This week we’re finishing up our in-depth look at energy metabolism. In the first segment of this series, we looked closely at the biochemistry of energy metabolism and focused specifically on the equations of the phosphagen system, glycolytic system, and aerobic carbohydrate and fat metabolism. In the second segment, we looked at what does and does not cause muscular acidosis. Now, in this segment, we’re going to look at how fats and carbs differ as fuels and how that influences their utility.
In my opinion, this is one of the biggest areas where a deep understanding (or at least a deeper understanding) can dispel pseudoscientific sports science before it really begins. When we start talking about fats and carbohydrates and how they’re used within the context of exercise, we’re really talking about fuel capabilities—and these two fuels have radically different biochemical profiles! It’s these profiles that lead to their relative advantages and disadvantages, and whenever someone claims that this or that fuel can overcome those built-in biochemical differences we know we’re dealing with a charlatan.
The only other thing I’m going to say to preface this final segment is to keep in mind the context. Right here, we’re talking about the basic attributes of fuels based on their biochemistry. In the wider context of human physiology and adaptability, we might see athletes compensate (not “overcome”) for these limitations. I’ll reiterate this further throughout, but for now let’s move onto the points themselves.
#1: Fat Is Better for Storage but Worse for Energy Regeneration
Fat is an excellent storage molecule. Not only is it much denser in energy than carbohydrates (9 kcal per gram for fat vs. 4 kcal per gram for carbs), it also doesn’t require water to be stored. Thus, it makes perfect sense why our body stores energy surpluses as fat and not as carbohydrate. It’s not because fat is the “perfect energy source”—it’s because fat is the perfect storage molecule!
When it comes to regenerating ATP in a performance context, however, fat loses against carbohydrates on two counts:
Fats Are Slow
Metabolizing a molecule of fat requires more reactions than aerobically metabolizing a carbohydrate (which itself requires more steps than anaerobic carb metabolism, which requires more steps than creatine phosphate metabolism). The complexity of fat metabolism ensures that it is always the slowest method of regenerating ATP—which is fine when we’re talking about fueling our day-to-day life, but problematic when we need to maximize ATP regeneration during exercise.
We’re always burning a mix of fuels when we exercise, and we can only store limited amounts of carbohydrates anyway, so in practical terms we’re not talking about a two-fold difference between metabolizing carbs versus fats—it’ll be far smaller. The more available carbs are, however, the more energy you’ll be able to regenerate moment-to-moment, and that impacts performance.
Fats Are Oxygen-Inefficient
Unlike carbohydrates, fats do not contain any oxygen atoms in the fuel-part of their molecule. That means aerobically metabolizing a molecule of fat requires more oxygen than aerobically metabolizing a molecule of carbohydrates—about 10% more oxygen.
Since climbing so frequently occludes muscles due to its isometric nature, oxygen is actually less available (to the muscles) than would be assumed based on breathing and heart rate. When muscles regain blood flow, they have a limited amount of oxygen to work with before they become occluded once again, and thus oxygen efficiency is greatly important.
This isn’t to say fats are not an important fuel source or that climbers cannot perform well without carbohydrates, as neither are true. Rather, what this suggests is that carbohydrates have inherent performance advantages over fats that are built into the molecules themselves and their metabolism. If you choose to limit or eliminate carbs from your diet, you lose these advantages without gaining any (performance advantages) in return, which isn’t a recipe for climbing success.
Aerobic Metabolism Is Always Occurring
The interplay of energy systems is often illustrated as linear:
- You begin with simple ATP hydrolysis, which lasts for a couple seconds.
- You then shift to creatine phosphate, which lasts another 7-12 seconds.
- After creatine is exhausted, you shift to anaerobic glycolysis, which lasts another 45-90 seconds.
- Finally, you reach aerobic oxidation, which will carry you indefinitely (or until you succumb to fatigue).
Simple, but Inaccurate
This is a convenient simplification, but it misses the big picture that all these energy systems are constantly working together to regenerate ATP. There is no “passing of the baton” from one system to another, there is only the increasing reliance on faster and more powerful (but more limited) systems.
Your body is fairly good at monitoring how much energy it is using and recruiting the proper systems to keep ATP at the correct level. When energy needs are low, there’s no need to recruit fast and limited systems and so the aerobic system dominates (particularly aerobic fat metabolism). When energy needs are extreme, aerobic metabolism is still occurring, but the amount of ATP it regenerates is dwarfed by the amount of ATP regenerated by the faster anaerobic systems.
In this way, time is actually the wrong measure for which system is used when; the proper measure is exertion. Since your level of exertion predicts how long you can exert yourself for, there is some correlation—but presenting time as the primary function by which your body determines energy system use is inaccurate. Let me illustrate:
Right now, as I’m typing this article out, I have very low energy needs, and those needs are easily met by aerobic fat oxidation; for every ATP hydrolyzed, there is an abundance of oxygen to metabolize a molecule of fat through the citric acid cycle to regenerate it. If I close my laptop and start walking, those needs will still be easily met, but a greater percentage of my aerobic capacity will be used, and carbs may be burned at a greater rate (though this will heavily depend on diet and so is variable person to person).
