I know you’ve seen the charts on machines. Your fat burning zone, your cardio zone. Companies love to put that stuff on there. Of course it doesn’t work like that.
You see your body burns calories of fat and carbs all the time, just at differing rates. When you’re at about 50 percent of your max heart rate then you burn about 50 percent fat calories and 50 percent carb calories. As your heart rate increase (when you work harder) then you burn percentage wise less calories of fat and more of carbs. That’s where the myth of fat burning zone comes from.
But it also breaks down too. If you workout in that “fat burning zone” for half an hour and much harder for half a hour, you will burn way more calories when you work harder. Sure percentage wise you might burn more carbs, but you will have burned so many more calories overall that you will burn many more fat calories working hard than going slower.
That’s why it’s best not to dog it. When you work out, bust your butt! You’ll get in better cardio shape and you’ll burn more fat too.