I like telling people that I'm fasting.  The blank, quizzical stares that meet this statement are always amusing.  I mean, who fasts?  The very idea calls up images of Ghandi, political protests religious self-abnegation or radical dietary desperation.  Clearly I'm not a monk or a radical, so what the hell am I doing not eating?

Intermittent Fasting (or "Intermittent Feeding," as some are now trying to re-brand it) is nowhere near as extreme as it sounds at first, and has some solid reasoning behind it.  It isn't a diet in the traditional sense, as it makes no effort to tell you WHAT to eat, or how much to eat - simply WHEN.  Lots of folks try to overthink it, but it really boils down to one simple rule:  Eat.  Don't eat for a while.  Eat.

How long is "a while?"  Well, that's up to you.  Probably at least 16 hours, and probably not more than 24.  A couple times a week, at least, but not more than five days.  The exact timing is really a matter of personal preference and your own schedule and needs, but there is a method behind this madness of minimums and maximums.

The biggest question:  Why?

So why would you do this to yourself?  Well, it turns out some pretty interesting and beneficial things happen to your body when you stop eating for a while.  Well, the definitive source on the matter is a long article by Scott Kustes called "What Happens to Your Body When you Fast?"  It's certainly worth reading in its entirety, but I'll paraphrase some key bullet points here.

• Lipolysis.  Everyone loves burning fat, right?  Well, depending on your activity level, you'll typically burn through your body's glycogen stores in 16-18 hours.  When your body has no more glycogen to burn, it turns to gluconeogenesis (converting proteins to glycogen) and lipolysis (releasing bodyfat stores) for energy.  In other words:  you burn fat.
• Hormone manipulation.  With blood sugar at baseline, the body has no need of insulin, so those levels are minimal and insulin sensitivity is improved.  ACTH, epinephrine and glucagon (which aid in stimulating lipolysis and muscle growth) go up.  Cortisol goes up, too, but not enough to really worry about. BHB (Beta-hydroxy butyrate) -  a ketone shown to be beneficial against Parkinson's and Alzheimers, insulin resistance and free radical damage - goes up, as does hGH (Human Growth Hormone), which is primarily responsible for tissue repair and hypertrophy.
  
• Reduced inflammation.  A study of muslims fasting during Ramadan showed "significantly low" levels of IL-6, CRP and homocysteine (all markers of inflammation) during fasting, despite the inflammatory counter-effect of reduced sleep.
• Longevity and healing.  Most of the studies here are on rats and mice, since IF is still a fairly new concept to science, but the upshot is that by forcing the body to look elsewhere than the gut for energy, you encourage cellular repair - a cell will turn to its own damaged or older proteins for energy.  Not good in the long run, obviously, but when you eat again the cell will be able to use the new resources to replace the older one it consumed.  At a macro level, that leads to higher resistance to Cancer, Heart Disease, Alzheimer's, Parkinson's, and simple old age.
• Faster recovery.  As I've had it explained to me, "energy not spent on digestion can be spent on healing."  I don't know that the body's extremely complex systems and resources can be reduced to a zero-sum game, but it does make a certain amount of sense and is born out (anecdotally, at least) in practice.
• Inefficiency.  Routine is the enemy of training. The body adapts extremely well to routine, which is why we are constantly throwing new things at it in CrossFit - by keeping on our toes and always mixing it up, we provoke the greatest response in our bodies.  The same goes for diet.
  

Not too much, though

IF, like training, is a measured stimulus intended to provoke a beneficial response.  A little damage is a good thing, as it leads to supercompensation.  But, like training, you can push the stimulus too far and do more damage than you intended.  Which is why it isn't really recommended that you fast for more than 24 hours, or why you shouldn't fast every day.  By all means, mix it up - keep your body on its toes! - but know that you are, in effect, hacking your body's natural processes and that can have unintended consequences.  (Having attempted a 24-hour fast, though, I'm not really too concerned about anyone overdoing this one.)

How do I do this?

The how is up to you, but here are a couple places to start:

• Stop eating at 8pm.  Don't eat again until noon the next day.  There's 16 hours.  Try it twice a week to start, and up it to 4 days or so as you get comfortable with it.
• Have a nice big early dinner at, say, 6pm.  Then don't eat again until dinner the next day.  I wouldn't do this more than twice a week, and I'd pick a busy day with a lot of distractions - nothing stimulates hunger like boredom.
• Remember, you're supposed to eat as many calories as your current dietary needs dictate in the eating window.  IF makes caloric restriction easier, but it doesn't have to be about weight loss.  I've gained and lost weight fasting.
• As always, quality matters.  IFOC (Intermittent Fasting on Crap) will not yield the results that a diet of real, whole food.  All that energy you're diverting from digestion will instead by spent by the body on filtering, processing, and generally just trying to figure out what the hell to do with the crap you've given it.
  

Can I eat <blank>?

No.  Fasting means not eating.  No juice, no milk, no pills filled with oil, no significant calories.  Black coffee or tea is acceptable, as is lemon in water (I hear that eases hunger pains).

My own experience

I found a noticeable performance increase when I started fasting, as well as improved recovery.  When I started doing it I was interested in weight loss, and after years of trying I was finally able to get under 10% bodyfat.  I've just finished a strength cycle where I ate insane amounts of food while doing IF, and gained a good amount of muscle with only a bit of extra fat.  (Now I get to ride the caloric restriction train again!)

I have never seen fat disappear as quickly as I did with the following protocol:

• While still in a fasted state, do a brief HIIT (High-intensity Interval Training) workout.  This can be Sprint-8 (running or rowing), or just 50 burpees in hard sets of 10.  Something to spike your heart rate a few times.  This will burn through any glycogen you have remaining in your muscles after sleeping, and stimulate your body to release some fat into the bloodstream for energy.
  
• Follow this up with about thirty minutes of easy cardio - something about 60% your max heart rate, so just light jogging or some biking.  Nothing too rough - all you're doing now is burning up the fat your body released.
• By now your body will be struggling to keep up with your energy demands.  The longer you can wait until you eat, the more you'll burn, but be careful - you can bonk pretty hard.
• Do this 2-3 times a week.