It sounds like a great idea: private insurance plans in the Medicare Advantage program offering gym memberships, a small investment that could curb health-care costs among seniors. But what some see as innovation, others view as a stealth campaign to lure the healthiest seniors into the private plans while leaving Medicare with the worst risks.

In research published in the New England Journal of Medicine, Alicia Cooper and Amal Trevedi looked at Medicare Advantage plans that added fitness club memberships to their benefits packages in the mid-2000s. They compared enrollees in those plans with other plans that didn’t add a gym membership and with the plan’s old membership before it started paying fitness club fees. The result? Medicare Advantage enrollees who signed up for a plan with a gym membership tended to be significantly healthier.

A gym membership as a health insurance benefit is a bit of a double-edged sword. The benefits are pretty obvious: Access to a gym could keep enrollees healthier and curb chronic conditions, such as heart disease and diabetes, that eat up about three-quarters of our health-care costs.

But a gym membership — plus a good advertising campaign — can also be used as a risk selection tool.

Where this could actually become most significant isn’t necessarily in Medicare Advantage but under a Medicare reform plan like the one that Rep. Paul Ryan (R-Wis.) and Sen. Ron Wyden (D-Ore.) proposed late last year, where seniors would get a set amount of money to shop for a plan, the government-run plan being one of their options.

It may also become relevant in the soon-to-launch health insurance exchanges. In those state-based marketplaces, insurance plans will have to cover a set of essential health benefits and accept every applicant. But they do not face any limits on what they can cover. There’s nothing to stop an insurance plan, for example, from offering a gym membership or, if they want to get really aggressive, covering entry fees for marathons.

The health reform law does contemplate the issue; last year, the Department of Health and Human Services rolled out regulations that spelled “risk adjustment” mechanisms that will be used to deal with “risk selection.”

Insurance plans will have to accept all applicants beginning in 2014, just as Medicare Advantage must take all seniors over 65. But advertising a plan as the go-to health insurance source for marathoners could lure a healthier subscriber base, disrupting the rest of the marketplace in the process.

Kliff covers health policy for The Washington Post and is a regular contributor to Ezra Klein’s Wonkblog on