Depends on what a type of fast. There are many. Traditional fasting, no calories for a planned certain amount of certain days. There is intermittent fasting, where you fast for so many hours-not days. There is 5:2 fasting; 5 days normal cals, 2 spaced out days under 500 Cal's. There is fat fasting, where you eat 90% or more of cals only from fat & keep Cal's 1,000-1,200. Probably others I'm not familiar with.