Why Train With Restricted Carbohydrate Availability (RCA)
In this post, I’ll explain why training with restricted carbohydrate availability (RCA) might be useful even if you compete over durations where the body’s carbohydrate stores are not a limiting factor.
What is training with RCA?
Both elite and amateur cyclists have trained with restricted carbohydrate availability (RCA) for decades, albeit often unintentionally. This training might manifest in the form of early-morning sessions before breakfast, or twice-daily sessions, with minimal refuelling in-between.
In both cases, the availability of carbohydrates (in the form of blood glucose and glycogen in the liver or muscles) is below baseline. This impacts the energy systems used to produce power, and in turn influences the adaptations arising from the training.
Often these RCA sessions are borne out of convenience/necessity e.g. enabling workouts to fit around work or family commitments. However, for the last decade or so, there has been mounting evidence to support RCA training as a specific tool to elicit certain training adaptations.
A quick physiology refresher
To understand why RCA training might be useful, we need a quick physiology recap. There are two main systems used to produce energy: aerobic metabolism and anaerobic glycolysis*.
Aerobic metabolism involves breaking down carbohydrates and fats** using oxygen to produce energy. The aerobic system is the most efficient system (i.e. more energy is produced per gram of fuel). But it’s also slower, and is therefore dominant only at lower intensities.
Anaerobic glycolysis involves breaking down carbohydrate without oxygen. This process is much faster than aerobic metabolism, but is less efficient and also produces lactate and other metabolic by-products. As these byproducts accumulate, it’s thought that a signal is sent to the central nervous system that exercise is becoming unsustainable, and this induces a feeling of fatigue. Exercise demanding a high energy contribution from the anaerobic system (e.g. hard efforts) can thus only be sustained for short periods.
While lactate itself is not bad, it correlates strongly with the accumulation of these fatiguing metabolites, and this is why the concept of a ‘lactate threshold’ - a maximum power at which lactate levels (and importantly, fatiguing metabolites) remain constant - has received such attention, and is widely considered a key performance determinant in a wide range of cycling disciplines.
Impact of RCA training on fat oxidation
Now, it’s a little simplistic to think of the aerobic energy system as just one system. That’s because there are two main potential fuel sources: fats and carbohydrates.
Let’s say you were doing a long steady ride, where the majority of your energy is derived from the aerobic energy system. Depending on your propensity (or in other words ability) to oxidise fats versus carbohydrates, you may derive different amounts of energy from those two fuel sources. That’s the first key point: different people will use different amounts of fat and carbohydrates at a given exercise intensity.
A second key point is that RCA training can affect this balance between fat and carbohydrate oxidation. There’s now good evidence that at least some forms of RCA training promote adaptations related to improved fat oxidation (1-4).
So, when riding at a given intensity, you will derive more energy from fat, and less from carbohydrates. (Note, I’m skirting around the issue of what type of training session should be performed in an RCA state, and what exactly the best way to achieve this RCA state is – this is an important factor, which I will cover in a subsequent post.)
Ok great, but why does that matter?
The ‘Glycogen Sparing’ argument
The argument you will see time and again, is that improving your ability to oxidise fats is important because it conserves precious glycogen stores (i.e. the carbohydrate stored in muscles and the liver). The average person typically has sufficient glycogen stores to fuel roughly 1.5H of all-out racing. There’s also a limit to how quickly you can consume and absorb carbohydrates (~60-90g/hr depending on the type of carbohydrates and how well trained you are consuming carbohydrates while riding). That’s compared with fat stores, which could fuel exercise for days, even for a very lean person.
Together, these factors do mean that if you are racing for a sustained period longer than 1-2 hours, then it could be beneficial to improve our ability to oxidise fats, so that you do not run out of glycogen stores, which would force you to have to slow down significantly (this is what happens when you ‘bonk’ or ‘hit the wall’).
