State expansions of the Medicaid health insurance program for poor Americans reduced adult mortality rates by more than 6 percent compared to states that did not broaden eligibility for their plans, according to a study released on Wednesday.

The findings published in the New England Journal of Medicine could fuel a political furor over new plans for a nationwide expansion of Medicaid that erupted after the U.S. Supreme Court's ruling to uphold President Barack Obama's healthcare law in late June.

In an unexpected move, the high court ruling also left it up to states to decide whether to participate in the law's broader eligibility criteria for Medicaid that would extend insurance coverage to as many as 16 million more Americans starting in 2014. At least five Republican governors who opposed the healthcare law have vowed to opt out of the expansion, saying the program will pose a huge financial burden.

The lead author of the study was Benjamin Sommers, an assistant professor in health policy and economics at the Harvard School of Public Health who is temporarily working as an advisor to the U.S. Department of Health and Human Services. According to a disclosure note in the study, the paper was conceived and drafted while Sommers was employed at Harvard and the findings do not reflect official U.S. government policy.

The study examined three states that substantially broadened Medicaid eligibility for adults since 2000 -- New York, Maine and Arizona. They were compared to neighboring states that did not implement expansions -- Pennsylvania (for New York), New Hampshire (for Maine) and Nevada and New Mexico (for Arizona).

Adults between the ages of 20 and 64 years old were studied for five years before and after the expansion, using data from the U.S. Centers for Disease Control and Prevention.

Medicaid expansions were associated with a reduction in mortality from all causes, by 19.6 deaths per 100,000 adults, for a 6.1 percent decrease compared to the states without expansions.

The mortality declines were greatest among adults between ages 35 and 64, minorities and residents of poor counties.

The expansions also led to decreased rates of uninsurance, lower rates of delayed care because of costs, and an increase in the rate of people reporting their health status as "excellent" or "very good".

"The takeaway is that state expansions of Medicaid coverage to adults appear to be effective at improving both access to care and health for low-income Americans," Sommers said in an interview.

The results corresponded to 2,840 deaths prevented per year in the states with Medicaid expansions. That figure suggests that 176 additional adults would need to be covered by Medicaid in order to prevent one death per year, according to the study.