WASHINGTON -- In the largest one-year enrollment bump in program history, 8 million Americans are expected to gain health insurance in 2014 through Medicaid under the nation’s massive health care overhaul.

The Medicaid expansion is a signature provision of the Patient Protection and Affordable Care Act and will mark a symbolic turning point in the controversial public and private effort to provide health insurance for all Americans.

Yet questions about the cost of the expansion, whether states will cooperate and a potential shortage of care providers has cast a cloud of uncertainty over the move as the nation’s rapidly evolving health care system readies for this rare jolt of newly insured patients – and whatever surprises come with them.

“The challenge is you don’t know who will show up until they show up,” said Matt Salo, executive director of the National Association of Medicaid Directors.

He said that nearly every state that has undergone a large expansion of its Medicaid eligibility has seen two things: “More people show up than you think will show up, and the people that show up are sicker than you expected.”

Along with requiring individuals to purchase health insurance or face a fine, the law, which critics have derisively referred to as “Obamacare,” allows states to expand Medicaid coverage to non-elderly adults who earn up to 138 percent of the federal poverty level. That’s about $15,900 for an individual in 2013, or nearly $32,500 for a family of four.

It’s unclear how many states ultimately will embrace the Medicaid expansion, since there’s no time limit for them to decide.

So far, 21 states and the District of Columbia are set to make the move next year, according to the Advisory Board Co., a health care consulting firm. Another four states are leaning toward expansion, while 12 others won’t participate, and five more are leaning toward not participating. Eight states are still undecided.

Most of the 8 million new Medicaid enrollees will be low-income childless adults, a group that Medicaid has traditionally shunned. The rest will be low-income parents whose earnings exceed current Medicaid eligibility thresholds, and poor people with disabilities who don’t meet the disability requirements for the Supplemental Security Income program, a federal cash assistance program run by the Social Security Administration.

As a group, the new Medicaid enrollees will be fairly young, with an estimated median age of just 31, according to PriceWaterhouseCoopers’ Health Research Institute. They’ll also be poor. The institute estimates that only 26 percent will have a full-time job and that their median annual income will be roughly 65 percent of the federal poverty level.

That’s less than $7,500 a year for an individual in 2013, or roughly $12,700 for a family of three.

In addition, most of the new Medicaid enrollees will be female – about 54 percent – and the vast majority – about 74 percent – will be white, according to the group’s estimates. Blacks will make up about 18 percent, according to the institute, while the other 8 percent will be Asian, Native American or biracial.

Hispanics, who can be black or white, likely will make up 28 percent of the new Medicaid population, according to estimates by Avalere Health, a Washington research and consulting firm. That syncs with a Health Research Institute estimate that more than one-third will not speak English as their primary language, which can be a formidable obstacle to quality care.