Recall, carbohydrates are stored for energy in the form of glycogen in the muscles and liver, and the remainder is stored as body fat. The body uses both of these energy sources for fuel, but the rate at which they are utilized depends on the level of intensity and duration of exercise.
Fat Burning Zone
During high intensity exercise, the body uses mostly stored carbohydrates for fuel, as opposed to low intensity activity, which burns mostly stored fat as fuel. The “Fat Burning Zone” is the idea that the body burns a higher percentage of calories from fat during low intensity exercises. It’s a calculation that determines your optimal heart rate for maximum fat loss.
220 – (Age) = Maximum Heart Rate (MHR)
(MHR) * 0.65 = Fat Burning Zone (Beats per Minute)
Example: 26-Year-Old Male
220 – 26 = 194
194 * 0.65 = 126 Beats Per Minute
*Some critics argue that high-intensity exercise burns more calories overall, making it more effective than low-intensity cardio.
However, this will result in greater muscle loss, making low-intensity cardio the better solution if you’re concerned with preserving muscle mass.
There are two specific periods of time that are optimal for burning the most fat in your workout, first thing in the morning and immediately following an anaerobic workout. Glycogen levels are severely depleted during these times, so the body must use fat for fuel.
1. First thing in the morning on an empty stomach.
Recall, when we wake up our body is in a fasted state. The level of glycogen stored in your muscles and liver is extremely low because it has been 10-12 hours since you last ate any carbohydrates. The depleted level of glycogen is optimal because the body must tap into its reserve energy source, body fat. Studies have shown up to 300% more fat is burned in a fasted state.
2. Immediately following an anaerobic workout, such as weight lifting.
Typically, it takes 20 minutes for the body to deplete its glycogen storage. The body is in the perfect state to start burning fat because the weight lifting workout already burned off most of the carbohydrates. The switch to fat for fuel is the perfect time to start an aerobic workout.
* Aerobic exercise by itself would take twice as long to burn the same amount of fat because the first 20 minutes would be spent burning off carbohydrates.