What is Substrate Utilization?
Whether you’re sleeping or running an all-out sprint, your body constantly fuels itself with a combination of carbohydrates, fat and a small amount of protein. That fuel mix changes, though, depending on how hard you’re working: You burn a higher percentage of fat when your heart rate is low, and you burn a higher percentage of carbs when your heart rate spikes.
But there’s always a mix of fuel sources no matter what you’re doing.
You go for a one-hour run, and your fitness app reports you burned 700 calories. Knowing this, you might think there is some cushion in your daily caloric allotment. What the app doesn’t take into account, though, is the fuel source of those 700 calories, which depends on the intensity and duration of the workout.
During that one-hour run, you might burn a 50/50 ratio of fat to carbs—350 calories from fat and 350 from carbohydrates. The fat calories don’t need to be replaced if the goal is to shed fat, but the carbohydrate calories do, and 350 calories will naturally occur at your next timely meal.
Substrate utilization makes the calories-in, calories-out approach is a bit more complicated than merely entering your activities and meals into a clean equation.
(It should be noted the reported 100 calories burned per mile from some apps is a rough estimate and may not be accurate for everyone. Also, people who are more aerobically fit will burn a higher percentage of fat for fuel when a workout intensifies.)