Death rate in the U.S. has been falling for decades thanks to advances in medical treatments and public health measures but researchers have found that mortality rates in middle-aged white Americans have increased over the past 15 years.

In a new study published in the journal PNAS on Nov. 2, researchers Anne Case and Angus Deaton, from Princeton University, used data from a different reports and survey and found a sharp rise in the mortality rate for middle-aged whites.

Death rate for whites between 45 and 54 years old rose by half-percent per year from only 381.5 per 100,0000 in 1999 to 415.4 per 100,000 in 2013.

Deaton and Case reported that the pattern is not seen in other affluent nations or among Hispanic or African Americans in the U.S. The researchers linked the fatality rate to alcohol and drug use, suicide, chronic liver disease and cirrhosis.

"Rising midlife mortality rates of white non-Hispanics were paralleled by increases in midlife morbidity," the researchers wrote in their study.

"Self-reported declines in health, mental health, and ability to conduct activities of daily living, and increases in chronic pain and inability to work, as well as clinically measured deteriorations in liver function, all point to growing distress in this population."

While the mortality rates associated with alcohol, drugs and suicides have increased for middle-aged whites across all education levels, the researchers noted that the biggest increases were seen among those who have the least education.

In those with high school degree or less, for instance, mortality due to drug and alcohol poisoning rose fourfold. Suicides likewise rose more than 80 percent and deaths due to cirrhosis and liver disease increased by 50 percent.

The all-cause mortality increased by 22 percent for the least educated group but death rates were marked by decline in those with a bachelor's degree or higher.

Although it isn't yet clear why the mortality rate only rose among whites, experts offered a possible explanation.

"For whites their parents had done better economically and they had been doing pretty well. Then all of a sudden the financial floor dropped out from underneath them," said Joe Skinner, from Dartmouth College. "For African-American and Hispanic households things had never been that optimistic and so perhaps the shock wasn't quite as great."