@kalimm wrote:
I think the 220-age formula isn't necessarily a problem. You just have to take it for what it is - an estimation. For those who don't know their zones, it's a decent estimation to get the average person started. But once you start working out more and find out what you're capable of, it would be nice for Fitbit to allow us to change the labels when a custom zone is entered. Of course, you also have to remember that with anything that pertains to body physiology, there's no one formula that can be generalized for everyone as we're all so different.
The difference in the zones is the difference in what your body is burning as you work out. In a very general sense, yes exercise is exercise. But it depends on your ultimate goal. If you do cardio at too high of an intensity for too long, it won't be as effective because you'll burn mostly sugars instead of fat. More calories burned does directly correlate to more weight loss. Just as there are good and bad calories to intake (a 750 calorie Big Mac is far from equal to 750 calories of chicken and vegetables), there are also good and bad calories to be burned.
I guess I must disagree with you regarding the 220-age issue; it is bandied about as gospel which many-many folks swear by and which the Fitbit folks use as a pretty hard and fast rule for their trackers with a heart rate monitor. The net result of the adherence to the "rule" is many folks aren't working out hard enough, while others are working out too hard. Heck, this thread wouldn't even exist were it not for these poorly thought out zones. That's a pretty significant issue in my book.
As for burning sugar versus fat in the various zones, I've looked and looked and never found any compelling science to support that notion. The only thing I've come across even remotely in line with this is for marathon runners (not exactly Fitbit's target customer for most of their trackers).
Within the distance running community there is a growing awareness of the role mitochondria plays in fueling the muscles during a marathon or other very long endurance event. Basically, the more developed one's mitochondria, the better one can perform in such an event. Said another way, those with very well developed mitochondria will hit "the wall" either later in the race or not at all compared to those less developed. "So," you ask, "what does mitochondria have to do with the current discussion?"
Muscles can be fed with pure glucose, or a combination of glucose and fats provided by the mitochondria; the more developed the mitochondria, the longer one can run without exhausting supplies of glucose. The thing is, studies have shown the development of mitochondria requires at least one (preferably two or three) run(s) of at least 90 minutes per week; no mention of pace or "heart rate zone" need apply, just the duration. Said another way, a 90 minute run in the "Peak Zone" (supposedly impossible based on the common usage of the whole zone theory, but easily possible for many) is just as good as a 90 minute run in the "Cardio Zone".
If you have a link to any compelling science which supports the whole zone theory, I'd love to see it.
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, hope you're doing great today! I'd recommend taking a look at this discussion shared by my friend @SantiR, where he's explaining about being in the fat burn zone. 
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