I think chrisb1 would be the best person to answer this fully, but in the meantime I just googled Shelton and underweight and came up with
http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch31.htm
- I found one quote at the top of the page:
"The gain in weight after a fast is usually very rapid. Often it is almost as rapid as the loss during the fast. People that were formerly always underweight and emaciated, due to impaired digestion and assimilation, will become normal in weight."

Unfettered_Moth,
Mouseclicks post is quite right.
Even underweight people can fast safely to a degree and where Shelton is on record as stating that he fasted many patients who were deemed to be unsafe by other experts of fasting. He was often amazed at the ability of skinny and even grossly underweight people to fast for up to 20 days and more quite safely, and with distinct benefit.


Sheltons most comprehensive comment re' fasting while skinny or underweight is from his book " Fasting Can Save Your Life " in the Chapter: Fasting to Gain Weight.

Just one extract........
"Food and Nutrition are not synonymous. One is not nourished in proportion to the amount of food ingested, but in proportion to how much one digests and assimilates. When digestion and assimilation are impaired, overeating in an effort to put on weight defeats its purpose. Emaciation is due to impairment of health, rarely to lack of food. The emaciation is commonly proportionate to the extent of the impairment of health. What such patients need is not more food but a properly functioning detoxified system able to digest and assimilate food. Given this they have no difficulty in gaining weight".

" It should be known, also, that weak and underweight patients often stand fasting much better than strong and overweight patients".

http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch23.htm


"Real vital resistance is very rarely lowered by fasting. Temporary muscular weakness should not be classed as lowered vitality. Indeed, I have seen many cases of infection of different kinds recover completely on a fast. Take for example an advanced case of sinusitis after five or six painful operations--frontal, ethmoidal and antrum--with surgical drainage and irrigation two or three times a week, continued over a period of two to five years, with no relief or amelioration of symptoms. After almost unendurable suffering, such patients are, as a rule, THIN, and physically and mentally depressed. When they make complete recoveries after a prolonged fast, as the great majority of them do, is this not sufficient proof that fasting somehow or other raises the power of the organism to overcome infection, rather than that fasting renders them more susceptible? What is true of sinusitis is equally true of other infections, even those so situated anatomically that they cannot be surgically drained and must therefore be absorbed."--In Defense of Rational Fasting".

http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch20.htm


Regards

Chrisb1.

there is a little bit of info on this in the paul bragg book "the miracle of fasting" where he talks about how his sister gained weight after fasting.