I tried this in the past, I absolutely could not last past 36 hours, all I could think about was food. The easier way for me was to do the Atkins fat fast, which basically 800 calories of 90% fat for a few days to kick myself into ketosis. I’m not sure why I needed with a capital “N” to have food when I tried a “hard” fast. It could be a mental block, as a child I was never allowed to skip a meal, even if I only ate a little bit. Weight didn’t become an issue until post 30 :)