I get so many questions about this that I thought I would give a quick summary and guide to my approach to intermittent fasting (IF). I am not recommending this, it is what I do and find works for me. Discuss this with your doctor, if your doctor knows anything about the subject. I find it is rare to find one who knows the subject. IF is safer than crash dieting or even than most diets that are out there, but it is really your choice.
1. The research has not established the benefits of IF conclusively. There are many studies by now, most of them done on rats (poor lab creatures that seem to have only a bit in common with our species or even the wild species). One on humans confirms the rat research. There are doubtless others by now.
2. Wild animals do IF. They cannot avoid it. So did wild humans, when they lived more like wild animals than they do now. So do I.
3. It is well-grounded in the evolutionary evidence as well. My study on "Why We Get Fat" at the Research link above reviews some of the papers on this subject and gives a detailed statistical model on paleolithic energy expenditure and intake. I find that the energy landscape (peaks and valleys of energy balance) is rugged like a fractal. There were deep energy excursions into stored energy and other periods of great abundance. None of this was chronic. It was all episodic, random with a seasonal component, and highly intermittent.
4. About one third of the time our ancestors were in negative energy balance. This is about what I strive for over a period of about a month. You can eat sparingly every third day, every 10 days, or every 15 days, and so on. A good model might be half of your normal intake on the "lean hunting" day and enough to make up for that on the other two days in a three day cycle. But, again, there should be no routine cycles; it should be randomized.
5. The test of effectiveness is your body composition and your insulin sensitivity or serum insulin level. If your blood insulin level does not respond and decline, then you haven't got it.
6. Your total energy intake needn't decline, it should just be made more random. Research does confirm this point on rats and humans (studies are cited in earlier posts).
7. Stay active on your "hungry" day. This signals a GH response, which conserves your protein stores and burns fat. Don't jog or run any distance, but do walk. You do not want to metabolize protein by running very far. A sprint burst - walk form of playful walk is excellent.
8. The hunger ought to hurt a little bit in order to evoke a mild stress response. It is theorized in the research on IF that by triggering an acute, but not long-lasting, stress response IF maintains this essential genetic and metabolic response for really stressful incidents.
9. It is also theorized that IF turns on genes that maintain and repair tissues and some of these genes are now being discovered. The evolutionary element is that when you are hungry you will not successfully reproduce so the genes switch to maintenance programs to keep you alive until you can reproduce later when times are better.
10. You will generally be more active when you are a bit hungry. This seems to be an evolutionary program that makes you look for food when you are hungry. When you are full, you become inactive.
11. All this will work better if you have elevated your energy expenditures into the evolutionary range. This seems to be the range to which the active genotype is adapted. Twice basal metabolic rate is about right for an average energy expenditure.
12. Similarly, if you are active your appetite will become a better guide to your nutritional requirements.
13. Many people may have damaged their hypothalmus with excess food intake and neurotoxins (some doctors think or claim that aspartame attacks the hypothalmus and contributes to obesity but I am not so sure). Giving the hypothalmus some "rest" instead of a constant assault of energy dense food may help to restore its function. Like other organs, the hypothalmus responds to pulses of inputs and puts out pulsate responses. The information in the signal is diminished through neural fatigue when it is constant. Try it yourself with a scent; after trying three colognes at the department store, you won't be able to tell one from another.
14. You will not become weak and get sick. Obesity is far more dangerous to your health and your immune system. In fact, prisoners who are underfed in prisoner of war camps get sick less often than people outside the camps. Or even their guards. Only when they lose so much muscle mass that their organs begin to shrink and decline are they likely to become ill.
15. Finally, IF will help to alter your attitude toward food. Living through a hungry day will make you look at food and how people eat in a different way. It helps you realize that you don't need all that food to feel good. You will see that being, like Cassius, "lean and hungry" is the way to live.
Mom wasn't right when she said eat to keep your strength or you'll get sick. Quite the opposite, particularly in the present environment of energy excess.
One of the most difficult parts of IF or Evolutionary Fitness eating is the social pressure and conventions. Restaurants are particularly difficult. If you eat out often, you are bombarded by cues that evoke your evolved senses and behavioral patterns. These are patterns evolved during a time of uncertain availability of food. If it is in front of you, you will eat it. Our ancestors had to do it. We don't. So, be careful what you let your server put in front of you. Send the bread back or ask that it not be brought to the table. Ask for fresh vegetables in place of potatoes, rice, beans, and other dense energy foods. Or just skip the side dishes and eat the meat with the salad (no croutons). Or, just skip that meal altogether.