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 athletes have trained with restricted carbohydrate availability (RCA) for decades, often unintentionally. This training might manifest in the form of early-morning sessions before breakfast, or twice-daily sessions, with minimal refueling in-between. This causes the availability of carbohydrates (in the form of blood glucose or glycogen) to be below baseline.
For the last decade or so, there has been mounting evidence to support RCA training as a specific tool to elicit certain training adaptations, mainly being how well our body oxidizes fat for fuel.
A quick science refresher
There are two main fuel sources for creating energy. Fats and carbohydrates.
Exercise intensity is the greatest determining factor for what fuel we use. The higher the intensity, the more carbs we need to make energy (ATP). Most of the day we are utilizing fat for fuel. Even during low intensity exercise, fat is still the main fuel source.
However, once intensity becomes too high, a sufficient amount of ATP (energy) is need, and carbohydrates are prefered to make energy. Carbs are used in forms of glycogen or blood glucose. Rarely is the body only using one form of fuel, most of the time it’s a blend between fat and carbs.
As a benchmark, maximal fat oxidation is around 40-45% of Vo2 in Men and 45-50% in women. As intensity increases, less fat is used and more carbs are used to make ATP.
So why does it matter if we use more carbs vs fats? Why are using carbs bad?
Carbs ARE NOT bad, however:
There is a limited supply we can use.
Carbs have a byproduct of lactate.
Like the exhaust that comes out of your car from gasoline, lactate is the biochemical exhaust of using carbohydrates as fuel.
At submaximal intensities, lactate can be recycled to make more ATP but when exercise intensity increases to high, the cells get overwhelmed and lactate builds up.
This build up of lactate starts inhibiting lipolysis and fat oxidation. It can also interfere with muscle contractions.
Therefore we want to delay/minimize the use of carbohydrates during exercise for as long as possible. This will preserve more glycogen for intense hill climbs we want to achieve later in the ride, while keeping our blood lactate levels low.
Impact of RCA training on fat oxidation
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 ability to oxidize fats for fuel, you may derive different amounts of energy from fat vs carbohydrates compared to another individual at the same workload and FTP. This is because individuals are better at oxidizing fat for fuel relative to others.
Mobilization vs oxidization. When exercising, fat is first mobilized from its place of storage to the bloodstream where it hopes to find a cell and be oxidized for fuel. The average athlete will mobilize fat at similar rate compared to an elite athlete. What separates elite athletes are their ability to oxidize those fats for fuel. Maximum fat oxidation or “MFO” will determine how well your body oxidizes fat for fuel. We can find out your MFO with our metabolic assessments.
Key Point: Restricted carb training can affect this balance between fat and carbohydrate oxidation. To put simply, the body will use what’s available. If you consume 100g of sugar then hop on the bike, your body will burn through as many of those carbohydrates as possible before turning to fat. You still get all the great benefits of cardio except the first 20-40 min you will be burning through those sugars and not practicing fat oxidation to your full potential.
Carbohydrate intake prior to exercise can potentially inhibit fat oxidation. Since the best way to improve fat oxidation is to have your system practice burning fat for fuel you should not consume carbohydrates prior to training if the goal of the session is to improve fat oxidation. This is assuming your training session is less than 2 hours and in zone 2 the entire time. For more intense or longer rides, carb up!!!!
Glycogen Sparing
Improving your ability to oxidize fats is important because it conserves 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 (60-108g/hr).
Compare 1.5 hours of glycogen storage to 30+ days of fat storage (lean individual) it would make sense to want to save those glycogen storages for intense bouts and only when needed. This is another advanged of improving your MFO and getting better at using fats for fuel. Come race day you can spare more of your glycogen for when you need it the most.
Better Fat Oxidation = Better Lactate Threshold
The more you can oxidize fat for fuel the less carbs you will need. If you burn less carbs you produce less lactate, If you produce less lactate you can exercise for longer periods of time at higher intensities.
This is what separates professional endurance athletes from amature endurance atheltes. The ability to sustain a workload for an extended period of time. One can have the same Vo2 max and 20 min FTP but have significantly different outcomes on a 5 hour mountain stage. This is because its not your power output that matters, its what fuels are you using to get that power! If you have a higher % of fat utilization at the same intensity then that athlete will perform better the longer the event.
Don’t get me wrong, professional athletes also consume significant amounts of carbohydrates (100g an hour) and produce significant amounts of lactate. The difference is their ability to A.) shuttle the lactate/re use for fuel and B) produce less lactate at a given workload thanks to their higher MFO. Since we currently don’t have a metric for measuring lactate shuttle ability we correlate with MFO. The higher your MFO the better you shuttle lactate and can last longer at the given intensity.
So how do I train for a better MFO? according to research 80% of your time training should be in Zone 2 and with new RCA research, possibly do that zone 2 training Carb restricted.
Conclusion
In short, if you want to improve MFO, train without carbs in zone 2. But if you want to perform at your best, EAT CARBS!!! You need carbohydrates to perform at your best, only utilized RCA when training lower-moderate intensity days. If you have less time to train this method can improve sustainable MFO changes in the long run.