We know that full-on fasting - such as ultra-low calorie diets, or low-cal juice cleanses — tend to lower the metabolism and bring down the metabolic set-point, especially in people with thyroid issues such as hypothyroidism.
But research has shown that short-term fasting does not have this permanent impact on metabolism, while potentially offering a number of other benefits.
A 2013 study from the British Journal of Diabetes and Vascular Disease found that IF diets can help combat diabetes and heart disease, lowering blood pressure, reducing insulin levels, and resulting in weight loss.
More recently, Yale School of Medicine researchers discovered that a compound produced by the body when fasting can actually block responses that are involved in various inflammatory disorders.
Beta-hydroxybutyrate (BHB) is produced when fasting, restricting calories, or on a ultra-low carbohydrate ketogenic diet. BHB inhibits the production of a protein that plays a key role in inflammation in autoimmune disease, type 2 diabetes, Alzheimer's disease, and a variety of other inflammatory disorders.
If you're interested in how to implement an IF diet, you may want to look at Dr. Michael Mosley's book, The Fast Diet — which outlines a 5/2 plan — or Dr. James B. Johnson's Alternate-Day Diet Revised - which features a day on, day off approach.
James E. Brown, Michael Mosley and Sarah Aldred. Intermittent fasting: a dietary intervention for prevention of diabetes and cardiovascular disease? British Journal of Diabetes and Vascular Disease, April 2013
Yun-Hee Youm, Kim Y Nguyen, Ryan W Grant, Emily L Goldberg, Monica Bodogai, Dongin Kim, Dominic D'Agostino, Noah Planavsky, Christopher Lupfer, Thirumala D Kanneganti, Seokwon Kang, Tamas L Horvath, Tarek M Fahmy, Peter A Crawford, Arya Biragyn, Emad Alnemri, Vishwa Deep Dixit. The ketone metabolite β-hydroxybutyrate blocks NLRP3 inflammasome-mediated inflammatory disease. Nature Medicine, 2015; DOI: 10.1038/nm.3804