What will happen if you keep increasing the deficit beyond reasonable to 50% deficit of what your body would like to have?
My bet is ultimately you'll join the extreme dieters where 80% fail to reach or maintain goal weight.
You'll lose enough muscle mass to make this a mess trying again next year.
You'll yo-yo diet for a long time in life, because you'll keep thinking about the extreme method that worked in the past, and attempt it again.
How that can play out behind the scenes.
Your body needs so many calories for the base functions of life, if the exercise is knocking a large portion of that off the top for use purely for movement, leaving barely anything, the body will try to deal with it.
Your body will attempt to adjust what you burn so that the deficit isn't so big as to be so stressful.
You'll be more tired outside exercise and move less, you'll become tired during the workout and burn less.
If that doesn't drop your daily burn enough, it'll slow the rate of hair growth, nail growth, skin replacement, muscle repair, staying warm, ect.
If that doesn't help enough, your base metabolism will slow.
Your immune system will be shot, so when sick you'll burn much less.
Many times the body will get it's slowdown when due to lack of repair you get injured finally.
Stress induced cortisol will increase and water weight along with it, so scale weight can appear to stop dropping, even though fat and muscle is still being lost.
That's most of the stuff that will occur if you continue to eat the same despite burning more and more.
Now, you will burn a little less due to just weighing less, but not enough.
Now - one saving aspect, for a little bit of calories anyway - you aren't burning that much on the spin bike. Where the Fitbit is using HR-based calculated calorie burn.
850 Kcal would imply putting out 236 Watts for 1 hr solid. You could have that as your FTP, but doubtful.
You have a bike that gives a watts figure?
Watts x 3.6 x hours = kcal.
For 30 kg to healthy weight left, 1000 cal deficit is reasonable, but when you get to 22.7 left, 750 is more reasonable, than 500 at about 14, than 250 for final 6kg.
That's to avoid the body reacting as I've described above.
You could join a research study and be measured out the whazoo as to when the body takes the turn for the worse, then stop the diet to get over the reaction it's done already, then start diet again more reasonable.
Or take the common lines in the sand for what is reasonable and try not to cross over. And hope your body doesn't react the opposite way and adjust before you even get near the line.
But Fitbit is indeed trying to teach a life lesson about weight management.
You burn more you eat more.
You burn less you sure better eat less.
In a diet a tad less in either case.
Good question to ask. Some people think exercise doesn't matter - but when it takes so many of the calories available from eating, and your body isn't left enough for the basal metabolism functions - it will find a way.
Oh, you'll lose fat, and muscle. Sadly the heart is a muscle and people have done permanent damage to it.
But it won't be the body you'll really want when you get there.
You'll wish you had done resistance training for instance.
Nothing but cardio and pushing the deficit will mean upwards of 20% Lean Body Mass is lost as part of the weight.
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