Fat Burning Compared To Glycogen

We as endurance athletes are always looking for that edge against our competition and trying to cut that extra minute off our time for a given distance. There are many things we can do, like various types of running sessions, cross training, weights and using different fuels do keep our energy levels up. But using gels, sports drinks and even the Amazeballs, which are favored by Shaun and I, is a tricky business to get perfect in race conditions. So the question is, what are the alternatives?

Fat is an untapped source of energy in many athletes for a variety of reasons. First, we have been programmed to think that it is our most reliable source of energy from companies like Gatorade and Powerade etc while downplaying the effectiveness of fat as an energy source. But let’s look at a comparison between the two of them to see their shortfalls and advantages. But first, when I talk about them as individual components, we must keep in mind there is always a certain amount of each used in exercise, just the ratios vary for different people.

First off is the amount of each energy source we store in our body. Sugar is a very limited source of energy with roughly 2 teaspoons worth stored in the blood and the remainder of the 2000 calories (as an upper limit) we store as glycogen in the muscles. Once this starts to get depleted, which takes roughly *90 minutes to 2 hours at tempo pace for a trained athlete, we start to tap into other energy supplies such as the proteins in our muscles. Through a process called gluconeogenesis, we convert the proteins into sugar for readily available energy.

* Note – Running at about a 12km/h pace uses roughly 750-800 calories per hour. As we start to reduce our glycogen stores, our brain sends a signal out to slow down our body. Tim Noakes calls this our internal regulator. We are not out of energy, but we are getting low and the brain wants to protect the body from complete depletion.

Gluconeogenesis is a long slow process, so can’t really be relied upon for your primary energy source, so we must top up the supplies externally with our preferred fuel hopefully before this process starts kicking in. After all, who wants to metabolize muscle to run that extra few k’s? Certainly not me, I have enough trouble building the stuff to waste it on energy!

Let’s compare this to fat stores. If you look at the average man at roughly 75kg with 10% body fat, you can safely say that he has 7.5kg of fat on board to use as energy. With fat being a far more dense fuel than sugar (sugar at 4 calories per gram and fat at 9 calories per gram) we can say that the fuel stores for our subject are in the region of 67,500 calories. Remember what I said about our glycogen stores being close to 2,000 calories which lasts about 90 minutes. Imagine how long 67.500 calories could last for! Theoretically, that’s about 90 hours worth at 750 calories per hour.

So in this round, we can safely say that fat wins as the greatest amount of energy stored in our body.

Now let’s look at a different factor. Which can burn more effectively. Glycogen is able to be burnt at the rate that our body requires, which is why we look at it as the preferred fuel for exercise. If you need 1,000 calories per hour, you can burn that quite easily when using glycogen. Fat however is considered a little harder to burn. The most commonly recognized amount is 1.1g per minute in trained athletes as a maximum with most studies showing an average of 0.8-0.9g as an average. If we look at 1 gram per minute, that’s 60 grams per hour and 540 calories per hour. Which is a massive shortfall of our 750-800 calories needed per hour.

But is this research correct in their outcomes? Have they taken into account the athletes that have eaten to maximize their use of fat as energy? Let’s find out next week when I look into eating to increase our fat oxidation rates.

Run Well

Chris O’Driscoll

Leave A Reply (5 comments so far)

  1. Gareth Parker
    4 years ago

    Fascinating stuff Chris. Looking forward to your update next week.

  2. Chris
    4 years ago

    Thanks Gareth. Would have loved to have done it in one post, but it really is a huge topic…

  3. Stephen
    4 years ago

    Great blog Chris

  4. Delphine
    3 years ago

    Sounds fascinating…and very usefull – as I would love to put my fat stores to good use…despite running 5 half marathons and training hard for them…still have lots of fat to burn…

  5. Chris
    3 years ago

    Thanks for your comments Stephen and Delphine. Keep an eye out for the next one which should help you apply it a bit more…

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