If you exercise before breakfast do you lose muscle or body fat?

8 messages
25/03/2013 at 22:38

Something I have read about and trying to find out more. Very mixed and contrasting information around.

Is it true that if you exercise in the morning before breakfast, that the body breaks down muscle to fuel itself, or does it fuel itself off the body's natural fat stores?

Thoughts appreciated...

25/03/2013 at 23:05

What I think you are talking about is fat adaptation.... if you are interested have a read of this;


This is the second article;



Edited: 25/03/2013 at 23:08
cougie    pirate
25/03/2013 at 23:44
I'm no expert but I'd say not muscle. Why would a nights sleep use up all your energy reserves ? How many calories does sleep take ?
26/03/2013 at 08:24
cougie wrote (see)
I'm no expert but I'd say not muscle. Why would a nights sleep use up all your energy reserves ? How many calories does sleep take ?

Depends what else you are doing in bed 

26/03/2013 at 10:25

I think it depends on the intensity of the exercise, rather than the time of day.

Slower, steady exercise allows your body to break down fat stores and utilise them.  

26/03/2013 at 10:37

what Wilkie said. Most of us, even if we're lean, have enough fat to run for about 1000 miles, so long as it's slow enough that our body can supply the muscles with energy from fat (it takes longer to convert stored fat to energy your muscle cells can use, so if you train at higher intensities and there is glycogen to use, your body will just use that, as it's quicker to access).

26/03/2013 at 10:41

fat adaptation is perhaps only part of the story here and only really comes into play when you are doing long training sessions first thing in the morning

if you're doing short sessions, your glycogen stores will be sufficient (assuming you've ate and hydrated well the evening before) to take you through the session.  and bearing in mind that a lot of glycogen is stored in muscle, then that maybe what the OP is getting at??   so you're not breaking down muscle for energy but using up glycogen that's stored in the muscle.

in fact to use muscle to produce energy is the least efficienct energy conversion method compared to using glycogen (primary source) or fat (secondary source) and only really happens in any quantity when you have run out of both glycogen and fat.   

26/03/2013 at 10:44



I tend to do most of my runs before breakfast otherwise I can't run.  However when it's a long run I ensure I carb load the prevoius day.



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