[3][4] The book makes sensational claims of fasting curing practically all diseases, including cancer, tuberculosis, asthma, syphilis, and the common cold.

[5][6] The book was condemned in The Monthly Cyclopaedia and Medical Bulletin by gastroenterologist Anthony Bassler, who described treating many sickly patients who had followed the advice published in Sinclair's The Contemporary Review and Cosmopolitan Magazine articles.

The accompanying article in Current Literature criticized Sinclair as a "faddist pure and simple, one whose mind is obsessed by a series of notions one after another, none resting upon any basis that can be called scientific or even sensible.

"[5] Likewise, in the book Quackery: A Brief History of the Worst Ways to Cure Everything (2017), authors Lydia Kang and Nate Pedersen write, "Although modern doctors would strongly disagree with Sinclair's unsolicited medical advice, there have been some recent promising studies on the impact of fasting on mice with cancer.

"[8] Sinclair appears in T. C. Boyle's novel The Road to Wellville (1993), which is built around a historical fictionalization of John Harvey Kellogg, the inventor of Corn Flakes and the founder of the Battle Creek Sanitarium.