THAT LITTLE VOICE nagging you to get in shape is increasingly coming from your employer and is likely to grow louder with a looming change under the federal health-care overhaul.

More companies are starting or expanding wellness programs that aim to reduce medical costs by improving employees' health. They're asking workers to take physical exams, complete detailed health assessments and focus on controlling conditions such as diabetes. Many companies also are dangling the threat of higher monthly insurance premiums to prod workers into action.

The Affordable Care Act is one reason the programs are spreading. The federal law calls for a 40 percent tax on expensive benefit plans starting in 2018, and many companies that offer employer-based coverage already have begun to look for ways to lower costs and avoid that tax.

Businesses see wellness programs as a win for themselves and workers. But studies have shown that the programs have a limited ability to reduce costs. They also raise concerns about privacy and discrimination.

Penalties also can hit lower-wage workers harder than they would executives because premiums already consume a larger portion of those workers' paychecks.

Benefits consultants say federal regulations help guard against that. Companies can be penalized under the overhaul for offering coverage that is considered unaffordable.

Businesses also are required to offer alternatives that help workers avoid penalties like a higher premium because they can't meet a wellness program goal.

Despite employee concerns, the idea of prevention as a way to reduce health-care costs has been largely embraced by employers, who provide the most common form of health insurance in the U.S.

For years, they have offered rewards to employees who agree to take steps to monitor their health. The goal is to at least make workers more aware of their health, and it worked for Roy Simmons, 55, a nuclear power-plant manager for Dominion Resources Inc.

Dominion started offering a $400 premium credit for employees who agreed to a health assessment, so Simmons had basics such as weight and cholesterol measured. He then forgot about the numbers until a reminder arrived last year. Another physical told him he had gained 40 pounds and his cholesterol was up.

"That was a bit of a wakeup call for me," Simmons said. "I didn't know it had happened to me. I know that sounds stupid, but I wasn't paying attention to it, and it just sneaked up on me."

He cut junk food from his diet and asked his college football-playing son to become a workout partner over the summer. He has since dropped the weight.

Benefits experts say companies haven't seen enough cases like Simmons', in which an incentive helps nudge an employee to participate in a wellness program, so some employers have started using penalties.