The Fast-5 protocol is quite simple. You fast every day for 19 hours that leaves a 5-hour window within which you can eat. Fast-5 does not specify any type of diet or eating program. It centers mainly on meal timing.
The Fast-5 book does a good job describing the two major forms of hunger and how they relate to this style of eating. Limbic hunger is related to overeating. The limbic system is the primitive part of the brain, driving emotion and memory. The book states that limbic hunger is the drive that ancient humans (hunter-gatherers) felt that drove them to eat as much as possible while food was available (to store as fat for times of scarcity). Limbic hunger would have been a useful survival tool in times when food was more difficult to come by. Now, however, that food is abundant, we need to watch out for limbic hunger. When fasting, we are not engaging our limbic hunger system,
Instead, we feel somatic hunger. The book defines this hunger as the “sensation of discomfort in the stomach that is commonly called hunger or hunger pangs” (page 21). This sensation is the result of many, complicated hormonal interactions. It can be unpleasant, but hardly debilitating (which doesn’t make evolutionary sense – if hunger knocked you out, you would surely die of it). The book emphasizes, and I agree, that as you continue to practice fasting, somatic hunger sensations become less and less intense/distracting.
For more information on this approach (and to download the free ebook), check out the Fast-5 website.