I don't think 1400 is unsustainable? it'll lead to losing weight continuously, maybe a half pound a week ish, but that's sustainable for as long as you have weight to lose, isn't it?
This depends on your size and activity level. The last time I dieted, I was losing a pound a week on 2700 calories. 1400 would have been a severe crash diet.
Depends on the person. I'm an active, 6' male with a TDEE of 2600. 1400 calories is absolutely unsustainable. That's nearly 3lb of weightloss per week.
If I was shorter, female and sedentary, sure. But that's not me.
There's physically sustainable, and then there's mentally sustainable. Based on what I've read about long term fasts, I suspect 0 would be easier than 1400.
