Why isn't Mitch Snyder writing a diet book? With half the American people trying to shed weight on any given day, it's time.

Snyder is the nation's premier weight watcher. His most recent fast -- which ended after 33 days on water only when the Reagan administration promised to give $5 million to renovate Washington's largest homeless shelter -- left him nearly 50 pounds lighter. The president had reneged on a 1984 promise to turn the shelter into "a model" facility, a pledge Snyder had fasted 51 days on water to extract. The latest fast meant that as Snyder kept losing weight the administration kept losing face. It gave in before Snyder gave out.

With another $2.5 million needed for a proper renovation, Snyder is talking of appealing to friends in Hollywood to raise the money. They are filming a movie of his life and support his work. The appeal isn't needed. Snyder should take to a typewriter and spin out "Mitch Snyder's High-Stakes Low-Weight Diet Book." It would bring in royalties as surely as previous waistland best-sellers: the Scarsdale diet, the Pritikin diet, the Beverly Hills diet, the high-fiber diet, the low-fat diet.

Not to deny a calorie of credibility to any of the authors of those books, but Snyder is no mere theorist. Since 1978, he has shown the nation six times the guaranteed ways of thinning. He has fasted 42 days, 64 days, 30 days, 51 days and 33 days, all on water only. Once he went 11 days on nothing. In all, that has been a shedding of between 250 and 300 pounds for someone whose normal weight is about 150 pounds. The National Enquirer, which runs weight-loss tales on its front page as often as Pravda denounces American imperialists, would have the ultimate headline: "How Mitch Lost Twice as Much as He Weighed."

On talk shows, Snyder might need coaching in the art of banter. "Don't you go overboard?" he might be asked by Phil or Johnny. In a 15-minute answer, Snyder would say, no, he is a moderate, although something of an emphatic one. Phil or Johnny would then do the book promo and ask Mitch for a story or two on how he once took tea with Mrs. George Bush at the vice-president's mansion or how he felt when he forced Reagan to back down after the second fast. If Carl Sagan can gab about nuclear winter on the "Tonight" show, Mitch Snyder can talk about life on the grates.

Should lessons be needed on keeping it light, there is the example of America's other world-class faster, Dick Gregory. In the early 1970s, he went for more than a year without eating any solid food. During that time, he was running between five and 10 miles a day. In "Farewell to Food," a chapter in "Up From Nigger," Gregory shows how to appeal to a public eager for new taste thrills. He tells of getting a starvation high from his fasts: "At the end of three weeks, the body begins consuming itself, thoroughly ridding itself of stored-up poisons which have been accumulating for a lifetime. And there is an accompanying resurgence of energy."

At the end of the fast -- which the word breakfast, as in break-the-fast originally meant -- there are more thrills: "A person must gradually return to eating by remaining on a diet of juice one day for every ten days of fasting. During the fast, the villi in the stomach have gone to sleep, and they must be awakened gradually. But that first sip of juice is a real trip! You can feel those stomach villi waking up. You can almost hear the stomach sending a message to the brain, "Hey, he's eating again. Get ready. Here it comes!" You close your eyes, and it seems like every star in the heavens comes down to introduce itself to you personally. It is Mother Nature's trip, a real mind-blowing experience."

On a full heart and empty stomach, Gregory wrote "Dick Gregory's Natural Diet for Folks Who Eat." It is a nutritious stew of a book, but nothing that couldn't be surpassed by Mitch Snyder's. Who, if not Snyder, knows as much about the cooking of quality soup? Anyone who has been to his soup kitchen knows that, except for the absence of minced parsley garnishings and garlic croutons over the top, what he ladles out is the equal of any soup du jour served in the best of Washington restaurants. Or better. Snyder's unwatery soup is thick with vegetables and deep with rice or whatever else came in that day from the suburbs. If Edwin Meese had Snyder's soup kitchen in mind, he was right: the poor are there by choice, for the delicious soup.