Americans may be aging more slowly than they were two decades ago.

A new study by University of Southern California and Yale University researchers suggests that at least part of the gains in life expectancy over recent decades may be due to a change in the rate of biological aging, rather than simply keeping ailing people alive.

"This is the first evidence we have of delayed 'aging' among a national sample of Americans," said senior author Eileen M. Crimmins, University Professor and AARP Professor of Gerontology at the USC Leonard Davis School of Gerontology.

As noted in the study by Crimmins and lead author Morgan E. Levine, assistant professor at the Yale Center for Research on Aging: "A deceleration of the human aging process, whether accomplished through environment or biomedical intervention, would push the timing of aging-related disease and disability incidence closer to the end of life."

Using data from the National Health and Nutrition Examination Survey (NHANES) III (1988-19994) and NHANES IV (2007-2010), the researchers examined how biological age, relative to chronological age, changed in the U.S. while considering the contributions of health behaviors. Biological age was calculated using several indicators for metabolism, inflammation, and organ function, including levels of hemoglobin, total cholesterol, creatinine, alkaline phosphatase, albumin, and C-reactive protein in blood as well as blood pressure and breath capacity data.

While all age groups experienced some decrease in biological age, the results suggest that not all people may be faring the same. Older adults experienced the greatest decreases in biological age, and men experienced greater declines in biological age than females; these differences were partially explained by changes in smoking, obesity, and medication use, Crimmins and Levine explained.

"While improvements may take time to manifest, and thus are more apparent at older ages, this could also signal problems for younger cohorts, particularly females, who -- if their improvements are more minimal -- may not see the same gains in life expectancy as experienced by the generations that came before them," said Levine, who received both her PhD in Gerontology in 2015 and her BA in Psychology in 2008 from USC.

Slowing the pace of aging, along with increasing life expectancy, has important social and economic implications. The study suggests that modifying health behaviors and using prescription medications does indeed have significant impact on the health of the population.

"Life extension without changing the aging rate will have detrimental implications. Medical care costs will rise, as people spend a higher proportion of their lives with disease and disability," Levine said. "However, lifespan extension accomplished through a deceleration of the aging process will lead to lower healthcare expenditures, higher productivity, and greater well-being."

The study first appeared in the journal Demography on March 6, 2018. This research was supported by the National Institute on Aging grants P30 AG17265 (Crimmins), T32-AG00037 (Crimmins), and 4R00AG052604-02 (Levine).