EARLY findings from the most comprehensive large study ever undertaken of the relationship between diet and the risk of developing disease are challenging much of American dietary dogma. The study, being conducted in China, paints a bold portrait of a plant-based eating plan that is more likely to promote health than disease.

The study can be considered the Grand Prix of epidemiology. Sixty-five hundred Chinese have each contributed 367 facts about their eating and other habits that could ultimately help them and Americans preserve their health and prolong their lives. The data alone fill a volume of 920 pages, to be published next month by Cornell University Press. Among the first tantalizing findings are these: Obesity is related more to what people eat than how much. Adjusted for height, the Chinese consume 20 percent more calories than Americans do, but Americans are 25 percent fatter. The main dietary differences are fat and starch. The Chinese eat only a third the amount of fat Americans do, while eating twice the starch. The body readily stores fat but expends a larger proportion of the carbohydrates consumed as heat. Some of the differences may be attributable to exercise. The varying levels of physical activity among the Chinese were measured, but the data have not yet been analyzed.

Reducing dietary fat to less than 30 percent of calories, as is currently recommended for Americans, may not be enough to curb the risk of heart disease and cancer. To make a significant impact, the Chinese data imply, a maximum of 20 percent of calories from fat - and preferably only 10 to 15 percent - should be consumed.

Eating a lot of protein, especially animal protein, is also linked to chronic disease. Americans consume a third more protein than the Chinese do, and 70 percent of American protein comes from animals, while only 7 percent of Chinese protein does. Those Chinese who eat the most protein, and especially the most animal protein, also have the highest rates of the ''diseases of affluence'' like heart disease, cancer and diabetes.