Converts from junk food to health food will tell you, at the drop of a tofu grain, how they saw the light. The more graphic will describe how they saw the fat. These are the fortunate ones who have seen the waste vats of grease at the fast-food restaurants and realized that the hamburgers and fries are soaked with the heart- threatening fats they were cooked in.

Some of the converts are at the Center for Science in the Public Interest, a Washington-based health-advocacy organization supported by 70,000 members. Last week, it released the fast-food eating guide. If Americans once sought to live off the fat of the land, now they get by on the high saturated fat of the fast-food joints. The center, which has a taste for measuring the tasteless, reports that Burger King's double beef whopper contains 12 teaspoons of fat. Across the street at McDonald's, a small order of chicken McNuggets is laced with nearly twice as much beef fat as a hamburger. At Wendy's, the fat from a triple cheeseburger fills 15 teaspoons.

With these chains, as well as the others that dispense high-fat items in 60,000 fast-food eateries, America is the world's greasiest greasy spoon. It is also the nation where the swallowed fat is part of the reason that heart or blood-vessel diseases killed more than 980,000 people in 1983. The total of cancer deaths was less than half that. The American Heart Association reports in "Heart Facts" that cardiovascular diseases account for one of every two deaths in America.

The cardiologists, rescue-squad ambulance drivers and other health-care workers who are called on to save the victims of heart diseases have no chance for equal time. All they have are warnings.

In the center's "Nutrition Action Health Letter," Dr. William Castelli, the medical director of the Framingham, Mass., federally sponsored heart study, spoke about the results of the fast-food chains' appetite for advertising high-fat and high-cholesterol diets: a direct link is present between the greaseburgers and the killer diseases. In Framingham, one man in eight between ages 40 and 44 develops heart disease, a rate that increases to one in four for those over 55.

Americans who travel abroad feel a burst of home-country pride when they see a golden arch in such a place as downtown Tokyo. Castelli the cardiologist sees it as the exporting of misery: "The Japanese traditionally had a very low-fat diet, consuming only about 30 grams a day. Now that we're sending the Japanese our beef in return for cars, urban Japanese are eating 60 grams of fat a day -- Americans get about 90 -- and the heart-disease rate for city-dwelling Japanese is about four times higher than it was 35 years ago."

Since 1961, when the American Heart Association first spoke about diet and health, studies have appeared almost as often as fast-food chains have incorporated. They have advised the public of the obvious negatives and positives. Eat less saturated fat and less cholesterol. Eat more fruits and vegetables. Yet the people who want to eat fast keep getting their grease jobs at the McDonald's and Burger King.

The death rates remain high because eating habits are tied to cultural habits, which aren't changed by governmental studies. Big brother may be the national nanny, but he's not the national chef. Meat-eating is embedded as deeply in mass culture as baseball, where the staple in both box seats and bleachers is the hot dog. That it is a highly processed and chemicalized concoction of sodium chloride (salt), sodium nitrite and fat is about as troublesome to the fans as George Steinbrenner's firing another manager. A generation ago, a good family man presided over a Sunday dinner of meat and potatoes. Today he s the fast-food line for a burger and fries. It's still meat and potatoes, so his Americanism isn't under threat, only his health.

The meat, dairy and egg industries -- which Michael Jacobson of the center calls "the cholesterol lobby" -- have a history of resisting national nutritional policies. That, coupled with the advertising might of the $50-billion-a-year fast-food industry, means that the citizen who keeps to a rational diet is portrayed as the odd one. We have health-food freaks, but no one greasing his throat and stomach at the fast-food stand is called a sickness- food freak.

That's what he deserves to be labeled, just as the hamburger he's eating should be called the heart-attack special, with a side order of coronary-bypass fries.