Quote:
it doesn’t really matter whether you burn fat or carbohydrate for fuel during the workout itself. Rather, total energy expenditure, metabolic stimulation, and 24-hour calorie balance is what matters. If you burn glucose during exercise, you tend to burn more fat the rest of the day. If you burn
fat during exercise, you burn more glucose during the day.
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Like I said conservation of energy.
I can think of only two reasons why fat burn zones matter:
1 The demands on the heart are low. It is safe and easy to train in this zone, even if your are seriously over weight or have bad physical condition. It's better to burn few calories safe then to burn more calories while risking heart-failure. Fat burn is a stage to go trough from couch-potato to regular excercise.
2 The body stores only a limited amount of glycogen for immediate, intense action. If you practice endurance sports like long distance running or cycling, you want to keep your glycogen for the final sprint in the last kilometer. Those athletes train their body to burn fat and preserve glycogen.
There are plenty of ways to work in the fat burn zone that don't involve fancy equipment: gardening, going for a walk, walking the dog,... . Unless you actually live and work in the gym, you'll get plenty of "fat burn" workouts in ordinary life.