Explanation gets deep into exercise physiology, probably more than you want, but short answer, fitbit doesn't know, and heart rate zone titles, not just by fitbit, were designed for top athletes and names are not really accurate for normal people, but are still useful as guidelines. But it doesn't really matter whether or not you are burning fat because if you are, say, burning glycogen stores instead, then your body will eventually have to burn fat to replace those glycogen stores. So a calorie is still a calorie and your body will balance out where it comes from.
Hope I didn't just confuse you more.
For athletic training, fat burning is more efficient than burning glycogen/carbohydates, but glycogen is needed for high intensity effort. So for distance running you might want to stay in fat burning zone to be able to run further, but getting into aerobic zone will have some other training benefits, and peak zone will have max training benefit but you won't last long. Simplified.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.
Platinum Fitbit Product Experts share support knowledge on the forums and advocate for the betterment of Fitbit products and services. Learn more
Best Answer