When does body turn to fat for fuel?

I have read that it isn't until after the first 30 minutes of a cardio workout that your body starts burning fat for fuel.

My question is, on the days when I lift weights before I run, is the fat burning threashold reached sooner do the the activity before the cardio workout?

I hope my question makes sense...

I think there are many schools of thought on this.  I've read that don't eat before a cardio workout so you burn fat instead of fuel - then I've heard to eat before weights so you build muscle.  From my basic understanding the "fat burning zone" for cardio is when you are in the 60 - 70% range of cardiac.  So for me being 47  my heart rate needs to be between 105- 121 - so if you are getting your heart rate up while doing weights I'd say yea you would start burning fat.  I know for me its hard to get my heart rate up doing weights.  I have heard also you should do cardio first then weights - once again it all depends on who you listen to seems lots of different websites related to fitness have all different ideas.

This wiki explains the fuel your body burns based on your heart rate:

http://en.wikipedia.org/wiki/Heart_rate#Train ing_zones

My understanding is that your body burns both carbs and fats in all exercise, it's just the ratio that changes depending on your heart rate.  I think what you eat just prior to working out may factor in as well, but I am not sure how.  The "fat burning zone" is the zone where your body burns a higher % of cals from fat, but if you are in a higher zone and burn more overall calories, it is possible to burn a higher number of actual fat cals even if your body burns a lower %.

Yea its kind of funny how if you work out in the 60-70% range you may burn 60% of the cals will be fat - so if you burn 200 cals 120 will be fat - but if you work out in the hard zone 80- 90% and only 40% burned are fat cals but you would only have to burn 300 cals and I know for me burning 200 cals at the low would take about 45 minutes but if I am working in the hard zone it would only take about 30 minutes so if time is an issue work out hard for a short time and you will get the same benefit in fat cals burned.

dbacker is right on about the % fat burned at low heart rate vs % fat burned at high heart rate.

If you know your excercise herart rate than you are burning at about 50/50, but like dback said you burn less fat at high heart rate but more cal so if it's short duration go hard.

As for when to do your cardio?weight lifting.  The studies I have read say that the muscular benifit is greater if you lift after cardio.  I haven't read anything I can remeber that mentions caloric benefits.

I think there are studies showing lifting and higher intensity cardio keep you burning calories well after you stop the activity. Easy cardio  just stops right after you do.

Hi, I'm usually just a reader here in calorie-count but this topic really sparked my interest :) so my questions are:

the wiki site mentioned about 60-70% being the fat burning zone.

1. For how long do you maintain this? 10-30 mins?
2. How does this relate to the weight-loss running strategy of 2 minutes walk, 2 minutes madman-like sprint, 2 mins walk, 2 minutes of madman-like sprint again?

kwayzee -  you are referring to HIIT which will burn more calories and will burn more calories after you stop working out-   like what has been said earlier - if you are looking for calorie burn work hard and fast if you are looking for fat burn work steady and low

Also although the "fat burning zone" is low intensity so you basically have to work out longer to burn the same "fat" cals as if you worked hard.  Its kind of a "wash"  see post #3 above