I’m often asked by clients how to nail down a fueling strategy for racing. One of the points I often make is to dial in your calories per hour, and I suggest a trial-and-error method in finding the right amount to eat during an event. Let’s go into a little more detail on figuring out how many calories you should consume during a race.
We’ll only consider races of two hours or longer, as the intensity is lower in events of that duration and you will likely want to replace some of the calories you burn. I would like to lead off by reminding you that there is no ideal formula that works for everyone and there are a lot of variables (lean body mass, metabolic efficiency, intensity, race distance, and environmental conditions) that can affect your personal nutrition strategy. What I can do is provide a basic understanding of things to consider and a general methodology to try in order to figure out your personal fueling plan.
One very useful piece of information in determining how many calories to consume during a race is knowing how many you burn during your activity. Heart rate monitors often have a feature to estimate this, as does TrainingPeaks. However, if you really want to nail it down specifically, get yourself tested on a metabolic cart. This information will also tell you how “efficient” you are at burning fat vs. carbohydrates at different intensities.
Above is a test that I did with Bob Seebohar of Fuel4mance in January 2011. These calorie numbers were calculated from my O2/CO2 exchange rate, so they should be pretty accurate. Please note that this chart shows my unique test results – your ability to burn calories at various intensities will differ from mine due to the factors listed above.
Each bar shows the number of calories that I burn per hour at that particular pace. The red portion of the bar represents calories from fat and the blue represents calories from carbohydrates (we’ll just consider the total for the purpose of this article). Knowing how many calories per hour I burn at particular paces gives you a starting point for determining how many calories per hour you should consume when running. One recommendation is to consume somewhere between 15-25% of your caloric expenditure.1 If we use my recent Ironman run pace as an example, which was about 11:20min/mile, I burned approximately 516 calories per hour at that pace. Thus my range to experiment with fueling started at about 75 to 130 calories per hour (15-20% of 516 calories per hour). Other common recommendations are more general, at 30-60 grams of carbohydrate per hour (that’s 120-240 calories from carbohydrates)2,3 going all the way up to 90 grams per hour (360 calories from carbohydrates), especially for longer endurance races.4
You can see there is quite a range of recommendations. So how do you know where your target caloric consumption falls in that range? Keeping the above information in mind and knowing that all athletes are going to be different, we can consider two current and general approaches to endurance sports fueling based upon how many calories you burn:
Take in as few calories as possible
Our body gets energy from muscle glycogen, fat, and if necessary, protein. We have a limited amount of glycogen stores (about 1,400-2,000 calories worth, depending on your lean muscle mass), and a relatively unlimited amount of calories from fat. The body is highly trainable, and through nutrition and specific training, you can make your body more “metabolically efficient” so that it burns more fat. The more efficient you are at burning fat, the slower you will go through your muscle glycogen stores, and the fewer calories you will need to consume. While the controlled research on this approach is limited, there are Kona qualifying athletes who have successfully implemented it.5
Take in as many calories as you can
Since carbohydrates are our primary source of energy, we can maximize our consumption to make sure there is plenty of fuel for the fire. There is quite a bit of research on carbohydrate intake vs. performance, and it does indicate that the more grams of carbohydrates you ingest, the better you will perform.6
I will note that these approaches are not mutually exclusive. It is a good idea for all endurance athletes to improve metabolic efficiency. You can manipulate your metabolic efficiency by diet and doing some longer sessions at an easy pace with minimal calories. Your metabolic efficiency training should be in your prep and base phases when your training intensity is low. You can also “Train Your Gut” to tolerate more calories if you need them in order to stay on top of your game. So add training sessions to test what you can tolerate.
Here are some questions you can ask yourself to see if you might want to modify your race nutrition approach:
Do you find that you have enough energy for your workouts and races?
- You should finish strong but spent, not crawling home or hitching a ride!
- Eat more often if you bonk!
Do you ever get “grumpy” during a long session?
- If so, you likely aren’t consuming enough carbohydrates.
Do you experiencing GI distress?
- You might be consuming too much or need to combine different sources of carbohydrate (i.e., glucose, fructose, sucrose, maltodextrin).
- You can try fewer calories or read labels to find products made with multiple carbohydrate sources and try different brands of products.
- You might also work on metabolic efficiency to see if you can reduce the number of calories you need.
Do you seem to be able to eat whatever you want, even when the intensity is high?
- No reason to back off if it is working for you!
If you make changes, start small (about 10-20% of current hourly intake in either direction depending on your circumstances) and keep track of how it worked for you. If you are really struggling to dial in your caloric intake, one of your best resources is a Registered Dietitian who is also a Board Certified Specialist in Sports Dietetics (CSSD).
Despite the many variables involved in race fueling, if you know how many calories you are burning and what your specific “nutrition issues” are, you have a starting point for how many calories you will need and can then begin the tweaking process.
References
Seebohar, B. (2014). Metabolic Efficiency Training: Teaching the Body to Burn More Fat (2nd Ed.). Fuel4mance.
Clark, N. (2019). Nancy Clark’s Sports Nutrition Guidebook (5th Ed.). Human Kinetics.
Ryan, M. (2012). Sports Nutrition for Endurance Athletes (3rd Ed.). Velo Press.
Hutchinson, A. (2010, November 22). Maximizing carbohydrate absorption during exercise. Retrieved from https://sweatscience.com/maximizing-carbohydrate-absorption-during-exercise/
Hutchinson, A. (2011, October 26). Higher carb intake = faster Ironman finish. Retrieved from https://sweatscience.com/higher-carb-intake-faster-ironman-finish/
