Body's "preferred" fuel source...

[mindspin]

Which statement below is true?

When you are on a low carb diet (50g or less per day), glycogen stores have been depleted and you are doing both resistance training (4x week) and HIIT cardio (7x week), your body will fuel itself as follows: 

A) first fats/proteins consumed, then stored body fat then protein (broken down muscle tissue).

B) first fats/proteins consumed, then protein (broken down muscle tissue) then stored body fat.

C) first fats/proteins consumed and then protein (broken down muscle tissue).  No stored body fat will be used.

D) none of the above.

This isn't a trick question.  I'm just curious what everyone thinks and if there are any studies out there that actually shed light on this.

Thanks :-)

[monsieurjkb]

It depends if you're sitting on your ass or in the middle of one of those insanely frequent HIIT workouts. BUT if your glycogen stores are depleted, then surely you'd be running on fats/ketones and however much glucose your body/brain required from protein/muscle tissue.

Your body does not work on one substrate at any given time. It varies depending on what you demand from it:

high intensity requires more sugar, whether it's from stores or muscle or dietary protein.

lower intensity/rest will use more fat or ketones than glucose.

Be more specific?

[mindspin]

So if the goal is maximum fat burning while maintaining muscle, will the body break down muscle tissue for fuel (during resistance training and/or HIIT cardio), or will it use fat reserves?  Again, assuming glycogen stores are depleted.

Thanks :-)

[Draugluir]

that depends... when you reach the glycolytic threshold you have no choice but to burn carbs, but that point can be moved quite a bit. At this point, if you have no glycogen, you have to convert amino acids to glucose, or stop AFAIK. I don't know all the specifics, but there are a lot of variables. How used is your body to using fat for fuel? Doing a ketogenic diet seems to push the threshold higher, and you can train for it as well. Kiefer has mentioned this in some of the podcasts.

[mindspin]

Maybe this is a better way to ask the question:

If someone's glycogen stores are depleted and they are on an ultra-low carb diet, what will their bodies use for fuel when:

A - doing resistance training
B - doing continuous cardio - 40 minutes treadmill 4mph
C - doing HIIT cardio - 20 minutes with four 30 second sprints
D - Sitting at a desk

Thanks

[monsieurjkb]

The answer I first gave still applies. High intensity exercise at any length of time over a few reps will require glucose. If your body has no stored glycogen, it'll make glucose for the type of activies that require it: higher reps with heavy weights will require more glucose than lower reps with heavy weights.

A - doing resistance training answered above
B - doing continuous cardio - 40 minutes treadmill 4mph brisk walking will use mostly fat for fuel
C - doing HIIT cardio - 20 minutes with four 30 second sprints the sprints themselves will probably use a bit of glucose but the effects of HIT like sprints is supposedly what happens 24 hrs after the sprints themselves. Sprints will help fat loss assuming your diet is on
D - Sitting at a desk if your on a keto diet, then you'll use ketones probably

HERE: http://www.exrx.net/Nutrition/Substrates.html Just apply all that's here to a situation with no stored carbs.

[mindspin]

The answer I first gave still applies. High intensity exercise at any length of time over a few reps will require glucose. If your body has no stored glycogen, it'll make glucose for the type of activies that require it: higher reps with heavy weights will require more glucose than lower reps with heavy weights.

A - doing resistance training answered above
B - doing continuous cardio - 40 minutes treadmill 4mph brisk walking will use mostly fat for fuel
C - doing HIIT cardio - 20 minutes with four 30 second sprints the sprints themselves will probably use a bit of glucose but the effects of HIT like sprints is supposedly what happens 24 hrs after the sprints themselves. Sprints will help fat loss assuming your diet is on
D - Sitting at a desk if your on a keto diet, then you'll use ketones probably

HERE: http://www.exrx.net/Nutrition/Substrates.html Just apply all that's here to a situation with no stored carbs.

cool...thanks.