The scientist often quoted in the original Fast Diet Dr Krista Varady claims her ten years of research has been misrepresented by the authors and has asked all her work be taken out.
Varady's work focused on alternate day fasting, which she says is nothing like 5:2 and can't be used as evidence for its efficacy. This is worth reading:
www.healthista.com/nutrition/fasting-diets-science-research-studies-52-diets/