Re:Heart beat for fat loss and cardio confusion!
As you move from lwo intensities to higher intensities, the amount of fat vs. carbs burned shifts from one to the other
At low intensities, you may burn near 100% fat
At the highest intensiy (acually just about anything above lactate threshold), you burn 100% carbs
at any intensity between, you burn a proportion of the two. As you move from lower to high intensity, you burn proportionally less fat and proportinally more carbs until you reach a point taht the body can only burn carbs.
The issue with the 'fat burning zone' concept is that people confuse %ages with absolutes
Say you're walking at 3mph and burning 5 cal/min, but you're burning 100% fat. That's 5 cal/min of fat.
Say you're running at 6 mph and burning 10 cal/min but you're burning 50% fat.
Ruh roh, that's less fat, isn't it? No, it's not. 10 cal/min * 50% 5 cal/min of fat. It's the same amount of fat in absolute terms although it's a lower percentage. But you're also burning 5 cal/min of carbohydrates.
Say that at 6 mph you're burning 10 cal/min but still 65% fat. That's still lower by %age than at 3mph. But yo'ure burning 6.5 cal/min of fat which is higher. And you burn more total calories. And you deplete some of the carbohdyrate in your muscle.
Some studies have shown that that maximum absolute amount of fat burned occurs right around the lactat tehreshold (the highest, hardest, most painful intensity that you can sustain for an extended period) although it depends on training status and some other factors
When you deplete muscle glycogen (via burning it during exercise and/or carbohdyrate restriction), this increases whole body fat oxidation. And, for the most part, what you burn during exercise is less relevant than than what you burn the rest of the day and none of this matters if you aren't in a deficit). So say you do a hard session where you burn a combination of fat and carbs. Not only did you burn those calories, by depleting muscle glycogen
a. your body will burn more fat for the rest of the day (I'm not saying more in terms of 'metabolic rate' is increases, but more in terms of the proportions used)
b. incoming carbohdyrates tend to go to refilling muscle glycogen instead of being used for energy
Which is why, to a certain degree, it doesn't matter what you do as long as the calorie burn is roughly similar
Low intensity activity is sort of a direct fat burner, you burn mostly fat for fuel but that's all you get out of it.
Higher intensity burns some proportion of fat/carbs but impacts more greatly on what you burn later in the day
Intervals burn only carbs during training but the glycogen depletion and other factors may make you burn more fat later in the day
I think the bigger issue is that, if you do too much high intensity activity too frequently, you get overtrained and that causes too many problems.
Elite athletes do 75% or more of their volumes at low intensities, what makes fitness people think that they can handle more than this?