IT TAKES a mighty shove to change a population's eating habits. And an iron chain of evidence to corral nutritional scientists into agreeing on anything. That's why it's remarkable to see the ever-growing public and scientific trend against high-fat foods. After some forceful kicks from former senator George McGovern and Dr. Mark Hegsted, the Agriculture Department's top nutritionist, it seems we're awakening to the fact that some of our most cherished foods -- notably dairy and meat products -- are doing us in.

Even if you don't buy the cholesterol-is-bad-for-you argument, there's other evidence against fatty foods. Perhaps the most damning: You gain the most weight from fat. One gram of fat contains 9 calories, nearly twice as many as the four calories in a gram of protein or carbohydrate, even sugar. Nevertheless, most of us know distressingly little about fatty foods. And our desire to know is dampened by the fact that many high-fat foods carry a mystique of Americana or wholesomeness: milk, cheese, butter, beefsteak, hot dogs.

Enter into the picture a new book that is not merely a tirade against such foods. "Jack Sprat's Legacy" (Marek, $12.95) by Patricia Hausman, a nutritionist formerly with the Center for Science in the Public Interest. It is the first comprehensive, sobering, thought-provoking book about fat in foods. This is not an ordinary book. It is one of scientific sophistication, well-written and persuasive, with much practical information on how to save yourself, which is necessary, the author says, because your government won't do it for you. The subtitle is: "The Science and Politics of Fat and Cholesterol".

First, there is a fascinating historical look at why dietary fat is regarded as so valuable in our society. Perversely, the greater the fat content of milk in most states, the higher its market price: The folly of an affluent nation killing itself on fat.

Hausman shows no conflict in her mind over the dangers of cholesterol and fat. She cites numerous studies, dating from 1908, showing that high-saturated fat and cholesterol diets clog the arteries. A famous study at the University of Chicago found six times more atherosclerosis in monkeys fed a typical American high-fat diet than in monkeys fed a low-fat diet. One point Hausman makes is sure to surprise many: Some high-saturated, fatty foods can hype the blood cholesterol more than some high-cholesterol foods. She stresses that it is not just one kind of fat, but a combination of cholesterol, saturated fats (mainly in meat products) and polyunsaturated fats (mainly of vegetable origin) that determines the blood-cholesterol level. She figures, for example, that three ounces of shrimp (exceedingly high in cholesterol) will not raise your cholesterol as much as three ounces of veal, beef or pork. One egg yolk, she says, raises your cholesterol about the same as a cup of ice cream or two ounces of swiss cheese.

She also claims that sneaky public relations maneuvers by the meat and dairy industries have tried to obscure the saturated-fat danger by putting the entire blame on high-cholesterol foods. To illustrate, Hausman cites a news release from the National Livestock and Meat Board: "Steaks, roasts and chops are not full of cholesterol, as many have come to believe . . . meat is lower in cholesterol than a number of poultry, fish and seafood items." True, but there is no mention that saturated fats found in meat may be worse for your blood cholesterol.

But this is not just a book on the dangers of animal fats. Hausman, herself a fallen-away vegetarian, comes down hard on all kinds of fats including vegetable oils. That judgment encompasses the staple of the health food mystique, the avocado. Despite those ads showing Angie Dickinson keeping thin and healthy on avocados, they are high in fat and calories. One cup of California avocado pure'e has about 40 grams of fat and 400 calories; 90 percent of the avocado's calories come from fat.

Nor, should you assume that if butter is bad, margarine and vegetable oils are good. Some new studies suggest that trans-fatty acids in margarine may also cause atherosclerosis, although Hausman rightly reports that many scientists are still skeptical about the danger. And experts do not think it is wise to replace all your dairy fats with polyunsaturates such as corn oil. Hausman recommends the more neutral olive oil, noting that the Greeks, who use lots of it, have "remarkably low heart disease rates."

Moreover, several studies link both animal and vegetable fats to higher rates of cancer.

Although Hausman makes a dismayingly thorough case against fatty foods, hers is not a doomsday book. She does not say: Never eat fatty foods. She advises eating less of them. She does not ask you to substitute low-nutrient foods such as non-dairy whipped toppings and creamers for whole milk and cream. (Incidentally, these non-dairy replacements often contain palm or coconut oil, both of which are as highly saturated as lard.) She does not advocate synthetic eggs over real ones. She stresses you can eat so-called "wholesome" foods made more wholesome by removing the fat. Substituting skim or low-fat milk for whole milk still provides the same nutrients, including protein, minus the fat. She also points out that many low-fat foods are exceedingly nutritious. In one instance, she compares the "incredible, edible pea" with an egg, and the pea -- that is three-quarters of a cup of them -- comes out ahead.

The fat-is-dangerous-theory has its detractors, and not only among industry. Dedicated consumer advocates such as author Beatrice Trum Hunter ("The Great Nutrition Robbery") and Ruth Desmond, head of the Federation of Homemakers, fear that substituting imitation foods may be far worse than eating high-fat dairy foods. Noted nutritionist Dr. Roger Williams sees no defects in eggs. The respected Consumers Union doesn't believe the cholesterol issue is settled. Certainly, it would be surprising if the fat controversy did not turn out to be more scientifically complicated than anticipated. And I personally have the nagging suspicion that eggs can't be all that bad for you, perhaps because of my own childhood mystique about eggs.

But cutting back on fat seems so common-sense compelling, especially when most of us have nothing to lose but fat itself. Even though one may not agree absolutely with Hausman, she is an author to be trusted and listened to. Few who read her book can still think it is anything but lunacy to put a high premium on fat in a nation where fat-laden products pose a major health problem.