As I increase my walking speed to a jog and then a run, I will eventually maximize use of my aerobic systems (determined by my VO2 max, cardiac strength, and mitochondrial density) and begin to draw lightly upon the anaerobic systems, primarily anaerobic glycolysis. At this point, I now have a “timer” on how long I can exercise at the current intensity. If I’m only slightly above my anaerobic threshold, it may be an hour or two, but eventually my anaerobic sources will be exhausted and I will literally not have the ability to regenerate enough ATP to maintain my current level of exertion.
Picking up the pace, my ATP needs grow greater and greater beyond the capacity of the aerobic system. I still have the exact same amount of oxygen coming in (because my respiration rate cannot increase further), but the amount of ATP I can regenerate by running carbs and fat through the citric acid cycle isn’t enough to maintain my high level of exertion and so the remainder is provided by anaerobic systems. As I near an all-out sprint, my energy needs are now so high that aerobic fuel oxidation is only able to supply a small percentage of my total fuel needs.
Thus, the true picture for how the energy systems play together might look something more like this:
There’s never a moment when we stop using our aerobic system—or even really when we begin to use our anaerobic glycolytic or phosphagen systems—there’s only moments when the different systems provide more or less of our total ATP regeneration. At rest or a low exercise intensity, the aerobic system is more than enough to meet our needs and any ATP regenerated by creatine phosphate or anaerobic glycolysis is minimal (or, perhaps more accurately, they’re still being used but the anaerobic portion of glycolysis will always be finished with the aerobic portion and all creatine phosphate molecules that are hydrolyzed are near-instantly regenerated by the aerobic system). At high intensities, we are generating the same amount of ATP from our aerobic system but it’s not enough to meet our needs and so other systems are being used as well. The transitions are smooth, not jagged.
Shrinking One System Doesn’t Increase Another
One final insight and then this series is finished: when you shrink or eliminate one system, you do not get much compensation (if any).
Since our systems work together commensurate with our current level of exertion (see “Aerobic Metabolism Is Always Occurring” above), and since they’re limited by their capabilities (see “Fat Is Better for Storage but Worse for Energy Regeneration”, also above), we cannot increase any system by decreasing another—there’s no scale shifting.
Solely within the aerobic system, we can shift our preferred fuel from carbs to fat (or the other way around), but that doesn’t change the total amount of energy we can produce using the aerobic system so it’s a zero-sum game. Picture it like this:
As we shift the ratio of fats to carbohydrates in our diet, our body shifts from preferring one fuel to the other—but never does the total size of that energy pool change. We merely train our body to burn one fuel better while it simultaneously “forgets” how to burn the other fuel.
In a similar way, if we reduce the capacity of any system, our other systems do not “make up” for that loss. Yes, they will compensate to the extent that they can, but your total effective energy pool size will only be reduced. If you lost a lung, your VO2 max would shrink and your aerobic energy pool would shrink with it. At some levels of exertion, you’ll still be able to fully meet your energy needs, but as your energy needs increase you will eventually hit a wall. A similar (if much less devastating) thing would occur if you cut carbohydrates out of your diet—you would be able manage fine below a certain level of exertion, but the reduced total energy pool size will start to trip you up when your exercise intensity demands more ATP regeneration.
The reciprocal is also true, though: you can increase certain energy pools without decreasing others. If you train and increase your aerobic capacity, your anaerobic potential remains the same, but since you can now regenerate more ATP using the aerobic system you rely on the other systems less at a given level of intensity. Similarly, if you supplement with creatine, you increase your pool of creatine phosphate, allowing for slightly greater amounts of high-intensity exercise—you still only use that system when the situation calls for it, and your aerobic capability remains the same.
Taken together, this is probably the biggest reason why I cannot support ketogenic diets for performance—you reduce the size of one energy pool (anaerobic glycolysis) while merely shifting fuel use around in another. There is no advantage to ATP regeneration because the ketone bodies ketosis creates must also be aerobically oxidized. Thus, it’s difficult to imagine a way in which ketosis would improve performance, and as of yet no study has demonstrated any advantage to a ketogenic diet while a few have noted significant decreases in anaerobic ability—which is exactly what we would expect!
That wraps up this series on energy metabolism. It’s a fascinating topic, and I’m sure I’ll write more about it in the future, drawing even more insights out of this base knowledge! For now, it’s time to move on.
From this final installment, I hope you’re walking away with the following key points:
- Fats and carbs have different molecular structures and their fueling advantages and disadvantages are derived from these differences.
- Level of exertion is the most important factor in fuel selection.
- You cannot increase your aerobic system through diet, but you can affect the sizes of your anaerobic systems.
As always, I’d love to hear your thoughts! If there’s anything that is still a little unclear (or that seems contradictory), let me know in the comments.