The studies show that if you will get 50% of your energy from fat at workout intensities of less than 65% and only 25% from fat of intensities greater than 65%. So if you want to maximize your fat loss, they tell you to work out at 65% because if you are going to burn 300 calories, you will metabolize 150 calories of fat if you work out at < 65% or 75 calories if you work out at greater intensities.
To burn 150 calories of fat at intensities greater than 65% you will need to burn 600 calories.
In terms of calories we can conclude that it takes twice as many calories at the higher intensity than it does at the lower intensity to burn the same amount of fat.
Now realize that the body becomes marginally less efficient as it is working at higher intensities. So as the intensity increases 300 calories will power your body for less and less time.
300 calories @65% you get 32 minutes of activity – fat burned 150 cal
300 calories @75% you get 20 minutes of activity – fat burned 75
300 calories @85% you get 16 minutes of activity – fat burned 75
You can see that 32 minutes of 65% or 32 minutes of 85% will allow you to metabolize 150 calories of fat. In this example, the slowest way of burning 150 calories of fat was @75%; it would take 40 minutes to achieve.
Also see in this example that after 32 minutes, you will be burning more fat if you work at an intensity of 85% than at 65%.
This is the math of it and it deals ONLY with the calories metabolized during exercise. It does not factor in the cost of anything after the exercise – energy burned through metabolic increases or muscle repair. In fact, the metabolism has been shown to be elevated for up to 2 hours following exercise at intensities of 85%. High intensity exercise is also more damaging to the body and requires more energy for repair. This means that you will continue to burn energy long after the high intensity exercise has stopped.
The math also only deals with fat calories metabolized. Remember that it takes twice as many calories to metabolize the same amount of fat working at a high intensity. What is the overall impact of this on weight? It is increased fat loss. There is a caloric deficit of 300 calories if you workout for 32 minutes at 85% vs. 65%. This means that at 85% in your 12th workout, you will have burned 3500 more calories. Since each pound contains 3500 calories, you have an extra pound of fat loss.
This is why I recommend high intensity calorie burning activity for people who want to burn fat.