The treadmill, bike, and elliptical machines in gyms typically display some type of heart rate zone training chart. This chart groups heart rates into "fat burning," aerobic, or anaerobic zones. The fat-burn zone represents 60-70% of the maximum heart rate, estimated by subtracting age from 220. It's true that the body predominately uses fat for energy at lower heart rates, and burns sugars or carbohydrates at higher rates. Therefore, the logic in training with lower intensities is that you will burn more fat calories. However, this often leads to inflated expectations when it comes to actual weight loss.
The truth is that our bodies do primarily run on fat storage while at rest and during low-level physical activity. However, the actual amount of fat burned (in pounds or grams) is tiny. A gram of fat has 9 calories. Compare this to 4 calories in every gram of protein and carbohydrate. Fat is extremely energy-dense, which is why using it up takes time and effort! When deciding on cardiovascular exercise for weight loss, focus on total calories burned.
High Intensity vs. Low Intensity Cardio
To use an example, let's say a person walks for 45 minutes in the "fat burn zone" and burns 200 total calories (120 of those calories from fat). If this same person does a sprint or other high intensity interval workout with the same 45 minutes, it is reasonable to expect a total usage of 450-500 calories and the same 120 from fat. At the end of the day, the workouts that burn more total calories lead to the fastest weight loss.
Higher intensity work (75%+ of max heart rate) is also more effective thanks to excess post-exercise oxygen consumption (EPOC) also known as the "after burn." After challenging interval-based workouts, the resting metabolism actually runs "hotter" for 4-32 hours, burning more calories than usual. And yes, these calories will predominately come from fat stores.
To summarize with action items:
Emphasize higher heart rate zone workouts and go for total caloric expenditure. Try the Tabata Protocol and other interval routines.
Pair exercise with a lower calorie, whole food based diet.
Low intensity, long duration cardio is only recommended for the de-conditioned body or for other purposes besides accelerated fat loss.
Strength and muscle-building exercises also contribute to a higher resting metabolism.
While there is hard science behind the "fat burning" zone, the concept is often taken out of context and leads to more confusion. When weight-loss is a priority, do not get caught up in the minor details. Work hard, work smart, and reap the rewards!
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