Metabolic Testing
In this latest article, coach Kevin looks into metabolic testing. What is it, how do you do one, and what data can you get out? Furthermore, how can you utilise the information as an endurance athlete to better your training and racing approach?
In a few of my previous articles, I’ve talked about the different fuel sources we use to provide energy for athletic performance. Fat provides energy for typically lower intensity endurance efforts, and glycogen, which comes from eating carbohydrates, typically fuels our higher intensity efforts. This is a bit of an oversimplification, but how these two energy sources are used & refuelled can affect our triathlon performance significantly. Metabolic testing identifies how much of either fuel source you are using at a given intensity; it can therefore highlight potential problems, provide insight into fuelling requirements, and point us in the right direction for effective training.
To get into the subject in more detail, I’ll start with an analogy – imagine the human body is a hybrid vehicle. For ease of understanding, this hybrid has both a diesel (fat fuelled) and a petrol (glycogen fuelled) engine.
The diesel engine is typically not so powerful but has a huge fuel tank. The petrol engine can provide more peak power but quickly runs out of fuel. So we have a couple of options here if we want to increase performance – either make the diesel engine more powerful or make the petrol fuel tank bigger/refuel more often (to last longer).
Which option you focus on will probably depend on whether you need to go faster for a shorter time (sprint, Olympic distances) or if maintaining speed over a long duration (half and full IM distance) is a higher priority for you. Or if you are unable to refuel? But either way, metabolic testing can show you each engine’s contribution and highlight potential improvements.
So hopefully, that’s explained why you might want to get a metabolic test, but what is it, and how does it work? The equipment required will be the same, and the test protocol very similar to a VO2test. You’ll go to a lab/university/testing centre, get hooked up to a mask. This measures how much oxygen you are using and carbon dioxide that you are exhaling, and be asked to either ride a bike or run on a treadmill following a progressively difficult step test until you reach a certain level or can’t continue. If you are specifically looking to do a metabolic test, the duration of each step will probably be longer than the usual 1 – 2 minutes used in VO2max tests (therefore requiring fewer but bigger steps).
Your oxygen consumption vs expired CO2 gives a ratio called RER (or RQ) from this information. Your RER (or RQ) and actual calories expenditure at any given time can be used to calculate the total amount of energy contributed by both fat and glycogen for each different step in your test. The output can be turned into a graph which might look something like this:
The bars in blue show the amount of energy contributed by fat, and in orange for carbohydrates.
So what can you take from this information? Well, there are several potential variations in results. If the vast majority of your graph is filled in orange, you tend to rely on carbohydrates – which could be a problem or disadvantage over longer distances if you cannot refuel at the required rate. And the more reliant on carbs you are then, the higher that replacement frequency or rate needs to be, which is where incidence rates of GI issues can potentially increase. The opposite result, a lot of blue but not much orange, means that perhaps you are limiting your potential overall output and could benefit from utilising more carbs. Just because you can put out a consistent speed while not needing to refuel much does not mean that this is the fastest speed you can achieve.
The way to resolve either of these problems is to focus on the weakness while maintaining your strength. If you are not burning enough fat, try more or longer endurance sessions at or near your aerobic threshold (check out this previous blog post to remind yourself what and where your aerobic threshold is). Alternatively, you could try carbohydrate periodisation, where you deliberately try to match your carb intake to suit the demands of your training session. Conversely, if you are in the ‘not utilising enough glycogen’ camp, try some more high-intensity work and perhaps be more generous with the carbohydrates before and after the higher intensity or more demanding long sessions. Lastly, suppose you have got the balance about right (as mentioned previously, this will depend on your preferred race distance). In that case, as well as patting yourself on the back, it can be a valuable validation of your current training practices.
The data can also be helpful to give supporting information to required fuelling rates during races and challenging training sessions. As we can see our carbohydrate consumption rates at race intensity, we know how many you’ll need to consume to maintain a positive balance, or more realistically, at least not reduce glycogen stores too rapidly for your race duration. Exactly when you’ll run out of energy for a given rate of decline in glycogen stores is more challenging to predict, mainly due to the variation in the potential starting point. Pre-exercise glycogen levels depend on several factors, such as gender, body weight, body composition, training status and diet, and might range from 1800 – 3000 calories. However, knowing the rate of potential decline and some experimentation/race review should help give you a good idea of what a sensible fuelling strategy could look like.
Of course, as triathletes, this assumes that you’ll need two metabolic tests – one for cycling and one for running, plus making an estimate for swimming. However, unless you are really keen, I’d suggest sticking with cycling – as it’s the longest duration discipline, you will need to get it right here. It’s also the part of the race where we have the best refuelling opportunities, so you can experiment with what you can manage to digest vs the best theoretical carbohydrate replacement rate.
So perhaps you’ve read this and are interested in getting a test done. The easiest way would be to search for VO2 testing local to you, then ask if they can do a metabolic test instead (or it might be called a substrate usage test). As the tests are typically longer due to the longer duration steps, it will likely cost more, but whereas VO2 max rates can be inferred by performance, how you are fuelling your exercise is otherwise a complete unknown. If you decide to give it a go, it would be great to hear from anyone who takes the plunge!
Kevin opened a B&B for cyclists in France in 2014, & then a year later decided to start a cycle coaching (level 3) qualification. This was mainly in order to be able to better support his guests (but also to make his own training more effective too). At the B&B he runs the odd training camp for cyclists but mainly offers coaching advice while guiding guests. An engineer by background, he happy diving into all sorts of training data, but also understanding that coaching is about much more than just the numbers!
After being introduced to triathlon by a good friend & then taking part in races for a couple of years he decided that a level 2 coaching course with British Triathlon was the way forward, completing this in 2019.
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