which macronutrient fuels burpees? and which fuels walking?
For high intensity, short duration exercise (i.e.: burpees), the body relies on carbohydrates as fuel. For more ‘steady state’ exercises (i.e.: walking or cycling on flat roads), the body tends to rely on fat as fuel.
Your muscles themselves do not store very much of the two main 'energy currencies': [ 1 ] ATP, and [ 2 ] and creatine phosphate. So especially during those first 10 seconds of exercise, your muscles immediately drain their small-rations of these. They must then quickly start breaking down the gates of carbohydrate and fat storage facilities to continue the workout.
Fats are take much longer for the body to break down than carbohydrates. Because carbs are transformed more quickly than fats, they provide most of the energy at the beginning of any workout.
An example of a "fat fuel" snack before a workout is peanut butter, nuts, or avocado.
After those first 10 minutes after your muscles have registered that the body movement at hand is more steady state (i.e.: walking), they go back to draining your fat storage facilities. Fun fact is that FAT contains much more energy per gram than carbohydrate...meaning it's more efficiently expended.
An example of a "carb-fuel" snack pre workout could be a piece of fruit, crackers/pretzels, or toast.
In more-speedy bouts of exercise, like interval training, carbohydrates storage facilities remain the dominant ingredient feeding your muscles, brain, and blood. Fun fact is that "carbohydrate metabolism can take place anaerobically -- without oxygen. Fats, however, can only be metabolized using oxygen." What does this mean? When you're gasping for air after doing high knees followed by pull ups, your circulatory system cannot provide oxygen to your muscles fast enough. So, because fats require oxygen, they're not an option! Leaving carbohydrates as the available pool to grab from!
"Recent evidence suggests that low-fat high-carbohydrate diets may increase blood triglycerides and reduce high density lipoproteins. This suggests that the chronic ingestion of diets that are extreme in their composition of either fat or carbohydrate should be approached with caution in health-conscious athletes, as well as in sedentary individuals."