But the research published Tuesday in the Journal of the American Medical Association also noted that the the gains in health coverage and access to care for low-income adults were particularly striking in states that expanded their Medicaid programs under the ACA to include more poor people.

The study found marked gains in the number of people with insurance—as other research has repeatedly confirmed—as well as improved access to doctors and medications, affordable health care and good health status after implementation of the Affordable Care Act.

Students from around the country celebrate in front of the Supreme Court after justices decide for the Affordable Care Act, on June, 25, 2015, in Washington, D.C.

The improvements nationwide came after all six measures used in the study "showed worsening trends in the 2012-13 period, particularly between March 2013 and September 2013," according to the study, which looked at the experiences of about 500,000 adults age 18 to 64.

All but one of those measures—days limited by poor health—improved significantly after Obamacare plans went on sale on government-run exchanges in October 2013, according to the study, whose authors are from Harvard's T.H. Chan School of Public Health, Brigham and Women's Hospital in Boston, the U.S. Health and Human Services Department and the Commonwealth Fund.

The number of people who lacked health insurance fell by 7.9 percentage points, and there was a 3.5 percentage-point drop in the number of people who lacked a personal physician, the study found.

The number of people who reported being unable to afford care fell by 5.5 percentage points. People who said they were just in fair or poor health dropped by 3.4 percentage points after the ACA took effect, the study found.

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There was also 2.4 percentage-point reduction in the number of people who lacked easy access to medicine. For the category of days limited by poor health, the downward trend of people reporting that measurement stopped getting worse after the ACA went into effect, but it did not get significantly better as did the other measurements, the study found.

And "we found that the largest improvements in access to coverage occurred among racial and ethnic minorities," said Dr. Benjamin Sommers of Harvard, lead author of the JAMA report.

While whites saw their uninsured rates drop by 6.1 percentage points, there was an 11.9 percentage point decrease in the number of uninsured Latinos. The number of non-Latino blacks without health coverage decreased by 10.8 percentage points after the ACA went into effect.

Those results "suggest that the ACA may be associated with reductions in long-term disparities in access to care, one of the goals of the ACA," the report said.

The authors noted that they could not determine whether the changes "are directly related to the ACA's coverage expansions," particularly given the fact that the economic recovery that was going on at the same time may have also played an role. But, they added that the analysis adjusted for several potential economic factors, including income, individual employment and state unemployment rates.

And they also said the analysis that looked at the differences in Medicaid expansion states "represents a stronger research design because it included a control group" of states that did not expand Medicaid.

