It is amazing just how much food reserve that even the most underweight person has.
Shelton exclaimed that he was amazed that an even an underweight person can go for 20 or more days on water and receive great benefit therefrom.
The organs of digestion are rested during a Water Fast and become much more efficient post-fast with much better digestion and assimilation thereafter.
Post fast, you will find that graduated aerobic exercise will also greatly improve digestion and assimilation so that weight-gain is much improved.
However, enervation can also be a key to a lack of weight-gain and a person remaining underweight.