Refeeding syndrome can be a problem. I recently had a two week fast that left me deathly ill but also I've done a 3 month fast and that was okay till the end when I became too dizzy to stand. In that one I did occasionally drink a sugary drink and took multivitamins. I've talked a woman who did a four month fast but she ended up needing hospitalization.

Although I get your point that most Americans are fat enough to not immediately starve.