Perhaps your fox can perform that trick, but that is not the way of Zen. My miracle is that when I feel hungry I eat, and when I feel thirsty I drink.

-- Bankei

Partake of food at regular intervals. Eat with moderation and never to the point of satisfaction.

-- Soyen Shaku

First of all, a heads-up: I'm using the term Zen diet here only as as shorthand for "the diet of monks at Japanese Zen monasteries". This is not an officially sanctioned The One and Only True Zen Diet(TM), and it is emphatically not the same as Zen macrobiotics, an extreme macrobiotic diet loosely based on these practices and responsible for a number of deaths. Also, while in the West this is associated with Zen Buddhism, almost all (traditional) Buddhist monasteries will follow some variation of this.

The Virtues

Over the centuries, Buddhist monasteries in Japan have developed a food regimen suited to their way of life. This diet is:
  • vegetarian (or to be more precise vegan), because Japanese interpretations of Buddhism forbid killing animals
    • most monasteries use the strict definition, forbidding all meat, fish and dairy products; some go further and ban eg. onions as well
  • healthy, so that monks stay fit, and
  • simple, so that everything required can be produced on the temple grounds.
From the modern Western point of view, it is also:
  • low-fat, being strictly vegetarian and avoiding the use of oil in cooking
  • low-calorie, largely due to the small portions
  • cheap, since the main ingredients rice and miso are, and
  • boring, due to the simplicity and lack of variation in the diet.

The Regimen

The Zen diet repeats the same cycle every day. A day's meals always consist of the following:

Breakfast (shoujiki, 小食)

  • Rice gruel
    • known in Japanese as okayu if made from scratch and zosui if made from cooked rice
    • see congee for recipes and serving suggestions
  • Pickles (Jp. tsukemono)
  • Salt
Lunch (chuujiki, 昼食) Medicine Stone (yakuseki, 薬石)
  • Miso soup
  • Rice
  • Pickles
  • Two side dishes
In the bad old days, there were only two meals a day and the yakuseki was an actual hot stone placed on the stomach to quell hunger. This turned into a small bite of food and is nowadays a full-fledged meal, usually the biggest of the day at that. However, in some monasteries yakuseki is only a rice gruel containing the leftovers from lunch, and fresh rice is served in the morning.

The side dishes can be prepared in any way and from any (vegetarian) ingredients, but deep-frying is usually frowned upon. Quantities are also kept small, the bulk of calories comes from the rice. Emphasis is also placed on being frugal, so that as much as possible of the ingredients is used: for example, vegetable peelings are often recycled as pickles.

The usual accompanying drink is green tea, downed in copious quantities.

The Practice

In monasteries, the meal is accompanied by a heavy amount of ritual, including chanting the gokan-no-ge before eating and maintaining complete silence through the (communal) meal. The traditional way of washing your bowl is to leave one pickle uneaten, pour in hot water, wipe the bowl with the pickle (held with your chopsticks) and finally eat the pickle. Presto -- no waste! More useful in lay practice may be keeping the habit of eating slowly, one mouthful at a time, which both allows you to concentrate on the taste and stops you from overeating.

If followed strictly, this diet will result in weight loss for almost all adults, stabilizing at some point within a year or so when extra reserves have been burned off. Physical exercise is an obligatory accompaniment to prevent loss of muscle tissue.

Other than being vegetarian, this is in fact very close to the traditional Japanese diet outside the monasteries as well, and in my impoverished student days I lived on this for about a year (excluding the occasional weekend splurge). After getting over the initial hunger pangs during the first month or so, I was quite happy and lost over 10 kg without really even noticing it, this off a frame that was never terribly horizontally enhanced in the first place. Ever since -- including at time of writing -- I've returned to the Zen diet whenever I've felt myself slipping out of shape.

Some Tips

There is no need to take the regimen above as gospel, it's just the way the Japanese do it. Instead of three meals of rice a day, try porridge for breakfast, noodles/pasta for lunch, potatoes for dinner, bread, whatever. It's all good. Also, some monasteries still maintain the habit of mixing barley into the rice, but should you try this you'll soon find out why the rest of Japan abandoned this poverty-induced habit as soon as they could.

Miso soup, on the other hand, is a very good idea because it supplies the bulk of your protein. Tofu should also be used liberally for the same reason.

Your biggest problem will be calcium, since vegan diets without dairy products and fish tends to fall well short of the recommended amounts. See Vegan Calcium for some suggested workarounds, or if you have no moral objections to squeezing cow tits, add dairy products back to your diet. (I, for one, cannot envision a life without yogurt!) Also keep a bit of an eye on vitamin C and B12.

And because the Buddha does recommend the Middle Way instead of the extremes of ascetism and hedonism, I see nothing wrong with adding a little dessert to your medicine stone. In Japan this was especially easy as all convenience stores sell various types of Japanese confectionery in little Y100 packs, which are perfect for this and also avoid the temptation of "Just one more cookie/slice/...". Or, perhaps even better yet, eat fruit (hideously expensive in Japan and thus avoided, but less so elsewhere).

The Non-Simple, Non-Boring, High-Expense Version

An elaborated, ceremonial version of temple fare called shojin ryori (精進料理) also exists, and many temples double as shojin ryori restaurants. Should you stay overnight at a temple, the food you will be offered is almost always shojin ryori. Essentially vegetarian kaiseki, you will rarely get away with less than Y2000 for a shojin ryori meal and prices go up into the stratosphere. The shojin ryori banquet I treated myself and a friend to on my 21st birthday remain the best meal I have ever eaten -- even if it did cost me half a month's salary. (But that's a story for another node.)

References

http://web.kyoto-inet.or.jp/org/saikosha/English/zen4-E.html
http://www3.kannet.ne.jp/~gengen/shojin/english/eng1-4.htm
http://www.bccancer.bc.ca/pg_g_05.asp?PageID=1699&ParentID=2