In April of last year my OH was 5st overweight, had uncontrolled diabetes and was taking statins for high cholesterol. The diabetes was a particular worry because her older brother, who is also diabetic was starting to have problems with his eyesight and her condition had mirrored his, just a couple of years later, up until that point.
We don't live in the UK and have reasonable insurance, so she was seeing the dietitian regularly and attending diabetic clinic but didn't mention starting the diet to either because she'd failed with weight reduction plans so many times before. In the first two weeks she'd lost 16lbs, so then felt confident enough to bring it up.
The dietitian was delighted at what she was doing, she felt that although 800 calories a day isn't sustainable forever, it was working for her and her blood sugar results were amazing.
Many, many people told her that her diet was unhealthy but none of them were able to back it up with anything other than thinking they might have read something somewhere because there is no research that proves eating 800 cal a day for a short period is bad for you. Every medical professional she met was delighted with her, to the point where she was asked to attend a training session for dietitians.
My OH is now 5st lighter, has a healthy BMI, is off the meds and is back to eating a more normal diet. Her diabetics isn't cured, as happens with some people, but it's controlled to the point where she doesn't need medication and that's bloody amazing.
Do your own research OP, talk to real medics and ignore people who are repeating what is little more than unresearched gossip.
Good luck to you.