Also, BMR is not how much you need to eat, it's what you'd burn if you were effectively in a coma. RMR (resting metabolic rate) is what you'd burn if you were as lazy as humanly possible (one can dream), often 1.15xBMR, and actual output is typically 1.2xBMR for an extremely sedentary person.

Practically speaking if you do things with your life, like deliberate exercise, it'll be more like 1.3xBMR or more. Which means 3770 for a BMR of 2900 - a ton of food if you're not eating calorie dense things like e.g. ice cream.

If you were to eat, say, 1000 calories, and maintain activity levels (most people can't), youd be losing the better part of a pound of mass a day, and close to 50/50 muscle and fat.