The Dual-fuel system revisited…
In one of the earlier parts, I talked about how muscle contraction is fueled either with glucose (glycolysis) or fatty acids (beta oxidation). I have since realized that there is more detail that I’d like include, so this is a part II of that post.
The phoshagen system uses stored creatine phosphate in the cell to create the ATP that ultimately drives the muscles. This can provide a *lot* of energy, but is limited to durations of 10-15 seconds because of limited supply of creatine phosphate in the cell. This is the system that is used for explosive power – weight lifting, sprinting, etc.
The glycolytic system uses glucose to create ATP. Strictly speaking – and this is a point I glossed over earlier – glycolysis only goes from glucose (or glycogen) to pyruvate. This step can occur both in the mitochondria and in the the cytoplasm of the cell, which means it can produce quite a bit of ATP quickly, but it’s only the first step in the full process of burning glucose. To keep the energy production high, something has to be done with the pyruvate, and it is converted to lactate. That lactate accumulates quickly, and that is what is making our muscles hurt during hard efforts.
The oxidative system – also termed “oxidative phosphorylation” or “mitochondrial respiration” – is the main energy provider for aerobic exercise. As discussed earlier, there are two feeds into the TCA/citric acid cycle:
Beta oxidation takes fatty acids and converts them to Acetyl CoA to feed into the TCA cycle. Or, we take the products from glycolysis and feed them into the same cycle. The important point is that those products can be pyruvate that was just created by glycolysis, or it can be lactate that was created during glycolysis elsewhere in the cell.
That’s enough context to now discuss different intensities:
At a moderate aerobic pace, we are getting all of our energy from the oxidative system; we are either converting fatty acids to Acetyl CoA or performing glycolysis to get pyruvate which is immediately fed into the TCA cycle.
As we increase the intensity, we need more glycolysis to produce more energy, and for that to keep working, we need to convert the pyruvate to lactate. That means we have more lactate around that we will need to feed into the TCA cycle to get rid of. That can happen both in the muscle that is exercising, but also in other muscle that is resting and in the liver and heart. At some point, the amount of lactate produced exceeds the body’s ability to get rid of – or “clear” – it, and tissue and blood levels of lactate increase much faster. That is what the term “lactate threshold” means, and a lot of high intensity training is looking to improve lactate clearance.
If we increase the intensity above the lactic threshold level, we accumulate lactate very quickly, and that means we cannot continue for very long.
Here’s a cool image that shows the relative contributions of the different energy systems:
Training intensity and fat loss
Knowing that is only burned by the oxidative system tells us something important about intensity; we can only burn fat at moderate intensity. The definition of “moderate” is going to vary quite a bit based on the availability of glucose during our endurance training, but one we have hit an intensity that maxes out beta oxidation, we will not be burning any additional fat. Extra intensity above that level will just lead to more glucose being burned.
For the longer term, we could improve our ability to burn fat by improving our ability to perform beta oxidation.
Muscle Fiber Type and energy sources
Muscle fibers can be categorized in a number of different ways; you may have heard them categorized into “slow twitch” and “fast twitch”. Slow twitch (type 1) fibers are utilized in longer-term exercise, and – since they have a lot of mitochondria – they can do a lot of aerobic metabolism, and therefore can be powered on fat. Fast twitch (type 2b) fibers have few mitochondria, and run mostly on glucose. And then the type 2a fibers are hybrids, though it probably makes more sense to think of muscle fiber type as a continuum rather that a set of discrete types.
What this means for our topic is that if you are doing the sorts of activities that recruit a lot of fast-twitch fibers, that’s going to require a lot of glucose. And therefore, you won’t burn a much fat from those sorts of activities, nor will you be able to do those activities well if you are very limited in glucose.