Protein I think would be the issue. There was a case study on a morbidly obese guy that did this with doctors. What I remember is he basically told his doctor he was going to stop eating entirely. They could either help him and give him medical advice along the way, or try to just advise against and he was going to do it anyways. He would take a daily vitamin for general vitamins and I believe some yeast paste to give him protein. I think the dude lost like 400lbs in a year. Had small bowel movements every 30 to 40 days.
Edit: Apparently it wasn’t protein but amino acids.
he consumed only vitamins, electrolytes, an unspecified amount of yeast (a source of all essential amino acids)