The "Fat Burning Zone" and Why It's Misleading
My Fitbit shows a breakdown of minutes of my workout that I spend in the "fat burning" zone. What does this even mean? I was intrigued, and then... confused... because when my heart rate exceeded a certain threshold, I would find myself out of this magical zone and into the "cardio" zone... and then my heart rate would climb higher into "peak" zone. Did this mean that higher intensity workouts weren't burning fat? Did this mean that I was working harder for nothing? I didn't understand.
What is the Fat Burning Zone?
The fat burning zone is the theory that if you stay in this zone during exercise, you'll burn more fat. Though there is variance between sources, it is commonly considered to be the time that your heart rate is at an intensity of approximately 60-80% of your maximum heart rate.
How Our Bodies Metabolize Fat
The 2 main sources of fuel are glycogen (a substance that stores carbohydrates) and fat. Both of these break down to form glucose and ultimately CO2 and water. Oxygen is required to oxidize, or break down. I will talk in a minute about why this is important. During a workout, your body requires more energy than when it is just doing normal body stuff; your heart pumps faster and harder to send oxygen to your muscle cells to break down more glycogen and fat to fuel your muscles.
1 gram of carbohydrate contains 4 calories of energy, 1 gram of fat contains 9 calories. This makes glycogen, or carbohydrates, a MUCH less dense form of energy storage that is readily broken down into glucose, as compared to fats. As such, glycogen is your body's first source of energy during exercise. Since high-intensity workouts require more energy quickly, you tap on glycogen rather than fat in your body for fuel. Your body only taps onto the next fuel, fat, when you start to run out of glycogen.
It is argued by this "fat burning" theory that the body burns a greater percentage of fat with lower-intensity exercises than at higher intensities because the body does not require "fast energy" from carbohydrates. However, during a high-intensity workout, where you'll be well above the "fat burning zone," your glycogen stores are depleted rapidly, and your body then taps into the fat storage. This means that high-intensity workouts are more efficient than restricting yourself to the "fat burning zone." Ultimately, the caloric burn is what matters in fat loss.
Using EPOC to Burn More Calories All Day
Earlier I briefly mentioned oxidation, or the breakdown of glycogen and fat. In other words, in order for your body to break down either carbs or fat, you need oxygen. When you work out at an intensity that you can’t sustain for a long period of time, your oxygen stores become depleted. Think about it... remember a time you found yourself in the middle of a workout that kicked your butt. How did you feel? Were you curled over, panting at some parts? WHY do you think the body responds this way? It is desperately trying to replenish the oxygen that your body is requiring at the moment to break down carbs and fat for energy. Workouts like these that literally take your break away-- when your intensity level is above 80% of max capacity, you force the body to work harder to build oxygen stores back up. In fact, your body may be working hard to do this for up to 24 hours afterward. This is important because the entire time your body is building it's oxygen stores back up, it is burning more calories-- EVEN IF YOU ARE SLEEPING! This is called excess post-exercise oxygen consumption, or EOPC. It is basically just an afterburn effect that increases caloric burn long after the workout ends.
What's the Takeaway?
If you're trying to lose the most amount of fat as quickly as possible, ignore the "fat burning zone." Do some HIIT training: circuits, sprints, etc. If you know that intense workouts aren't for you (yet), then just workout and stay in the "fat burning zone." Ultimately, what matters is calories in, calories out. Food vs. energy expenditure. If you're being consistent with low intensity workouts and eating more healthfully, then you'll lose fat. If you're able to be consistent with high intensity workouts, then go for it. Do whatever will make you successful at building sustainable habits for your health.