In one of great social changes in the nation's history, Americans have drastically changed their lives over the past 20 years to try to be healthier.

Surgeon General Julius Richmond, the Carter administration's top medical officer, documented the huge -- and surprising -- changes in eating, smoking and exercise habits yesterday as he presented the administration's last annual report on the country's health.

The changes show, he said, that people will rapidly change their health habits in response to new information.

Many medical authorities have called this impossible, and some still say that. But it is possible, Richmond said, and in fact "the American public has a growing understanding" of what affects health, and people are placing increasing emphasis on protecting their health.

There is still "far to go" in preventing illness and there are many minuses as well as pluses in the American health picture, Richmond emphasized. These remain challenges to a new administration.

But the assistant secretary for health in the Carter Department of Health and Human Services said "it is clear our prevention initiatives," both recent efforts and those going back many years, "are bearing fruit."

Since the 1960s or early 1970s, he reported:

Americans are consuming 21 percent less milk and cream, 28 percent less butter and 10 percent fewer eggs in a vigorous attempt to eat a less fatty, less cholesterol-laden diet. Most doctors advocate this to try to avoid heart disease, though they advocate only lower-fat milk, not no milk.

The number of adult men (20 and over) who smoke has dropped 28 percent, the number of adult women 13 percent and even the number of teenage boys by 20 percent. Only the number of teen-age girl smokers has increased -- by 51 percent since 1968 -- and this increase may be leveling off.

The number of adults who exercise is up 92 percent.

Those who ignore their high blood pressure or fail to have it diagnosed and treated dropped 10 percent by 1974 and has certainly dropped more since. Uncontrolled high blood pressure often causes strokes or heart attacks.

Only drinking habits have remained largely unchanged, as measured by the number who take two or more alcoholic drinks daily. People could be influenced here by growing evidence that one or two mid drinks daily may help prevent heart attacks.

Many people, especially teen-agers and young adults, are drinking and driving, however. This, Richmond said, has helped cause a reversal in what seemed for a time to be a healthy downturn in highway deaths.

Even worse, says Richmond's report, the overall "death rate for the 40 million adolescents and young adults -- unlike that of Americans in every other age group -- is higher today than it was 20 years ago," though it has declined some in the past decade.

The major reasons: the auto crashes (the main cause of death among young white males), homicides (the main cause among young black males), alcohol and drug abuse and a growing number of suicides (cause of more than one youthful death in every 10).

The young are also afflicted by sexually transmitted diseases and unwanted pregnancies. The diseases cause much sterility among young women or, if they become pregnant, much pneumonia, death and retardation in their babies.

Every year a million teen-age girls, two out of three unmarried, become pregnant. Their physical immaturity, poor nutrition and failure to seek medical care produce many babies with poor chances, said Richmond, and the mothers' disrupted schooling and dependence on welfare add up to a grave national problem.

Probably because of better health habits and better medical care, the age-adjusted death rate from heart disease dropped 20 percent between 1970 and 1978. The death rate from strokes dropped a third between 1970 and 1977.

Cancer mortality has dropped for those under 45, and recently began dropping for those between 45 and 49, though women are getting more and more lung cancer as a result of smoking.

Also, largely as a result of federal programs, there has been "a marked trend toward equity of access to medical care regardless of income," with the number of people seeing a doctor within the past two years up in every group, "with the greatest increase for the poor."