I don't know. I've always heard that the "keeping your metabolism" up thing may be a myth. I just don't know.
I do think that 1000 cal/day is too low to be sustainable long term. A calorie restricted diet known as CRON or CRAN (calorie restriction with optimal/adequate nutrition) is very low in calories, but does stop at about 1200. Plus, it's something you must commit to doing the rest of your life. Most who do it aren't necessarily trying to lose weight...just live longer (the premise of the whole diet is based on lab tests on rats that indicates that rats fed on fewer calories live substantially longer than rats fed more...the trick is that the lower calories must be very nutrionally dense...i.e., you can't just eat 100 cal snack packs of "lite" Oreos and expect to have optimal results).
At any rate, my mother went on every crash diet known to man, including one where she drank nothing but shakes all day. The upshot? She gained it all back, plus a lot more later. Too-low calorie diets NOT recommended.