"One out of five American women have had their health either getting worse or at best not getting better," said Dr. Majid Ezzati of the Harvard School of Public Health.
The joint Harvard School of Public Health and the University of Washington study found that 4 percent of the male population and 19 percent of the female population experienced either decline or stagnation in mortality beginning in the 1980s.
Researchers say it comes down to preventable diseases and lagging public health care. The downward trend of life expectancy was most pronounced in just a few parts of the country, including the deep South, the southern portion of the Midwest, parts of Texas and Appalachia.
In these parts of the country, doctors say they're finding alarming increases in cancer, diabetes, heart and lung disease.
"A big part of this story is around what has happened to patterns of smoking, patterns of blood pressure and obesity. … The set of diseases that are caused by things we know well how to control are not being controlled well enough," said Ezzati.
Researchers say access to health care, lack of education and poor eating habits are partially to blame. They also believe the Harvard study could serve as a possible indicator of what's to come for other parts of the country.
"So something about the public health system is not functioning as well as it should be and it may well just be a matter of time before larger parts of the population are effected," said Ezzati.
Doctors say women are more susceptible because they have a higher rate of obesity. Many don't have the equivalent financial means to gain access to health care, and it took 20 years for female smokers to catch up with the males.
As a whole, though, the long-term study found that the American population is living longer than ever.
Between 1960 and 2000, the average life expectancy increased by more than seven years for men to age 74, and more than six years for women to nearly age 80.