What are you burning?

Fat? Sugar? Protein? Nucleic Acids? (Don’t worry about the last one. I put it in there for the geeks)

What’s your fuel source?

In the past I’ve tried to explain that the benefit of exercise is not what you burn DURING the exercise. The benefit is caused by the adaptations your body undergoes as a result of the stress of exercise. The changes to your muscle and bones and blood vessels that take place in the days following exercise are where the benefits manifest. Now I want to explain how your muscles are fuelled DURING different kinds of exercise intensities. But to understand this fully I need to explain the 4 different ways you make energy.

I need to stress that we don’t use fat, sugar, or protein directly as fuel. We have chemical pathways which turn the food we eat into ATP, the energy which our bodies use for pretty much everything, not just muscle contraction.

The 4 dimmer switches

I want you to imagine 4 dimmer switches, all side by side, on the wall of a newly build house. I’m from North America but I’m pretty sure other countries have these: just think of little toggles you can slide up or down about 6 inches. Under each switch is a label.

Beta Oxidation       Aerobic Gylcolysis       Anaerobic Glycolysis       Creatine Phosphate

The first two are aerobic pathways. The second two are anaerobic pathways (follow the links to read my articles on these). First, let me explain what a ‘pathway’ is:

Just in case you weren’t sure what one looked like. (Source: Flickr)
Think of a pathway as a series of chemical reactions happening inside your muscle cells that lead to the production of ATP (energy). ATP is the molecule our body uses to fuel any type of muscle contraction. The reason I want you to associate each of these words with a sliding dimmer switch is to illustrate that the pathways don’t simply switch on or off. In reality they can ramp up or down to meet the requirements of the working muscle. After all, we don’t really store ATP. There is always a few seconds worth of ATP ready for muscle contraction (remember resting tonus?) because in reality we make ATP on demand. These are probably very new concepts for most of you, and this is pretty complicated biology. That said, I feel that if you have read all my other posts you are ready to start thinking about muscle contraction at a cellular level and then you’ll be able to relate it to real- world exercise. Let’s continue…

Now, a brief explanation of each dimmer switch:

Beta Oxidation – This is the breakdown of fatty acids in the presence of oxygen. It happens inside the Mitochondria. It is a slow chemical reaction but it produces lots of ATP. (When you see beta oxidation, think of fat as supplying the raw material for ATP production)

Aerobic Gylcolysis – This is the breakdown of Sugar in the presence of oxygen. It occurs partly in the cellular space around the Mitochondria (Google ‘cytosol’) and then moves inside the mitochondria. It is much faster than Beta Oxidation at producing ATP. However, it produces less ATP per cycle when compared to Beta Oxidation. (When you see aerobic glycolysis, think of sugar as supplying the raw material for the ATP production)

Anaerobic Glycolysis – This is the breakdown of sugar in the absence of oxygen. It happens in the cellular space outside of the Mitochondria (I said Google ‘cytosol’!!), but it does not carry on inside the mitochondria like the aerobic version. It produces ATP super-fast but overall less ATP per cycle than aerobic gylcolysis. (When you see anaerobic glycolysis, think of sugar burnt halfway to supply the raw material for ATP production; and because it is only burnt halfway there is a by-product – lactic acid)

Creatine Phosphate – This takes place right inside the cell as well. It is a one-step chemical reaction that results in very little ATP but it generates it SUPER INCREDIBLY FAST!!!! (When you see creatine phosphate, think of a molecule in the muscle in very limited supply so it won’t last. You’re only going to get maybe 10 seconds of strong/fast muscle contractions with this pathway).

What happens during exercise: Jacob gets some bad news

happy guySo let us look at a regular, pretty fit person. Let’s call this person Jacob. Let’s imagine what Jacob is doing and how his dimmer switches change in response to his activity. Picture Jacob in a chair reading a letter. When Jacob is siting still doing nothing his beta oxidation pathway is supplying much of the ATP his body requires. If he slowly gets up to walk around then the beta oxidation dimmer switch ramps up a bit in response because his muscle requires ATP to move him around. Now what is Jacob doing? He is opening the door and going for a walk outside. He saunters down the street and his beta oxidation dimmer switch is sliding up denoting a further increase in ATP production via this pathway. Jacob still has the letter in his hands and he reads it as he walks. It is a letter from his friend Bella. She informs him that she is getting married. Jacob feels the urge to jog a bit as a way to process this information. He starts slowly, barely faster than the walking pace he was already traveling at. Now his beta oxidation dimmer switch is rising even more and as it does his aerobic glycolysis switch is also rising. In fact the faster he goes the more aerobic glycolysis starts to take over. He is at a medium speed jog now and about 50% of the ATP is being provided by beta oxidation and 50% by aerobic glycolysis.

As his speed increases, that letter from Bella turning over and over in his mind, and the amount of ATP required rises also. Now beta oxidation is ramping down and aerobic glycolysis is taking over providing almost 100% of his ATP production. Beta oxidation is too slow to provide ATP at the rate required to maintain Jacob’s running pace. Aerobic gylcolysis is the only solution. But Jacob wants to go faster because there is an anger is growing in his gut. There must have been something unsettling about the idea of Bella’s impending marriage. Jacob starts to run at a speed he cannot maintain indefinitely. Now he requires even more ATP than aerobic gylcolysis can supply and anaerobic glycolysis has to kick in. The problem here is that without oxygen to process the sugar completely, the by-products of anaerobic glycolysis start to cause a burning in Jacob’s muscles. You guessed it, lactic acid is being produced and Jacob’s muscular thighs are on fire!

He shrugs off the pain and pushes on and the anaerobic glycolysis pathway starts to collapse. He has two to four minutes of ATP in this pathway. If it collapses ATP production will drop down to a level that can be maintained by one of the other two aerobic pathways. This will mean that Jacob will be forced to slow down to a pace in sync with lower ATP production. But Jacob doesn’t want to slow down. He rips his shirt off and accelerates. All three of the first pathways were trying to keep up the ATP production to help Jacob maintain this all out sprint, but they can’t. All that is left is for creatine phosphate to dish-out as much ATP as it can for as long as it can. Creatine phosphate can do this for maybe 10 seconds of all-out effort. Jacob’s legs start to feel like quivering jelly and just as the last few second of ATP are created and spent via the Creatine Phosphate pathway, and just as that pathway collapses, Jacob springs into the air and turns into a wolf. He lands on the ground, chest heaving, legs wobbling. He staggers over to a tree and urinates against it. Balanced on three legs beta oxidation again is the major producer of ATP in Jacob’s muscles.

The End

Read this all again and in my next post I’ll go into more detail and explain more about the fuel sources and what roll protein plays in all this. You can find out why Jacob got so hot in a previous post here.


Matt is a personal trainer who looks nothing like a wolf. (Source: Flickr)

Front slider image: ‘One Plate, Many Food Groups’ via Flickr

Article by Matthew Linsdell

August 21, 2012

Matt is a certified personal trainer and has a degree in Environmental Science. He calls himself an evidence-based trainer, because training is a field which is littered with well-disguised pseudoscience – his emphasis is always on teaching the biology behind exercise. He lives at the edge of the beautiful and expansive Gatineau Park in Quebec and works across the water in Ottawa, Ontario. If he’s not out walking his two pit bulls, you’ll find him doing press ups with insanely large weights on his back. Follow Matt on Twitter at @smartfitmatt.

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