Will there be rapid weight loss with extreme calorie restriction (under 1000 a day) or will it slow the metabolism?

Yes and yes. Very low calorie diets do allow rapid weight loss. The decrease in metabolism is temporary and is the bodiy's attempt to compensate. Programs like medifast can help kickstart a weight loss effort and then transition to a healthy eating program, exercise, and regular support and encouragement/accountability.
Dangerous. It will let body use essential muscle for calorie with very high risk for developing sudden heart attacks, loss of muscle in gut leading to acute bowel problems and if you are a diabetic developing diabetic ketoacidosis - diabetic coma be very careful.

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