This [1] study was pretty thorough on different fasting lengths. They tested 1422 patients for fasting lengths between 4 and 21 days, with a maximum calorie consumption of 200-250kcal and a moderate-intensity lifestyle program.
It concludes that all fasting lengths are beneficial, and are likely going to result in a) reduction in weight and waist circumference, b) beneficial effects on blood lipids, regulation of sugar and other blood-related parameters, including lower blood sugars and higher ketone body levels, c) an increase in physical and emotional well-being and absence of hunger, d) a high probability of decrease of pre-existing health-complaints, e) very limited chance of side-effects.
To build on this post it is further beneficial to understand the importance of the interaction between clock genes and nutrient intake timing for optimal results
It concludes that all fasting lengths are beneficial, and are likely going to result in a) reduction in weight and waist circumference, b) beneficial effects on blood lipids, regulation of sugar and other blood-related parameters, including lower blood sugars and higher ketone body levels, c) an increase in physical and emotional well-being and absence of hunger, d) a high probability of decrease of pre-existing health-complaints, e) very limited chance of side-effects.
[1] https://journals.plos.org/plosone/article?id=10.1371/journal...