Pop in to any major fitness organization like the well respected American College of Sports Medicine (ACSM) and you will notice a training guide for cardiovascular fitness using heart rate zones.
This has sparked the question:
“Can running in the fat burning zone really help me to burn fat?”
To help answer that, we need to first understand how the figures were discovered.
Whether we are moving or not, humans primarily will use two main energy sources: Carbohydrate and fats. These fuel will not be used exclusively in any occasion, but as you can tell from the adapted simplified Picture 1 below, the body prioritizes different fuel usage at different exercise intensities.
As you would notice, fuel dependence is rather similar at rest, is more dependent on fat resources at lower intensities, and reliance on carbohydrate increases as exercise intensity increases. When translated to heart rate data, research found that exercising at 60-70% of your maximum heart rate (HRmax) allows the body to utilize fat as a fuel best, and it was therefore termed as the fat burning zone.
What this practically means is that your body will be able to use fat more efficiently at 60-70% maxHR, allowing you to sustain your training for longer at that Heart Rate (HR) zone.
It is also interesting to note that eating more fat in your nutrition allows you to increase your fat adaptation levels higher, allowing you to tap into higher percentages of fat at similar heart rate zone. This is also termed as fat adaptation.
Does this mean that I can have a leaner body if I train at an lower-moderate intensity (like the fat burning zone) compared to to training at a higher intensity?
Great question. It depends.
As far as fat loss goes, the first rule of thumb is still to create a calorie deficit. Assuming all other factors remains the same, an exercise session that produced 500 kcal worth of energy expenditure will not have a significant impact on your body composition whether you performing it with a higher or lower intensity.
The same can be said about your nutrition – assuming all calorie and protein component to be the same, a higher fat diet coupled with low intensity exercise does not burn more fat compared to a similar study in a normal carbohydrate meal.
It is interesting however to note that you will lose more total body weight at the early periods of a higher fat diet, as shown in Kevin Hall’s famous study comparing the difference in body composition changes between high fat diet and a traditional diet.
Should I run in the fat burning zone?
From a body composition perspective, while exercising at the fat burning zone may be helpful to help you use more fat resources in your body, it is also important to consider the practical aspects.
The unfortunate downside to training at the fat burning zone is that you will need to train longer to hit the desired calories you hope to expand per day. If time is tight and you only have 30 minutes to train, exercising in a higher intensity might be more helpful to get to your desired body composition. If not, definitely go for the fat burning zone!
Okay cool. How about food? Should I eat more fats to accommodate my training?
In the long term, you will not see any significant changes in body fat percentage whether you decide to increase your fat intake or not. The most important rule of thumb is to ensure that you are eating lesser than the amount of movement you make.
A big advantage you might see is the shift in metabolic adaptation – this means that you will likely be able to tap into your fat resources better while training at the lower intensity as a high fat diet would help increase fat adaptation. If enjoying a high fat diet is something you can see yourself doing for the duration of your fitness journey and beyond, then it might be an option. Otherwise, it’s okay to stick a whole-food balanced diet.