Guy Richard Godfrey Mackarness (17 August 1916 – 18 March 1996)[1] was a British psychiatrist and low-carbohydrate diet writer.

[4] Mackarness was an advocate of clinical ecology and was influenced by the research of Theron Randolph on food allergies.

[4] Mackarness met Theron Randolph in the 1950s and applied his methods to treat patients with mental illness at Basingstoke Hospital.

[7] Mackarness was a founding member of the Clinical Ecology Group, which later became the British Society for Allergy and Environmental Medicine.

[5] Dietitian Margaret A. Ohlson negatively reviewed Eat Fat and Grow Slim, describing it as "another book on diet, based on a minimum of fact but supported by many chapters of what can only be described as propaganda based on a badly digested series of half truths and some outright errors".

[15] In the book Mackarness listed four doctors who had used a Stone Age type diet to treat their patients.

They were: Ray Lawson, a surgeon from Montreal, Alfred W. Pennington of New Jersey, George L. Thorpe of Wichita, and Blake F. Donaldson of New York.