Improving Lactate Threshold
Literature advocating RCA training often stops with the above ‘glycogen sparing’ argument. However, there is a lesser-known benefit to this type of training, which arguably has much wider applicability across a much broader spectrum of disciplines and racing durations.
To understand this benefit, we need to return briefly to some more physiology. Specifically, the use of carbohydrates as a fuel. The aerobic and anaerobic break-down of carbohydrates are actually two stages of a single process, where carbohydrates are first broken down anaerobically to produce a substance called ‘pyruvate’, which is then either converted into lactate and hydrogen ions, or oxidised to produce more energy (the aerobic metabolism stage).
This leads us to my third key point: anaerobic and aerobic carbohydrate metabolism go hand-in-hand. You can’t have one without the other. And the greater your tendency to use carbohydrates for fuel, the greater the rate of anaerobic glycolysis.
Put another way: the greater your tendency to use carbohydrates for fuel, the greater your tendency to produce lactate, and thus the LOWER your lactate threshold (also known as ‘anaerobic threshold’, or ‘maximal lactate steady state’ or ‘FTP’***) (5,6).
This leads to my final and most crucial key point: if we can increase utilisation of fat and reduce utilisation of carbohydrates at a given power, this will lead to an improved lactate threshold. This, I believe, is arguably the most important benefit of RCA training – to improve utilisation of fats at a given power output, and thus increase the lactate threshold, meaning you can ride at a higher power for an extended period of time.
Conclusion
In summary, there’s evidence that some forms of RCA training can improve your ability to use fats as a fuel. For some, this is useful as it enables glycogen stores to be conserved, delaying the point at which you will run out of fuel or allowing you to ride at a higher power before running out of fuel. However, RCA training has an additional benefit, which is not as widely known.
By reducing reliance on carbohydrates, RCA training also reduces the rate of glycolysis at sub-maximal power outputs and the associated lactate production, thus INCREASING the lactate threshold.
Therefore, irrespective of whether fuel stores are a limiting factor (typically not the case for races less than ~2-3 hours, provided you refuel), RCA training might be beneficial by allowing you to ride at a higher power for a given lactate level.
* There is a third (the phosphocreatine system), but this makes only a very minimal contribution in cycling disciplines, so for the purpose of this article we can ignore it.
** Protein can also be broken down for energy, but across the vast majority of situations, the contribution of protein to energy generation is negligible.
***These terms can actually mean slightly different things, but broadly attempt to capture the same concept, which is the point at which lactate levels are no longer in steady state, and exercise cannot be sustained for an extended period.
References:
1) Hansen, A. K., Fischer, C. P., Plomgaard, P., Andersen, J. L., Saltin, B., & Pedersen, B. K. (2005). Skeletal muscle adaptation: training twice every second day vs. training once daily. Journal of Applied Physiology, 98(1), 93-99.
2) Lane, S. C., Camera, D. M., Lassiter, D. G., Areta, J. L., Bird, S. R., Yeo, W. K., ... & Hawley, J. A. (2015). Effects of sleeping with reduced carbohydrate availability on acute training responses. Journal of Applied Physiology, 119(6), 643-655.
3) Marquet, L. A., Hausswirth, C., Molle, O., Hawley, J. A., Burke, L. M., Tiollier, E., & Brisswalter, J. (2016). Periodization of carbohydrate intake: short-term effect on performance. Nutrients, 8(12), 755.
4) Marquet, L. A., Brisswalter, J., Louis, J., Tiollier, E., Burke, L. M., Hawley, J. A., & Hausswirth, C. (2016). Enhanced endurance performance by periodization of carbohydrate intake: “sleep low” strategy. Medicine & Science in Sports & Exercise, 48(4), 663-672.
5) Mader, A., & Heck, H. (1986). A theory of the metabolic origin of “anaerobic threshold”. International journal of sports medicine, 7(S 1), S45-S65.
6) Billat, V. L., Sirvent, P., Py, G., Koralsztein, J. P., & Mercier, J. (2003). The concept of maximal lactate steady state. Sports medicine, 33(6), 407-426.
