of it.