The Affordable Care Act has gotten some bad news lately. It's not a sign of impending disaster, as the law's critics say, but it may be a sign of some real problems on the horizon.

UnitedHealth Group announced on Thursday that it has lost $425 million on the policies it has been selling through the Obamacare exchanges. The reason is pretty simple, according to Stephen Hemsley, the companys chief executive officer: UnitedHealth hasnt attracted a sufficiently balanced mix of healthy and sick customers.

Instead, Hemsley said, the company was seeing a large number of people with high medical bills -- or, more precisely, a larger number than it had expected. As a result, the premiums UnitedHealth is collecting for these policies arent enough to offset the high medical bills the new enrollees are generating.

Insurance stocks fell , in part because UnitedHealth's disclosure signaled an abrupt change in tone. Just a month ago, another executive said his company was enthusiastic about the exchanges -- and expected its market share to grow, despite some early signs that enrollment was matching the company's low predictions. During an investor call on Thursday, Hemsley was decidedly more downbeat.

We cant sustain these losses, he warned, adding that the company was re-evaluating its commitment to the exchanges and may even pull out of the markets altogether, Modern Healthcare reported.

Investor calls are not always the most reliable indicator of corporate intentions and, by itself, UnitedHealths dour pronouncement doesnt say much about the health insurance exchanges as a whole. Although UnitedHealth is the nations largest insurer, most of its business comes from coverage it provides to or through employers, as well as to seniors in the Medicare Advantage program.

UnitedHealth continues to make hefty profits in those other insurance segments, and its business model, which emphasizes breadth and size, actually makes it a poor fit for selling directly to individuals. UnitedHealth approached the exchanges more cautiously than its competitors during their first year and, today, it has just a bit more than 500,000 exchange enrollees . Thats about 5 percent of the total market.