Healthcare reform's requirement that people buy insurance is constitutional because everyone participates in the healthcare system, the Obama administration argued in an appeal to a legal challenge in Michigan.

The brief was filed Friday in the Sixth Circuit Court of Appeals in a case brought by the conservative Thomas More Law Center. Thomas More says the mandate exceeds the government's constitutional powers by regulating inactivity.

The administration won the first round in that fight in October when Detroit-based U.S. District Judge George Caram Steeh, a Clinton nominee, upheld the individual mandate. Another Clinton-appointed judge in Virginia has upheld the law, while a George W. Bush appointee rule against the mandate in the case brought by Virginia Attorney General Ken Cuccinelli.

Separately, a ruling is expected as early as next week in a 20-state challenge in Florida.

Friday's brief argues that while the four plaintiffs in the case have opted not to buy insurance but instead pay out of pocket for healthcare costs, such decisions have repercussions on government budgets, insurance profitability and people's premiums. Uncompensated care provided to people without insurance reached $43 billion in 2008, the brief argues, creating "substantial adverse effects on the interstate health care market."

"Unlike in other markets," the brief argues, "consumers receive very expensive forms of medical treatment without regard to their ability to pay. Health insurance is the most effective means to restrict the extent to which individuals shift their health care costs onto other market participants."