Imagine you are preparing for a big road race or group ride or maybe a Gran Fondo. For this 100 mile event, you’re going to be riding for some 4-5 hours at least and the total amount of energy you use is going to be 3,000 or 4,000 kilojoules. Since the human body has an efficiency of about 1/4 and 4 kilojoules are equivalent to one kilocalorie, you can expect to burn some 3,000 to 4,000 calories of energy substrate, mostly fat and carbohydrates. One pound of fat is equivalent to 3,500 calories, so this event is literally removing 454g of fat from your body, assuming we don’t burn any sugar as fuel.
However, it’s impossible to complete an endurance ride without using carbohydrates. As exercise intensity increases, carbohydrates make up a larger portion of our total fuel profile. This is mostly due to two factors. One: our Anaerobic system cannot use fat for fuel and requires carbohydrates and two: our Aerobic system uses a combination of fat and carbs to fuel the majority of our energy needs. As total energy demands increase, carbohydrates are the easiest source to provide that energy. Not only that, our brains run on carbohydrates; simple functions like balancing the bike and having complex thought require glucose.
Here’s the catch: our bodies can only store 500g of carbs in our liver and skeletal muscle and we can only absorb some 90g of carbs per hour. For a five hour event, the most amount of carbs we could possibly use is 950g (500g stored + 90g x 5hrs). At 233w, each hour we burn 800 calories, or about 200g of carbs. So for a 5 hour event we’ need 1,000g of carbs. Anything above 233w, you'd require even more carbs!
Fortunately, we have another fuel source, fat. Fat is how professional cyclists achieve average power values of 300w or more for one day classic races like Paris-Roubaix and the World Championships. Since the maximum amount of carbohydrates our body can consume is capped by the following equation:
Total Carbs = Stored Carbs + Consumed Carbs per Hour X Length of Event
Total Carbs = ~500 g + 90 g/hr X Length of Event
It's crucial you have the ability to use fat if you want to achieve total energy usage above this total carb value. Not only that, if you are optimising for maximising the total available energy throughout the event, it is in your best interest to save the carbohydrates and try to use as much fat as possible. This process of emphasising fat utilisation and decreasing carbohydrate use as much as possible is called carbohydrate sparing.
Maximising Carbohydrate Sparing
There are two key ways to improve the capacity to spare carbs in long events. The first is psychological; abstain from efforts that rely heavily on carbohydrates. You may notice professional cyclists do a great job of not doing work. They are always hiding (drafting) behind other riders, waiting for others to do the work to close a gap, and even going back to the team car for a chance to hold the car door for a bit of a pull. These are all strategies to help lower the total work, but also the intensity of the work done.
The X of carb and fat utilisation. Experiment values are not so linear, but overall, as intensity increases, fat use decreases and carb use increases.
As can be seen in the diagram above, as exercise intensity increases, the contribution of carbohydrates as an energy source increases. The exact values are not important, the big takeaway is that at VO2max there is no fat utilisation! Even at threshold, some 85-90% of VO2max, there is still almost no fat utilisation. At even lower intensities, such as the bottom of the tempo zone, fat stores are the source of about half of the energy for the given effort.
If you want your carbohydrate stores to last longer, figure out how to get the exercise intensity of the effort down. Maybe the overall energy use during an event is dictated by external forces such as other racers pushing the pace or friends in your group ride dictating the speed of the event, but there are more subtle ways to save energy.
Don’t sprint/speed up quickly following stop signs or red lights. This 10-15 seconds of high intensity uses our inefficient Anaerobic system to produce energy.
Tackle climbs more slowly. Climbing is a time when it’s easy to go hard, but intentionally holding back can save those precious carb reserves
Complete high intensity efforts as quickly as possible. If you have to bridge up to a group up the road, do it as quickly as possible. Getting stuck in between groups on your own in the wind is a great way to blow through carbohydrates.
Race conservatively. Do you think the race is going to come to a sprint? Why waste energy attacking if it’s going to get pulled back in. Conserve energy and go all out when it matters.
Getting into the mindset of preserving energy and racing conservatively is a key aspect to carbohydrate sparing.
The other key area is training. We can actually train to improve our use of fat for fuel and professional cyclists spend a lot of time in this area.
Base training is a period of training normally completed in the early season where long, slow rides are used to dramatically increases fat utilisation over a 6-8 week period.
The long, low intensity rides of the base season utilise fat stores as the main source of energy and help reinforce that metabolic pathway. Over time, as more low intensity rides are completed, fat utilisation increases across the board, including as intensity increases. This increase in fat metabolism at higher intensities is the reason professional cyclists can use primarily fat at mid to high tempo and ride all day at pretty high intensities. One thing to note is that the high VLamax explosive sprinter type riders will bake any of their top end work into the start of an endurance rides why glycogen stores are topped up. Typically the GC riders will do intense work towards the end of a endurance session to facilitate fat combustion after a long day. Simply put you train the energy system most relevant to your chosen discipline and avoid the type of training that reduces it's effectiveness.
Another way to improve fat utilisation is through carb depleted riding. Similar to fasted riding, it involves going out for a 1-2 hour ride following a depletion of your carbohydrate stores. This is normally done directly after waking up, but rather than skipping breakfast completely, I suggest a protein rich small meal. As fat metabolism is maximised following a significant carbohydrate fast, such as sleeping all night. One fasted ride a week can help increase fat utilisation. Make sure to bring some carbohydrate rich food with you and don’t stray to far from home if you are using this technique. You don’t want to find yourself in the middle of nowhere without food or energy. Also make sure to refuel properly after the ride.
Can you see the connection between these two types of rides? Base training drains your carbohydrate stores in the first few hours of riding, leaving the last hour or two with efforts done in a low carbohydrate state. Fasted riding achieves the same low carb state by taking advantage of an overnight fast. The key here is fat metabolism is trained and improved through riding when carbohydrates stores are low. Remember LOW, not EMPTY. For base training, remember to continue to eat at the end of long rides.
Conclusion
Sparing carbohydrates is important to success in long events. If you are a criterium rider and your longest event is 45 minutes, or a track cyclist, there’s no reason to have higher fat metabolism. The goal of sparing carbs is to maximize the use of a limited supply of sugar. If we spend a whole race saving our carbs, it’s also important to remember to use them at the end! Once the finish line starts to get closer, be willing to use up those carbs to get a race winning tactical advantage or to unload an all out sprint for the line.