When researchers adjusted the data for economic factors like income and unemployment rates and population characteristics like age, sex and race, and then compared those numbers with neighboring states, they estimated that the Medicaid expansions were associated with a decline of 6.1 percent in deaths, or about 2,840 per year for every 500,000 adults added.

While the data included all deaths, not just deaths of Medicaid recipients, the decline in mortality was greatest among nonwhites and people living in poorer counties, groups most affected by expanded Medicaid coverage.

“I can’t tell you for sure that this is a cause-and-effect relationship,” that the Medicaid expansion caused fewer non-elderly adults to die, Dr. Sommers said. “I can tell you we did everything we could to rule out alternative explanations.”

Several experts with varying views on the Affordable Care Act said the study, which was completed long before the Supreme Court hearings on the law, was conducted by highly qualified researchers who carefully analyzed the available data. Still, they and the study’s authors pointed to limitations in the data, noting that the mortality figures represent county-level statistics, not individual deaths.

“They are trying really hard with the data that they have available, but at the end of the day it doesn’t really compensate for the fact that you don’t have the data you want, which is individual mortality rates and what happens to people with change in coverage over time,” said Gail Wilensky, a health economist who headed Medicare and Medicaid during the administration of the elder President George Bush. In addition, when the researchers looked individually at each of the three states, the only state with a statistically significant decline was in the largest state, New York, and she questioned whether every state would have the same experience.

Douglas Holtz-Eakin, president of the American Action Forum, a Republican-oriented group, said the study was “well done” and “brings more evidence in about the benefit side” of Medicaid, but he wondered if the results could be generalized. The three states studied voluntarily expanded their Medicaid programs, presumably confident they could pay for the expansion, and had enough doctors accepting Medicaid to treat additional beneficiaries. Other states may be less able to afford it, he said, and it is possible that “having a piece of paper that says you’re on Medicaid doesn’t do any good because they can’t see anybody.”

Nonetheless, experts said, the results support those of another Medicaid study being conducted by some of the same researchers in Oregon. Oregon expanded its Medicaid program in 2008, but, without money to cover everyone at first, chose 10,000 people by lottery. Dr. Baicker and her colleagues, comparing those who got Medicaid with those who did not, have so far found that Medicaid recipients see doctors more often, and report better health and better financial stability.