NOTE: I started this post a long time ago, and I have lost a bit the train of thought. I am not sure I’ll ever finish it, but I do not want to delete it either as it contains quite a lot of interesting info (and it is related to one of my recent posts re fat loss and HIIT). So I’ll just put it out there the way it is…
Today a post that was in the making for a while, on a topic that I find fascinating: metabolism & energy systems, and how they relate to certain types of exercises (especially of the anaerobic kind). The ultimate prize obviously being the answer to the question what the most effective exercise protocol is for burning fat and building muscle.
[spoiler: I dont know, and IMHO noone really does – but I will make an educated guess]. To give you the skinny first, in this post I am in particular looking at lactate, ie the main pathway that is involved when engaging in any kind of anaerobic activity (eg sprinting, lifting). What I am particularly interested in is the metabolic impact of the various recovery/load ratios when engaging in any kind of HIIT (which in my definition me includes weight lifting, at least for the purpose of this post).
This post is a bit longer, so I’ll give you an outline of its organisation. The first section is an introduction to the various metabolic pathways. I would ask you not to skip it: if you are not an expert the reasons for this are obvious; and if you are an expert I would really appreciate feedback whether I got everything correct. I think I did (obviously), but having trained as a physicist, not a physician, my pool of relevant background knowledge is obviously rather on the thin side. In the second section I will zoom into the anaerobic glycolytic system, ie the one going via lactates, and especially how those are recycled in the liver. In the third section finally I will discuss the implications of this in particular for programming and training, and relate this to my individual n=1 experience. I will also discuss a number of health implications, especially with respect to Type 2 diabetes.
Section 1. The energy systems of the body
Not being a scientist in this field, I am horrible with my sources. I read a lot of stuff (mainly on the web), but unless it is on Wikipedia I never seem to be able to find it again. So when all the information is digested in my brain, and ultimately the pieces of the puzzle fall together, I find it very hard to source the underlying information. Not ideal, I know, but unfortunately a necessary compromise given that this is a blog and not a scientific publication. Having said this – if I do remember the sources I do add them, and I would ask my kind readers who might have sources at hand to just mention them in the comments so that I can incorporate them.
Energy sources and energy stores
Our bodies have four main sources of energy
Not all those sources are created equal though: Firstly ATP is the ultimate source of energy, in the sense that all processes in the body that require significant amounts energy to use ATP. All the other sources of energy (CP, glucose, fat) need to be first “converted” into ATP as they can not be used directly.
Also ATP and CP are not really energy sources, they are rather energy stores. What I mean with this is that whilst carboydrates and fat are “burned” (ie converted into CO2 and water) to provide energy once, ATP and CP are recycled: rather than being burned and excreted they are transformed into something else, and this something else is then re-transformed into ATP and CP respectively by burning carbs and/or fat. You can compare those systems to springs: when loaded they do provide energy to the body, but what ultimately matters is by which mechanism they are loaded.
So the two systems that provide energy to the body are the systems based on carbohydrates and fats respectively. As for the carbohydrate system it can operate in two different modes, the aerobic mode, and the anaerobic mode. In fact – and because this is central to this post and I will describe this in more detail in the next section – it is only the aerobic systems that are always energy sources in the aforementioned sense: whilst the anaerobic carbohydrate system does use glucose to operate, this glucose is not always “burned” but often it is recycled back into glucose, either by burning fat, or by burning other glucose molecules.
A final important piece of information is the oxygen use. Now as alluded to earlier, ultimately all energy needs of the body are met by ATP, and all other systems feed into the ATP system. So when it comes to resource efficiency, the ATP yield is the key unit in the denominator. When exercising intensely, oxygen supply usually becomes a limiting factor (think VO2max), and it is important to understand how many units of oxygen are needed to produce one unit of ATP. It turns out (unsurprisingly) that those numbers are different for fat and carbohydrates [and I would love to have the sources / actual numbers for this – I will make an effort to find those]: carbohydrates use significantly less oxygen per unit of ATP then do fats.
This is of course the reason why the fat metabolism tends to be more pronounced at the low activity levels, whilst when turning it up a notch the body needs to burn more carbs in order to efficiently use the limited oxygen that is available. In theory, at the anaerobic threshold the body would only burn carbs, because every unit of fat that is burned is wasting valuable oxygen. In practice this is probably not true because the body is not that efficient, but my guess is the better trained an athlete is (in this particular mode of operation), the closer he or she comes to only burning carbs at this level – just think of tri-athletes with their carb gels.
Section 2. The anaerobic energy system, lactates, and recycling in the liver
Alright, anaerobic glycolysis and the lactic acid system. An initial reference here is Wikipedia:
The increased lactate produced can be removed in a number of ways, including:
So depending on the specific environment, the anaerobic system can provide an energy source for the body in the above sense (ie the lactate is eventually “burned” into CO2 and water), or it acts like one of those “loaded spring” energy storage systems (ATP/CP) that are re-loaded into their original state by burning other fuels (ie fats, or other carbohydrates, or both).
Whilst the ATP and the CP systems all work inside the (muscle) cells however, the mechanism here is more involved. The steps here are as follows
- Glucose (most likely from the muscle’s glycogen stores) is converted into lactate, thereby producing a number of ATP’s that are providing the energy for the movement
- The lactate is transported through the bloodstream into the liver
- Inside the liver, the lactate is converted back into glucose following the Cori cycle. The energy for that is – as usual – provided by ATP’s, and those can eventually come – as usual – either from the aerobic oxidation of carbohydrates, or of fats.
- The glucose is released from the liver back into the bloodstream (it could in principle also be stored inside the liver as glycogen, or converted into fat, but given the scenario considered those pathways should be subordinated)
- The glucose is absorbed by the muscle cells, where it is mainly stored in the form of glycogen (unless it is needed immediately)
One warning is probably appropriate at this point: if relying heavily on the lactate system then it is worth keeping in mind that all the energy used will be channelled through the liver. For example if we assume that a particular session needed 500cals of “anaerobic” energy – this should be the right ballpark for one hour of heavy exercise. Then those 500cals need to be recycled in the liver, which means the liver itself needs to burn 125g of carbs (4cals/g) or 55g of fat (9cals/g) plus whatever is lost in the conversion.
I dont know what the capacity of the liver is in this respect. My assumption is that it is rather high, but nevertheless the amounts mentioned would start to put some stress on it, especially in the untrained, or if the liver function is impaired for any reason. So care should be taken not to overdo it.
Section 3. Implications for training and programming
General training and programming
To come later
Insulin resistance training
Without wanting to go into the topics of Type 2 Diabetes and Insulin Resistance, it is hypothesized that exercise can help addressing the latter, and therefore also the former. There are a number of mechanisms proposed. Suffice to say that emptying the glycogen stores and refilling them with blood glucose is believed to be a key part of this.
It is clear that all anaerobic protocols are very well suited to this, simply because they by definition rely 100% on glucose within the cell, regardless of whether the energy is ultimately provided by fat or by carbohydrates, and all this glucose has to cross the cell border which is arguably the key mechanism to address insulin resistance. There is a slight caveat in that activities that can be fuelled by the ATP/CP stores alone (ie those lasting less than about 10 seconds) and that happen in an environment of plentiful oxygen and little average effort might be (partially) refueled by burning fat within the muscle cells, thereby not contributing to the glucose transport.
So to put some numbers to it, any HIIT protocol with load-sessions of between 30secs and maybe 2-3min (to avoid too strong an aerobic component) should do the trick for improving insulin resistance. Rest should be adequate so that the exercise can be maintained for quite some time, maybe 15-30min time under load in total.