By Rebecca Haight
When we exercise, our body burns off energy in the form of calories. Those calories are burned off from fat and carbohydrate reserves.
Just as the number of calories burned depends on your level of activity, the relative number of fat and carbs burned varies by activity as well. Both aerobic and anaerobic exercises burn fat and carbs at different rates.
Whether an exercise is considered aerobic or anaerobic depends on the energy system your body is utilizing to fuel it.
Anaerobic exercise calls upon immediate energy reserves to fuel your body for short bursts of high intensity activity, while aerobic exercise needs to utilize multiple energy sources to keep your body fueled for longer durations of moderately intense activity.
Carbs vs. Fat
In any given workout, you’re burning a combination of fat and carbs. It takes longer to convert fat into energy than it does carbs.
Carbs only have 4 calories worth of energy per gram, whereas fat has 9 calories per gram. As you run out of energy from carbs, you start utilizing fat reserves.
So, the proportion of carbs vs. fat you’ll burn during a workout depends on how hard you’re working. Since high-intensity anaerobic exercise utilizes fast energy, you’ll burn a greater proportion of carbs for fuel.
Lower-intensity, aerobic exercise burns a greater percentage of fat for fuel. But don’t be fooled – this concept can lead to a large misconception.
The so Called Fat Burning Zone…
There is a common myth known as the “fat burning zone” used to explain the theory that doing aerobic workouts at a low intensity will help you burn more body fat.
It is true that aerobic exercise calls upon more energy sources, and therefore burns up more fat in order to provide that extra energy.
And aerobic workouts do burn a higher percentage of fat than carbs. That being said, high-intensity anaerobic exercises burn more total calories and therefore burn more total fat.
Here’s another way to break it down – about 60 percent of calories burned during aerobic exercise comes from fat. This is compared to about 35 percent of calories burned from fat during anaerobic exercise. However, the increased intensity of anaerobic exercise makes up for its lack of calories from fat percentage.
For example, if you perform 30 minutes of low-intensity aerobic exercise and burn 200 total calories, about 120 of those (60%) will come from fat. Exercising for the same amount of time at a high intensity will burn approximately 400 total calories and 140 of those (35%) will come from fat.
Even though fewer calories come from fat during anaerobic exercise, more total calories were burned at the high intensity and more overall fat was actually burned.
So if you’re looking to burn a high amount of fat in a shorter period of time, anaerobic exercise is the most efficient.
But still, it’s good to mix up your workout…a good mixture of aerobic and anaerobic exercise will ensure you’re burning calories from both carbs and fat.
Hobson, Katherine. “The 'Fat-Burning Zone': A Fitness Myth Debunked.” U.S. News, 3 Mar. 2009, health.usnews.com/health-news/blogs/on-fitness/2009/03/03/the-fat-burning-zone-a-fitness-myth-debunked.
Kelliher, Steven. “Aerobic Versus Anaerobic Fat Burning.” LIVESTRONG.COM, Leaf Group, 11 Jan. 2014, www.livestrong.com/article/431402-aerobic-vs-anaerobic-fat-burning/.
Tremblay, MSc Sylvie. “Fat Burning Vs. Carbohydrate Burning.” LIVESTRONG.COM, Leaf Group, 18 July 2017, www.livestrong.com/article/32587-fat-burning-vs.-carbohydrate-burning/.