Americans have become more casual about cholesterol and good nutrition in general, according to a Prevention magazine survey that shows a decline in health practices for the first time in five years.

"We don't know whether this year's score represents a temporary setback on the way to further progress or whether it is an early warning sign," said Thomas Dybdahl, director of the private publication's so-called prevention index.The 1988 score is 64.8 out of a possible 100 - down from 65.2 last year, but still 3.3 points above 1984 when the first index was compiled.

The latest sampling, issued Thursday, found that more Americans are using smoke detectors and getting strenuous exercise, but falling behind in the area of good nutrition.

For example, the number saying they are making efforts to limit high-cholesterol foods fell from 46 percent in the 1987 report to 42 percent, and there also was a four-point decline in the number who said they make an effort to get adequate vitamins and minerals - from 63 percent to 59 percent.

"One clear message from this year's survey is that the spread of good health habits is not automatic," Dybdahl said in remarks prepared for a news conference.

"People may know more than ever about good health, but they may not be acting on what they know. In key areas such as diet, exercise and weight control, there is still much room for improvement."

The sharpest gains from the 1984 report were the 57 percent who reported wearing seat belts now compared with 19 percent then and the 82 percent who say they have a smoke detector in their home compared with 67 percent then.

The biggest drop was in those who said they obey the speed limit - from 61 percent in 1984 to 55 percent.