Why You Need More Oxygen to Burn Fat than Carbohydrates (and How This Affects Your Climbing)

by Brian Rigby, MS, CISSN

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Carbohydrates Diet Fat

Why you need more oxygen to burn fat than carbohydrates.

I’ve written before about how carbohydrates are more oxygen-efficient than fat, which is a boon for climbing of all types; what I haven’t really addressed is why. Far from being a dry, biochemical fact of interest only to physiology enthusiasts, knowing exactly how carbohydrates derive their advantage can inform everyone about their diets, as well as help explain why the majority of climbers do not benefit from a low-carb diet performance-wise. Truly, it’s all in the structure!

Don’t worry if chemistry isn’t your strongest suit, I’ll do my best with descriptions and pictures to demonstrate how carbohydrates have an inborn oxygen advantage.

The Basic Structure of Carbohydrates and Fats

Nutritionally and biologically speaking, there are a huge number of unique carbohydrates and fats. Structurally, however, all carbohydrates resemble one another, as do all fats.

glucoseWithin the carbohydrate group, different types of chains made from different types of simple sugars create numerous carbohydrate-based macromolecules, from simple sucrose (table sugar, a glucose linked to a fructose) to amylose (starch, thousands of glucose molecules connected by links our bodies can digest) to cellulose (wood fiber, thousands of glucose molecules connected by links our bodies cannot digest). Despite their diversity, however, all these molecules are formed of simple sugars, and all simple sugars have the same basic formula: Cn(H2O)n. That means that every carbohydrate has exactly one carbon and oxygen atom for every two hydrogen atoms—or that carbon and oxygen are always found in equal measure in any carbohydrate.

In the picture to the right, you can see how the carbons (the black balls) equal the oxygens (the red balls) in number. This is important for a reason we’ll come back to in a bit.

Fats, too, have tremendous diversity. There are saturated fats, polyunsaturated fats, and monounsaturated fats, and each of these broad categories has numerous chain lengths ranging from only two carbons long to over thirty carbons long. And as with carbohydrates, all fats have the same basic structure with only a small amount of variation: CH3(CH2)nCOOH.* In reality, however, only the tail end—the “CH3(CH2)n” part—is important as a fuel, which means is that for all intents and purposes fats do not contain oxygen, and the body must provide all the necessary oxygen itself.

Palmitic Acid Chemical Structure*[Please note, this is only for saturated fats, or fats that are fully saturated with hydrogen atoms. When fats are unsaturated to some degree, they have fewer hydrogen atoms and therefore a slightly different chemical structure, but one that still only contains oxygen in the “COOH” (carboxyl) end. For the purpose of this discussion on fuels, oxygen, and fat metabolism, there’s no important difference.]

Ultimately, the body will break either of these fuels down into carbon dioxide (CO2), yielding energy in the process. In order to be able to break them down, however, it’s necessary to balance the input (the carbons and oxygens from carbohydrates or fat) with the output (carbon dioxide). When you start with a glucose molecule and its six carbons and six oxygens, you only need to add an additional six oxygens to balance the equation. On the other hand, when you start with a 16-carbon fat like palmitic acid above, you need to add thirty two oxygens, or one for each carbon that will go through the citric acid cycle (the means by which we aerobically metabolize energy).

Basically, all carbohydrates come equipped with oxygen in equal measure, which means it takes less oxygen to aerobically metabolize them. Fats don’t come equipped with oxygen, so you need to provide more to get the same benefit.

To sum up:

  • Carbohydrates come equipped with one oxygen for every carbon, and only need one additional oxygen per carbon to metabolize aerobically.
  • Fats do not contain oxygen (in the fuel-part of the molecule), and need two additional oxygens per carbon to metabolize aerobically.

When oxygen is plentiful—which is most of the time—that small advantage of carbohydrates over fats is meaningless. Oxygen is free, after all, and our body literally harvests it without needing to think about it. But during intense exercise, or any time your muscles are deprived of oxygen for any reason, then carbohydrates gain a distinct and incontrovertible oxygen advantage, estimated to be roughly 20% (as in you get 20% more energy per unit of oxygen when using carbohydrates as a fuel than when using fat as a fuel).

Oxygen Is at a Premium for Climbers

Okay, but climbing isn’t really the sort of sport that makes you breathe heavy in a desperate attempt to perfuse the blood with oxygen. You may find your heart beating faster by the end of a climb, but you rarely finish completely out of breath as you might during a run or swim. Is oxygen really at a premium for climbers?

Yes! Oxygen is a limited commodity for climbers—not because we can’t inhale enough, however, but because we can’t deliver enough.

Unlike most other sports, climbing relies heavily on isometric contractions—muscle contractions that neither shorten nor lengthen the muscle. While other sports have rapid muscle contract-relax cyles, where the muscle gets to relax at regular intervals, climbers often spend long chunks of time contracting their muscles before allowing them to rest. As a result, blood flow is occluded to the working muscles—and without blood, you have no oxygen.

Absolute occlusion is estimated to occur at about 65% of maximal isometric contraction for most people, but could be as low as 51% for stronger individuals (because their maximal strength is so much higher, and because blood vessels don’t become more resistant to contractile pressure as you get stronger, the overall percentage is lower even if the absolute force required to occlude vessels remains the same). This occlusion leads to an oxygen deficit, which drives greater reliance on anaerobic energy systems that ultimately drive down intramuscular pH and lead to fatigue—so even if it’s impractical to know exactly what force occludes your blood vessels, it suffices to say that you’ve experienced this occlusion.

That means that anytime you’re gripping onto something, particularly for more than a few seconds, you’re depriving your muscles of oxygen. The far greater consequence of this is the shift towards anaerobic fuel metabolism and the concurrent build-up of free hydrogen ions in the muscles (the cause of intramuscular acidosis, and one suspected culprit in fatigue)—but with sufficient carbohydrates, you can at least eke out a bit more aerobic energy from the remaining oxygen than you could with fat.

The Take Home Message

So how does all this play into your climbing? Let’s recap fast:

  1. Carbohydrates require less oxygen to metabolize aerobically than fat, giving carbohydrates a per-unit-of-oxygen energy bonus.
  2. Climbers rely heavily on isometric contractions that occlude blood flow, and therefore oxygen supply, to the working muscles.
  3. Without sufficient oxygen, the muscles must rely on anaerobic energy production, a process that rapidly increases intramuscular acidity.
  4. Since carbohydrates can produce more aerobic energy with less oxygen, they can make the limited supply of oxygen in a contracted muscle go further, helping prevent fatigue.

In other words, carbohydrates hold an advantage over fat in terms in of energy production during climbing because carbohydrates don’t need nearly as much oxygen to metabolize. It’s a small advantage considering how reliant climbing is on anaerobic energy sources regardless (because anaerobic energy sources are capable of far more energy output per second), but it’s an advantage worth taking nonetheless!

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1 comment

  1. Fred McWaid

    Would really like to have a printer friendly version

    Loved the info; never knew where that 20% extra oxygen number came from

    Thanks, Fred

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