What is high in protein, low in fat, cholesterol, calories and price—and hasn't a trace of hormones, pesticides or other gunk? It's tofu (or soybean curd), which is being hailed as the diet food of the '80s by such health nuts as Doris Day, Cheryl Tiegs, Bill Walton and Gloria Swanson.

Although America is the largest producer of soybeans in the world, tofu was virtually unknown in this country until the early 1970s, except to Orientals. (It has been a staple of Chinese and Japanese diets for centuries.) Now tofu is manufactured by more than 170 shops and soy dairies around the U.S. and sold in slabs floating in water-filled tubs at most health stores and even many supermarkets. Credit for the Americanization of tofu goes largely to California-born William Shurtleff, 39, who with his Japanese wife, Akiko Aoyagi, 30, wrote The Book of Tofu (Ballantine Books, $2.95), the bible of devotees.

"Tofu is not a new idea, but it is time for America to discover it," says Shurtleff. An eight-ounce serving contains only 147 calories, one-half as many as four eggs and one-third that of hamburger. "Tofu," says Shurtleff, "has the lowest ratio of calories to protein found in any plant food." Made from soybean milk that is curdled, compressed and then cooled, the pure white substance tastes as bland as it looks. But tofu picks up the flavor of most seasonings such as soy sauce and garlic and can be fried into hamburgers, scrambled into omelets and grilled, baked and broiled into other dishes.

Shurtleff does not expect tofu to replace yogurt, once an exotic substance itself, in the hearts and stomachs of Americans. But he would like tofu to "duplicate yogurt's success story. I would be happy," says Shurtleff, an ardent vegetarian, "if tofu replaced 30 percent of the meat in American diets."

The son of a retired construction company owner and a housewife, Shurtleff graduated from Stanford University in 1963 with honors degrees in humanities and physics and engineering. After spending two years in the Peace Corps in Nigeria, he returned to California, where he became interested in health and nutrition while working as a cook at the Tassajara Zen Mountain Center.

In 1971 he decided to continue his study of meditation in Kyoto, Japan. As a financially strapped student, he found he was able to feed himself for about 30 cents a day by building his meals around tofu. Shurtleff was introduced to Akiko, then a Tokyo fashion designer, by her sister. Akiko soon began to cook all her favorite tofu recipes for him, although at the time she could not comprehend his fascination with the food. "It is so common in Japan," she says, "like bread or water." The idea for the book came after a friend took them to one of Tokyo's haute cuisine tofu restaurants (the 12-course, all-tofu meal cost $2.80 per person). During four years of research in Japan, Shurtleff says, "We realized that we had stumbled onto a gold mine."

Their book, in its ninth printing, has sold more than 250,000 copies. Based on its success, the couple went on to write equally comprehensive books on two fermented soyfoods—miso and tempeh—as well as Tofu and Soymilk Production, a guide to those interested in setting up their own plants.

Shurtleff sees the acceptance of tofu in America as a major step toward his ultimate goal—introducing soyfoods as a practical, low-cost source of protein for underdeveloped countries. "I'm not doing this just for overweight people," he says. "They can take care of themselves. I want soyfood to be the most important source of protein in the world."