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Does Body Fat Burn as you Workout?

Hi guys,

Probably a noob question to some of you but its something i’ve been curious about for a while. Does your body burn fat and use it as energy as you are exercising or does it use it after? How long - time wise - does it take for you to see a noticeable difference in body fat after working out?


For you first question, try a search on energy pathways, both aerobic and anaerobic. I think there is a very good article on John Berardi’s site that will answer all of your questions.


Look there, I just did the grunt work for you.

For your second question, it varies too much by the individual to give you a good answer. Suffice to say that the results will not be noticed immediately, but can easily be noticable in a week, and substantial improvement in bf can be made in a month.

These are rough, arbitrary estimates and you should put little faith in them.

High rates of energy production (requiring high rate of ATP production) as occurs in weighlifting – any work rate that’s hard enough that it cannot be sustained for long at all – requires glucose as the principal fuel.

This does’t have to be from dietary glucose, but can be from glucose produced from some of the amino acids of protein.

(The reason I mention that is without a person knowing of this alternate source, they might assume that a low-carb diet would leave high rate energy production impossible, but that’s not the case.)

So during the time the muscles are working quite hard, fat is not their principal fuel. (Though they can use fat to so-to-speak recharge ATP stores, which when at maximum capacity can provide a number of seconds of high energy output. And this goes on while resting or working only rather easily. But they can’t use fat fast enough to support ongoing high-output work.)

However, the calories burned are calories burned and will result in fat being burned overall.

It shouldn’t really matter to the person exercising, other than theoretical interest, whether fat is being burned at the moment or will be consumed later.

You will not see a noticeable difference in body fat after any individual workout of any reasonable length.

For example, suppose you burn 1200 calories, which would be a lot of working out. Probably two hours of working quite hard. And suppose we wait enough time so that protein and glucose/glycogen levels are restored, so by now it’s all fat loss. That corresponds to about 1/3 of a pound of fat. You cannot see that.

I did once see a video someone produced of their losing 1 lb of fat in a day with something like 9 hours on the treadmill. This person was so lean in the first place that 1 lb actually was visible.

So if one wants to take things to the fairly ridiculous, then it is possible in such cases to have a noticeable small difference from “one workout” but certainly in any ordinary circumstance, no, as a shorter single workout cannot burn enough calories to represent a noticeable amount of fat.

It sounds like you’re hoping for something way too much.

Why not consider the cumulative effect of 3 or 4 workouts per week, thus about 13 to 17 workouts per month, over a few months so then say 40 or 50 workouts?

Now, individually invisible small losses of fat add up to something very noticeable.

On top of that, a fat loss program should involve fat loss on an ongoing basis as well, not simply from calories burned while working out.

Thanks for your in-depth answer and information.

Just to add another opinion, I’ve always liked Alwyn Cosgrove’s thoughts on this issue:

[quote]If you can imagine a big forest fire, you understand that it doesn’t just burn for an hour and then burn out - it gradually burns out so that over time there is no fire anymore. The “peak” of the fire may have been hours ago, but there is still flames being produced for a long time afterwards.

We call this Afterburn - metabolic disturbance - elevating EPOC to maximize caloric burn for the other 23+ hours per day. Is there much of a real world effect of burning 300 calories per workout (e.g. aerobic work) if I don’t elevate EPOC?

If we could elevate EPOC even an apparently insignificant 1/4 of a calorie per minute for the 38 hours that the study showed, then that 31 minute resistance training workout would burn maybe 300 calories during the session plus an extra 570 calories over the next 38 hours. That becomes very significant.

In the past - fitness professionals and researchers have looked at how much fat is burned during the exercise session itself. This is extremely short-sighted.

As my colleague Alan Aragon said:

"Caring how much fat is burned during training makes as much sense as caring how much muscle is built during training."

Think about that. If we looked at a weight training session that started at 9am and finished at 10am - how much muscle would we see built if we stopped looking at 10am? None.

In fact - we’d see muscle damage. We could make the conclusion that weight training does not increase muscle - in fact it decreases muscle right? It’s only when we look at the big picture - and look at the recovery from the session - that we find the reverse is true - weight training builds muscle.

Fat loss training is the same way. Someone talking about the benefits of the “fat burning zones” or “fasted cardio” is a sure sign that the individual has stopped looking at the end of the exercise session. They have come to the conclusion that fasted, lower intensity steady state exercise burns the most fat and made a massive leap of faith to suggest it is best for real world fat loss.

Using that same logic these same people would suggest avoiding weight training if you want to grow muscle.

Take home message - focus on the Afterburn effect not just what happens during the exercise session.[/quote]