Well, first of all, as Karky mentioned, the important factor isn't whether glycogen or fat stores get used but rather the calories that get used.
		
		
	 
Yes...I know.
That's why I earlier said,.... " 
As Karky pointed out, the issue isn't the % of fat burned during exercise, it is the total calorie deficit created..
..When it comes to fat loss, Karky is right, the issue is total calories burned overall...not how much fat ( vs carbs ) is burned " 
	
		
	
	
		
		
			If the HIIT and the 45 minute run use up the same number of calories then do you think they've had the same effect?
		
		
	 
Acutally, I'm curious as to -  "
 If the HIIT and the 45 minute run use up the same number of calories " - the ' effect '  a 45 minute has on the risk of muscle loss versus 30 minutes of HIIT. 
As I said before ( on the issue of both burning the same number of calories )  if you run at a slow pace for 45 minutes and burn 300 calories and do 30 minutes of HIIT and burn 300 calories, the suggestion was made, that there is a greater risk your body will break down muscle / protein running a slow pace for 45 minutes.   
	
		
	
	
		
		
			HIIT causes EPOC (Excess Post-exercise Oxygen Consumption) which causes your body to burn calories at a higher rate for long after the exercise has finished. The slow 45 minute cardio will do no such thing
		
		
	 
I know what EPOC is - but, I'm not asking about EPOC. 
	
		
	
	
		
		
			also, with such lengthly cardio you risk your cortisol levels increasing and causing muscle atrophy.
		
		
	 
This  is what i was curious about - why it is, a  ' slow '  45 minute run greatly increases the liklihood protein will be synthesized - i.e causing muscle atrophy  - to glycogen for energy vs HIIT.
For example, if you do a ' slow ' 45 minute run it could be at a ' slow ' pace in which you may only be exerting yourself at 60% - 70% of Max HR for 45 minutes. With HIIT, the work intervals could be at 90%+++ Max HR, and  the ' active ' recovery intervals may never go below 70%++ Max HR, so the bulk of your HIIT training is at a much higher ' average ' training heart rate than the 45 minute run.
Cortisol is a good point. I do understand that cortisol is released in response to ' stresses ' placed on your body.  That said, I would think that over the 30 minutes in which your body is under very high stress  ( as it is with HIIT ), your body would generate cortisol of at least the same amount / duration in your system ( if not more ) as a 45 minute ' slow ' run would. It just seems to me that from a Max HR point of view your body is under as much if not greater ' stress ' over 30 minutes of HIIT versus the amount of stress your body is under over a ' slow ' 45 minute run. 
Are you suggesting that the ' stress ' is much greater - i.e in terms of cortisol - over a 45 minute ' slow ' run ' than 30 minutes of HIIT? 
Welcome your thoughts.