Workplace wellness programs — efforts to get workers to lose weight, eat better, stress less and sleep more — are an $8 billion industry in the U.S.

Most large employers offer some type of wellness program — with growth fueled by incentives in the federal Affordable Care Act.

But no one has been sure they work.

Now researchers from the University of Chicago and Harvard have published one of the first large-scale studies that employs more sophisticated research techniques.

They randomly assigned 20 BJ's Wholesale Club outlets to offer a wellness program to all employees, then compared results with 140 stores that did not. The big-box retailer employed nearly 33,000 workers across all 160 clubs during the test.

After 18 months, it turned out that yes, workers participating in the wellness programs self-reported healthier behavior, such as exercising more or managing their weight better than those not enrolled.

But the efforts did not result in differences in health measures, such as improved blood sugar or glucose levels; how much employers spent on health care; or how often employees missed work, their job performance or how long they stuck around in their jobs.

"The optimistic interpretation is there is no way we can get improvements in health or more efficient spending if we don't first have changes in health behavior," said one study author, Katherine Baicker, dean of the Harris School of Public Policy at the University of Chicago. "But if employers are offering these programs in hopes that health spending and absenteeism will go down, this study should give them pause."

The study comes amid widespread interest in wellness programs.

The Kaiser Family Foundation's annual survey of employers found that 53% of small firms and 82% of large firms offer a program in at least one of these areas: smoking cessation, weight management and behavioral or lifestyle change.

A few employers tie financial incentives to workers actually lowering risk factors, such as high blood pressure or cholesterol — or making concerted efforts to participate in programs that might help them do so over time.

The Affordable Care Act allowed employers to offer financial incentives worth up to 30% of the cost of health insurance, leading some employers to offer what could be hundreds or even thousands of dollars off workers' deductibles or premiums to get them to participate. That led to court challenges about whether those programs are truly voluntary.

In the study reported in JAMA, the incentives were modest. Participants got small-dollar gift cards for taking wellness courses on topics such as nutrition, exercise, disease management and stress control.

An accompanying editorial in JAMA noted that "traditional, broad-based programs like the one analyzed by [Dr. Zirui] Song and Baicker may lack the necessary intensity, duration, and focus on particular employee segments to generate significant effects over a short time horizon."

In other words, don't give up entirely on wellness efforts, but consider "more targeted approaches" that focus on specific workers with higher risks or on "health behaviors [that] may yield larger health and economic benefits," the editorial suggested.