To fast is to go without food. We do this every time we go to sleep and then break our fast with the first food we eat that day (breakfast). Wikipedia defines Intermittant Fasting (IF) as an umbrella term for various diets that cycle between a period of fasting and non-fasting during a defined period. Some folks take a day off food, and then a day on. Some choose to eat only during a restricted period of time during the day over the course of the fast (Time Restricted Feeding – a whole other topic). How long you choose to fast is up to you and your metabolism. Some folks have unsteady blood sugar and would need to have a little something available, others don’t need anything and won’t feel hungry even if not eating for 2 days (me – I tried it. I was a little peckish at the end of day 2).
The question of “is it safe?” can be answered easily. Of course it is fine to take a day or some time off eating. We carry hundreds of thousands of calories and store other nutrients in our bodies, so we don’t really need to eat everyday. However, to get into this subject is to open a pretty wriggly can of worms involving habits, customs, culture, food addiction and lots of potentially conflicting information. As always, the answer is a qualified “It depends on you”. There is something called a metabolic switch which after a certain time following the onset of fasting (12hrs) the body switches fuel sources and starts to metabolize “fat through fatty acid oxidation and fatty acid-derived ketones, which serve to preserve muscle mass and function. Thus, IF regimens that induce the metabolic switch have the potential to improve body composition…” . But this is not true in all bodies (of course) or for all time courses because everyone metabolizes at different rates.
So the take away here is: Fasting is not unhealthy if you don’t go overboard, but it’s not a magic bullet for weight loss because you are also decreasing your intake of important daily nutrients. Choosing a consistently healthy diet and movement habits are the most sustainable things you can do for yourself in the long run.