He believes that fasting cures a lot of illnesses and that the reason some people didn't experience a cure was because their fast was shorter than 7 days. From his description, those extended fasts put a person in starvation mode, and their gaunt appearance alarms people around them.
"I believe that when the glad tidings of its miracles have reached the people it will lead to the throwing of 90 per cent of our present materia medica into the wastebasket." (Page 25)
We may have replaced 90% of the medical knowledge and practice in the 100 odd years since it was written, but another century will likely find another 90% change is medical belief and practice. There is still an awfully lot that we don't know.
"Following is the complete list of diseases benefited — 45 of the cases having been diagnosed by physicians: indigestion (usually associated with nervousness), 27; rheumatism, 5; colds, 8; tuberculosis, 4; constipation, 14; poor circulation, 3; headaches, 5; anemia, 3; scrofula, 1; bronchial trouble, 5; syphilis, 1; liver trouble, 5; general debility, 5; chills and fever, 1; blood poisoning, 1; ulcerated leg, 1; neurasthenia, 6; locomotor ataxia, 1; sciatica, 1; asthma, 2; excess of uric acid, 1; epilepsy, 1; pleurisy, 1; impaction of bowels, I; eczema, 2; catarrh, 6; appendicitis, 3; valvular disease of heart, 1; insomnia, 1; gas poisoning, 1; grippe, 1; cancer, 1." (Page 56)
For some of those diseases, I can imagine a mechanism by which fasting might effect a cure. For others? I recall that I was once acquainted with a David Taysum, a cancer researcher, who said if he ever got cancer he would totally eliminate vitamin C from his diet because cells, including cancer cells can't divide without vitamin C. He had conducted experiments upon mice (or was it rats), who lived to a normal life expectancy on the vitamin C free diet. He said he could recover from scurvy a lot of times. Perhaps scurvy would be less debilitating than chemotherapy. It would also be less expensive. For years I watched the literature to see if anyone else would advocate that tactic. I never saw a peep.
Some foods "should never be used, and that those who use them can never be as well as they would be without them. Such foods are all that contain alcohol or vinegar; all that contain cane sugar; all that contain white flour in any one of its thousand alluring forms of bread, crackers, pie, cake, and puddings; and all foods that have been fried — by which I mean cooked with grease, whether that grease be lard, or butter, or eggs or milk. It is my conviction that one should bar these things at the outset, and admit of no exceptions." (Page 75)
He went through a lot of diets, and found that what works for one person might not work for another. What worked for him at one time didn't work later on.
He experienced, and other people taking his advice experienced large weight swings both up and down. Current medical knowledge says that weight swings up and down are not healthy.
Many of the letters from people included in this book, learned of his recommendations from an article he wrote "Starving for Health's Sake," in the Cosmopolitan magazine probably early 1910. Another article of his was "The Truth about Fasting" in August 1910 Physical Culture. Both of those articles came out not long before this book was published in 1911. The book sold well. (That seems so similar to today's practice of authors making the rounds in magazine and podcasts drumming up support for their new book.)
It is interesting to read how differently disease was described 100 years ago, and especially the total lack of consensus on the proper treatment. Hmm, We don't have much consensus nowadays either. Although the words for common diseases are not the same, yet I notice that the approach of those who don't find conventional medicine satisfying, is much the same today as it was then. Perhaps the world hasn't change that much after all.