Doesn't need to be that extreme. A scot famously fasted for over a year, only taking a toilet trip once every fifty days or so.  https://coach.nine.com.au/2016/04/06/17/01/the-mystery-behind-the-207kg-scotsman-who-fasted-for-over-a-year the research paper itself https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2495396/ does kind of make the point quite clearly that it's calories in, calories out.