The endurance athlete’s guide to fueling and weight loss part 2: Energy Systems

Please read the introduction and earlier posts before reading this one.

In our last episode, we learned about how fat and carbohydrate get into our systems and the fundamental asymmetry between those two systems.

In this episode, we will look at how fat and glucose are used for energy. My focus is going to be on muscles since these posts are for athletes; there are similar concepts that apply to other tissues but they are beyond the scope of this discussion (see note 1)

I’m going to have to dive into some biochemistry to create a framework for further discussion; I have tried to simplify it as much as possible, and the post is relatively short.

This is a summary of the overall processes at work for aerobic energy production; there are others that are at work for non-aerobic production.

Each of the boxes is a complex series of chemical steps (note 2)


We’ll start at the bottom and work our way up.

Citric acid cycle

Our goal is to take fat and carbohydrates and create ATP, which is what powers our muscles. At the center of the diagram is a compound named acetyl coenzyme A, abbreviated as “Acetyl CoA”. You can think of Acetyl CoA as the common energy compound produced from either glucose or fatty acids. That’s not absolutely correct, but it’s correct enough.

The Acetyl CoA feeds into the citric acid cycle (sometimes known as “Kreb’s Cycle” after one of the discoverers). If you want to see the details, here’s a link:

Image result for don't press this button image

You pressed it, didn’t you?

Biochemistry is just ridiculously complex.

All of this takes place inside of each of most of our cells. You probably studied cells when you were in biology class, and the diagrams looked something like this:

Image result for simple cell nucleus mitochondria

Specifically, the citric acid cycle happens within the mitochondria, which you can see are those oval-shaped structures within the cell. I generally think of cells as being these little spheres or flat discs, and some are, but muscle cells are a bit different:

Image result for muscle cell nucleus mitochondria

Not at all like a globule or a flat disc; I especially like the nucleus stuck on the side as an afterthought.

Muscle cells need a lot of energy so they have tons of Mitochondria. Thousands of them.

And how are we going to get all the Acetyl CoA that we need to drive those muscles?

Glycolysis and Beta Oxidation

Glycolysis is the the process that converts Glucose to Acetyl CoA. Beta oxidation is the process that convert fatty acids to Acetyl CoA. Both of these processes happen in the mitochondria.

There is a very important point to be made about glycolysis and beta oxidation. Even though they both occur in the mitochondria and even though they both feed into the citric acid cycle, they use different chemical pathways and therefore use different machinery.

The citric acid cycle, glycolysis, and beta oxidation can all be trained. Not only can the body build more mitochondria, it can improve the rate at which each bit of machinery in the mitochondria works. That is one of the adaptations that makes us better at aerobic exercise.

But the body only makes improvement to the parts of the machinery that are being stressed, so if it’s glycolysis that is being stressed, improvements will be made in the glucose metabolism, and similarly if beta oxidation is stressed, improvements will show up in fat metabolism.

This point is going to be fundamental to our next discussion, so I’ll state it again in a different way – you can be an athlete with a very powerful glycolysis pathway and a crappy beta oxidation pathway. And vice versa.

Another interesting outcome is that to achieve higher performance, you need to improve either glycolysis or beta oxidation *and* the citric acid cycle. You can do a bunch of work to improve your beta oxidation and you’ll burn more fat and fewer carbs, but you won’t see higher performance if the citric acid cycle is unchanged (note 3).


This was a short post and I’m not sure I really need a summary, but the short one is that both fat burning and glucose burning have separate chemical reactions (glycolysis and beta oxidation) that feed into a shared set of chemical reactions (the citric acid cycle) that ultimately give the energy to drive the muscles.

That’s all for this post. The next post will take this post and apply it in real-world situations.

Part 2.5: More on muscles and energy systems


  • That would take us into ketone bodies and their usage in different tissues.
  • Like, ridiculously complex. In glycolysis, to get from glucose to pyruvate – which is fed into the citric acid cycle – takes a series of 10 different chemical transformations. Beta oxidation goes through 5 steps but repeats them for every two carbon atoms on the fatty acid, and it’s more complex for unsaturated fatty acids. The citric acid cycle has 9 steps.
  • This depends a bit on what you mean by “performance” and it ties into fueling, which will come up very soon in a future post.

  • One Comment

    • Reply Shaji |

      I think what you described is very important for endurance cycling. I theorize with limited evidence that if you deplete your glycogen stores (about 2 hrs of moderately fast cycling for the majority of folks with no carbohydrate intake) you crank up your beta oxidation, Initially it might be very inefficient and feel that you have no energy left, but as you keep repeating the process, I presume the body should get better at beta oxidation, I read some articles that in a serious endurance ride (200 km or more) it is difficult to consume sufficient food to compensate for what is burned, so the body turns to burning fat to compensate for the missing fuel. So if you have trained yourself to have better beta oxidation (particularly in older people, whose mitochondria in the muscle is naturally far less, but can be improved as you said), the better you will be at endurance events, Performance is a whole other thing and more difficult to modulate unless you have better muscle mass more circulation and tons of mitochondria,

    So, what do you think ?