Debunking the myths about the fat burning zone and weight loss. 

Myth: if you workout in your fat burning zone you will lose more body fat than if you workout at a higher intensity.

This is not the case.  To lose body fat and weight, you need to have a calorie deficit.  It is all about calories in vs calories out. If you consume fewer calories than you burn in the day you will lose body fat.   It doesn’t matter if you workout out in the fat burning zone or not.  It matters how many calories you burned in your workout.  If you are working out in the lower intensity, fat burning zone it will take you longer to burn the same number of calories than if you were working at a much higher intensity in the fast burning carbohydrate zone. 

You are never using only fat or only carbohydrates for fuel.  It is a percentage of each.  When you are sitting watching TV you are burning both fat and carbs for fuel.  This is not going to lead to weight loss because you are not burning very many calories, even though some are from fat.  When you go for a walk, this is an aerobic activity and you start burning more fat as fuel.

Once the intensity kicks up, you go anaerobic and you begin to use more carbohydrates for fuel.  Carbohydrates are easily metabolized and are a quick source of energy.  A study published in the Journal of Sports Science and Medicine (2021) supports this concept. The researchers found that during low-to-moderate intensity exercise, fat oxidation rates were higher, while carbohydrate oxidation increased with exercise intensity (1).  Since you have limited stores of carbohydrates, once they are used up you start to lose energy and it becomes difficult to maintain the intensity.  You then slow down, become aerobic again and start burning more fat for fuel, which is then converted into carbohydrates for energy.   A highly trained athlete can switch over to fat burning quite soon into their workout whereas someone less fit will burn carbohydrates and then burn out sooner.   Athletes are not lean because they burn fat sooner, they are lean because they train 2-4 hours a day and therefore burn more calories.

In a study published in the International Journal of Obesity (2022), the authors concluded that high-intensity interval training (HIIT) is more effective for weight loss and fat reduction compared to moderate-intensity continuous training (MICT) when matched for energy expenditure (2). The main reason behind this is the greater number of calories burned during HIIT workouts, as well as the increased post-exercise calorie burn due to the "afterburn effect" or excess post-exercise oxygen consumption (EPOC).

Bottom Line: If you only have 25 minutes, you will burn more calories doing a fast run or HIIT workout than you will doing a moderate pace walk for the same amount of time.  Even though you burned more carbs for fuel, the carbs will be replaced over time through diet and the breakdown of your body fat.  So in the end fat is broken down.  Therefore, regardless of the fuel source used during exercise, what matters most is the calorie deficit achieved that will result in fat loss.

If you aren't fit enough to push yourself to work at a high intensity, or you have a physical weakness that prevents you from doing so, you can still burn a lot of calories by doing low-intensity workouts for a longer period of time.

References:

Maunder, E., Plews, D. J., & Kilding, A. E. (2021). Intensity-dependent alterations in fat and carbohydrate metabolism during 60-minutes of moderate-intensity exercise: a cross-over study. Journal of Sports Science and Medicine, 20(2), 316-323.

Viana, R. B., Naves, J. P. A., Coswig, V. S., de Lira, C. A. B., Steele, J., Fisher, J. P.,& Gentil, P. (2022). Is interval training the magic bullet for fat loss? A systematic review and meta-analysis comparing moderate-intensity continuous training with high-intensity interval training (HIIT). International Journal of Obesity, 46(1), 1-14.