The number of middle-aged and older Americans who eat right, exercise and keep their weight down has declined substantially in the past two decades, a new study finds.

Using data from a large government health survey, researchers found that in 2006, only 26 percent of Americans ages 40 to 74 said they ate five or more servings of fruits and vegetables per day — down from 42 percent in 1988.

When it came to exercise, 43 percent said they worked out at least 12 times per month, versus 53 percent in 1988.

Perhaps not surprisingly, the rate of obesity went in the opposite direction, from 28 percent in 1988 to 36 percent in 2006, the researchers report in the June issue of the American Journal of Medicine.

The decline in healthy lifestyle habits is disturbing because it may translate into higher rates of diseases like type 2 diabetes and heart disease, said lead researcher Dr. Dana E. King, of the Medical University of South Carolina in Charleston.

What's more, he told Reuters Health, study participants who already had heart disease, high blood pressure or diabetes were no more likely to be adhering to a healthy lifestyle than those without the conditions.

"This tells us that many patients are not following the recommended lifestyle changes," King said, noting that this may be because many people instead rely on medication to control their health problems.

The findings are based on data from the National Health and Nutrition Examination Survey, a regularly conducted government health survey. King's team focused on surveys conducted between 1988 and 1994 and between 2001 and 2006, which together included more than 15,000 Americans between the ages of 40 and 74.

The researchers looked at rates of five lifestyle factors involved in the risk of a range of health problems: maintaining a normal weight, getting regular exercise, eating enough fruits and vegetables, not smoking and drinking moderate amounts of alcohol.

In 1988, just 15 percent of Americans in this age group adhered to all five recommendations. By 2006, that figure had dropped to 8 percent, the researchers found.

While regular exercise and fruit and vegetable intake dropped over time, the rate of smoking remained steady, at about 26 percent.

Moderate drinking — up to one drink per day for women, and two drinks per day for men — was the one lifestyle change Americans were willing to make, the study found. That rate increased from 40 percent to 51 percent.

The study cannot tell us why healthy lifestyle habits are on the decline, King said. "But one reason," he said, "may be that these changes are just difficult to make."

People's increasingly hectic lives may be making it hard to fit in regular exercise, or to sit down to healthier, home-cooked meals rather than eating take-out, the researcher noted.

However, he said, some simple steps can make a difference. "It's not hard, for example, to take some fruit to work with you, instead of going to the vending machine," King said.

He also stressed that "it's never too late" for middle-aged and older adults to make lifestyle changes for the better. In an earlier study, King and his colleagues found that when middle-aged adults newly adopted a healthier lifestyle — including regular exercise and eating more fruits and vegetables — their risk of developing cardiovascular disease or dying over the next several years fell by as much as 40 percent.