In 2018 BBC broadcast a programme entitled ‘The Big Crash Diet Experiment’ which took a group of obese and/or Type-II diabetic volunteers and put them on 800 calories a day. This programme was recently aired on RTE and is currently available on the RTEPlayer for another month or so. You get to see some nice sciencey stuff and the physiological effects of crash dieting. Also answers one of the questions (OK on a small number of volunteers) about sustainability and does the weight go back on as quick as it is lost.
Well worth a watch.