Carbohydrates are one source of fuel our bodies use to provide energy for our daily activities and for our runs. Fat is the other source of fuel our bodies use. Carbohydrates have to be replaced during training and racing, but fats don’t. How much and how quickly we burn through carbohydrates and fats depends on a variety of factors. Understanding how many calories we burn and where it comes from can help us dial in our nutrition plan for race day and for training.
When we run fast and for shorter distances we burn mostly carbohydrates. As our distance increases and our pace decreases we begin to use fat as a fuel source too. We begin to use fat at slower long distance events because we have a lot of energy stored up in fat, even very thin people. Fat takes more oxygen to burn so if you’re running hard, it’s not a good source of energy. Fat also takes longer to burn, so if you’re running fast, it’s not a good source. If you’re running a 5k-half marathon, you’re tapping into your carbohydrate stores and burning very little fat.
This is the core of the belief that endurance athletes can perform well on a low carbohydrate diet. The problem comes when the same athlete tries to push hard and go fast. I raced and trained on a low carbohydrate diet for 18 months and was fine, unless I wanted to push my limits and then…I died. Despite being low carb (under 50 g a day), my body still could not burn fat fast enough to sustain hard effort. So I switched back to a higher carbohydrate diet.
One way to figure out the ratio of carb to fat that you are burning is to use your heart rate. To do this you need to know what your maximum heart rate is (220- your age. This is a guestimate). During your run, check your heart rate. At 70 percent of your max heart rate, about 50 percent of your fuel comes from carbohydrates. At 75-80 percent of your max, it increases to 65% carbohydrate.
So a person who is 150 lbs, running at a 7 mph pace (8.5 min/mile) for one hour burns about 782 calories. If that puts the person at 70% heart rate that’s 391 calories from carbohydrate. At 75-80% heart rate that’s, 508 calories from carbohydrates. The same person running at a 5 mph pace (12min/mile) burns 544. Because they are going slower, the amount of calories coming from carbs is less and the amount coming from fats is more.
The American Council on Exercise has a physical activity calorie counter that is based on your body weight, duration of exercise, and intensity. You can find it here. There are a bunch of different tools there to try out, if you’re curious.
Once you know how many calories per hour you’re burning on average during your runs, you can begin to understand and build a strategy for resupplying your body with those calories. Your body cannot process enough food to be able to make up for the amount of calories lost through carbohydrate burn if you maintain a high intensity. The most you can hope to process per hour is about 352 calories. We’ll get into how to maximize that in the next post.