Thursday, May 29, 2008

Fat Burning Zone

Another interesting post by Lyle McDonald that I wanted to save here:

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 aoutanything about lactate threshold), you burn 100% carbs

at any intensity between, you burn a proportion

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. But you're also brning 5 cal/min of carbohdyrates.

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 dpends on training status and some other factors

when you deplete muscle glycogen (via burning it during exercise and/or carbohdyrate restriction), this increass 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 wheer 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 carbohdyraste 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 brns only carbs during training but the glycogen depletino and other factors may make you burn more fat later in the day

I think the bigger isse 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?

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