Low carb Diet and HR Zone

[Nico88]

I'm on a low carb diet and prefer to do cardio work outs above the Fat Burning Zone.  My question is, if I enter higher zones (like 85-90% of my HR or even HIIT) will my body first resort to carbs for energy and when it's done depleting them, it goes to fat for fuel?  If I am low on carbs to begin with due to a low carb diet (25g/day) won't these higher zones get to burning fat more quickly?

My ultimate goal is to lose weight fast but I want to be most efficient at it.  

Thanks - 
Nick

[Dominique]

If you are eating at a deficit, some fat from your fat cells will be used as fuel, regardless of the diet you are following (and regardless of what HR zones you exercise in). Your body can also break down protein from your muscles and use it as fuel, so it’s not like fat is the only fuel available once you have exhausted carbs in your glycogen stores. In fact, your body has a strong incentive to break down protein when in a caloric deficit, as maintaining lean mass is "metabolically expensive". 

The idea in the ketogenic diet (very low carb) is you teach your body to use fat instead of carbs as its main source of fuel. For that to happen, you need to restrict carbs below a very low limit (maybe 25 grams would meet the requirements) and keep it that low thereafter. It can take some time (a few days) before your body adapts. Keto enthusiasts may provide more information (I’m not one of them).