Programs in the workplace designed to get people to exercise can improve fitness, cut cholesterol levels, reduce job stress and even improve attendance, a new analysis of the medical literature shows.

But it's still not clear what makes for the most effective type of program, Dr. Vicki S. Conn of the University of Missouri in Columbia, the lead author of the research, told Reuters Health.

"We do have really good evidence that the interventions do work," she said. "What we couldn't say from this is that this intervention works better than that intervention."

Conn and her colleagues looked at dozens of studies of workplace physical activity interventions. The studies included about 38,000 people.

They found significant positive effects for the interventions on "physical activity behavior," meaning whether or not people became more active, and also on fitness level. The programs also helped fuel healthy changes in lipids (meaning harmful fats in the blood such as triglycerides), measures of body size, work attendance, and job stress, the researchers report.

The more effective programs had several characteristics in common: a facility for exercising on site; they were developed with the help of the company; and people were able to exercise during the workday rather than having to come in early or stay late. But it wasn't clear whether offering rewards helped.

While evidence is scarce on the long-term costs of workplace physical activity interventions, Conn noted, the fact that they reduce absenteeism suggests they could indeed save money.

The current investigation is part of a larger, National Institutes of Health-funded study of physical activity interventions in general, Conn noted. While there's no lack of evidence to show that exercise is good for you, she added, "what we don't know is how to get people to exercise," and the study may help answer that question.