I would like to understand the concept more!
IF is a simple concept. You basically go between 2 states: Feeding state, and the fasted state. The fasted state is longer than your feeding state. Actual time in each state varies. Some people fast for 24 hours, others do 16/8. The exact number do not matter as much.
Most people do not include non-caloric (or at least very low caloric) drinks as breaking the fast, so someone might have a cup a coffee in their fasted state and that would be okay.
if Time between 12h & 16h then EAT else PLAY
IF v1.1 - bug fix
if Time between 12h & 16h then EAT else PLAY; NO FOOD AND SOFT DRINK INCLUDING GAMES.
Do you want to know how to do it? Or what is happening in your body when you do it?
If it's the former, others have answered it already - reserve about 8ish hours of the day where you're allowed to eat and have no food or caloric drinks outside of that window. The most common is to only eat between noon and 8pm, as that give you a good chunk of your fasting while sleeping.
If you want to know what happens - here's the five-year-old explanation... Throughout your day proteins in your cell break down and become useless waste products just floating around. Normally your body will ignore them and they'll build up. But in periods of fasting, your body isn't getting any new food so it decides to recycle the damaged proteins which cleans up the waste lying around. The 16-hour fast is generally ideal because if you go longer you start to "recycle" muscle instead of just the waste products.
For me it is not worrying about eating on a fixed shedule. If I miss lunch that is OK. If I get home and dinner does not happen until 9 PM that is OK. For me it means freedom from snacking during the day and even before or after dinner.
This is an article on the leangains site about stubborn body fat, but it has very useful information when trying to understand what is happening inside your body while fasting:
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