Recently I started doing 72h salt-water only fasting once a month and my experience is that I get somewhat dizzy after 24h in the evening, but after a sleep I can perform normally without any need to eat, even including interval training and extreme uphill biking (slightly salty water is important here). This holds also for the 3rd day, again, absolutely no need to eat. Motivation were recent articles suggesting rejuvenation of one's immunity after 72h and subsequent feeding - will see if it works.

However, I believe I can do that only after I went on low-carb/no-carb diet for the past 2 years - I remember feeling horrible the first month on this diet with what you could name as "brain fog" most of the days - I guess at that point my body was learning how to survive on low carbs.

Anyway, fast forward, I am now far stronger, calmer (also due to 5 minute cold showers every day) and have brain clarity I haven't had in the past 10 years, which brings relax to what would most other people take as very stressful work, as the mind can perform and focus better and faster.

http://en.wikipedia.org/wiki/Ketone_bodies#Uses_in_the_heart...