Washington, DC….The nation’s history of heavy smoking is a major reason why lifespans in the U.S. fall short of those in many other high-income nations, and evidence suggests that current obesity levels also play a substantial part, says a new report from the National Research Council.

Over the last 25 years, life expectancy at age 50 in the U.S. has been rising, but at a slower pace than in many other high-income countries, such as Japan and Australia. This difference is particularly notable given that the U.S. spends more on health care than any other nation. Concerned about this divergence, the National Institute on Aging asked the National Research Council to examine evidence on its possible causes.

Three to five decades ago, smoking was much more widespread in the U.S. than in Europe or Japan, and the health consequences are still playing out in today’s mortality rates, the report says. Smoking appears to be responsible for a good deal of the differences in life expectancy, especially for women. The habit also has significantly reduced life expectancy in Denmark and the Netherlands, two other countries with lower life expectancy trends than comparable high-income countries.

Because there appears to be a lag of two to three decades between smoking and its peak effects on mortality, one can predict how smoking will affect life expectancy over the next 20 to 30 years. On this basis, life expectancy for men in the U.S. is likely to improve relatively rapidly in coming decades because of reductions in smoking in the last 20 years, the report says. For U.S. women, whose smoking behavior peaked later than men’s, declines in mortality are apt to remain slow for the next decade. Similarly, life expectancy in Japan is expected to improve less rapidly than it otherwise would, because of more-recent high smoking rates.

Obesity’s contribution to lagging life expectancies in the U.S. also appears to be significant, the report says. While there is still uncertainty in the literature about the magnitude of the relationship between obesity and mortality, it may account for a fifth to a third of the shortfall in longevity in the U.S. compared to other nations, the report says. And if the obesity trend in the U.S. continues, it may offset the longevity improvements expected from reductions in smoking. However, recent data suggest that the prevalence of obesity in the U.S. has leveled off, and some studies indicate that the mortality risk associated with obesity has declined.

Lack of universal access to health care in the U.S. also has increased mortality and reduced life expectancy, the report says, though this is a less significant factor for those over age 65 because of Medicare access. For the main causes of death at older ages — cancer and cardiovascular disease — available indicators do not suggest that the U.S. health care system is failing to prevent deaths that would be averted elsewhere. In fact, cancer detection and survival appear to be better in the U.S. than in most other high-income nations, and survival rates following a heart attack also are favorable.

Certain risk factors are unlikely to have played a major role in the divergence of life expectancy over the last 25 years, the report adds. Although a large body of emerging work suggests that there may be important connections between the strength of social ties and mortality, the committee that wrote the report found little compelling data to indicate that differences in social networks among people in high-income countries are related to the differing patterns of life expectancy. Similarly, little evidence supports the hypothesis that hormone therapy has played a part in the relatively lower longevity for American women.

The study committee also identified many gaps in research. While lung cancer deaths are a reliable marker of the damage from smoking, no clear-cut marker exists for obesity, physical inactivity, social integration, or other risks considered in this report. Moreover, evaluation of these risk factors is based on observational studies, which — unlike randomized controlled trials — are subject to many biases. While there is no perfect substitute for randomized controlled trials, studies that take advantage of natural experiments, such as increased cigarette taxes or a dramatic change in the use of hormone therapy, can sometimes serve as valuable supplements to them.

The report was sponsored by the National Institute on Aging’s Division of Behavioral and Social Research. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies. They are private, nonprofit institutions that provide science, technology, and health policy advice under a congressional charter. The Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering.