A new study by researchers in the US (the first to examine the effect of four preventable risk factors on life expectancy across the nation), suggests that smoking, high blood pressure, elevated blood glucose, and overweight and obesity reduce life expectancy in the US by 4.9 years in men and 4.1 years in women and lead to health disparities.

You can read about the study, led by researchers from the Harvard School of Public Health (HSPH) working with researchers from the Institute for Health Metrics and Evaluation at the University of Washington, online in the March 2010 issue of PLoS Medicine.

The authors explain in their background information that life expectancy (a measure of longevity and premature death) and overall health have increased steadily in the US over recent times. But some groups live longer and healthier lives than others, and disparities are large and persistent.

For instance, on average, the lives of black men and women in the US are 6.3 and 4.5 years shorter than those of the their white counterparts. And if you were to compare the counties with the lowest life expectancy with those that have the highest, you would see a staggering difference of 18.4 years for men and 14.3 years for women.

These disparities are mainly due to differences in deaths from chronic diseases like cardiovascular diseases (for example, heart attacks and stroke), cancers, and diabetes, and every year, hundreds of thousands of Americans die from these diseases, which are caused by smoking, high blood pressure, elevated blood glucose and obesity.

As well as estimating the effects of these four preventable risk factors across the nation as a whole, the researchers looked at how they impact eight subgroups of the US population: the "Eight Americas", which are defined by race, county location and the socioeconomic features of each county.

The results revealed that the four risk factors account for a big chunk of the disparity in life expectancy among the Eight Americas. For example, the largest reduction in life expectancy was among southern rural blacks (6.7 years less of life for men and 5.7 for women) while the smallest was among Asians (4.1 years less of life for men and 3.6 for women).

The Eight Americas are: (1) Asians, (2) Northland low-income rural whites, (3) middle America, (4) low-income whites in Appalachia and Mississippi Valley, (5) Western Native Americans, (6) Black middle America, (7) high-risk urban blacks, and (8) Southern low-income rural blacks.

Senior author Dr Majid Ezzati, associate professor of international health at HSPH said in a statement that:

"This study demonstrates the potential of disease prevention to not only improve health outcomes in the entire nation but also to reduce the enormous disparities in life expectancy that we see in the US."

For the study, Ezzati and colleagues analyzed data from the National Center for Health Statistics, the National Health and Nutrition Examination Survey, the Behavioral Risk Factor Surveillance System covering the year 2005. They also conducted an extensive review of epidemiologic studies on the effects of the four preventable risk factors.

They estimated the number of deaths that could have been prevented in 2005, and the impact on life expectancy, if the levels of the four risk factors in the population had stayed within the healthy range or optimal levels suggested by commonly used guidelines.

They found that a major predictor of a person's life expectancy and how healthy they were depended on two things: their ethnicity and where they lived. For instance, they found that:
  • Whites had the lowest blood pressure.

  • Blacks, especially those in the rural South, had the highest blood pressure.

  • Western Native American men and Southern low-income rural black women had the highest BMI (body mass index, a measure of obesity where you take person's weight in kilos and divide it by the square of their height in meters).

  • Asian American men and women had the lowest BMI, blood glucose levels and prevalence of smoking.

  • Western Native American and low-income whites in the Appalachia and Mississippi Valley had the highest prevalence of smoking.
These patterns of smoking, high blood pressure, elevated blood glucose and overweight and obesity, explain nearly 20 per cent of disparities in life expectancy across the US as a whole: they also account for three quarters of disparities in cardiovascular deaths and up to half of disparities in cancer deaths, wrote the authors.

If each individual risk factor were to be reduced to an optimal level, the benefit in increased life expectancy would be:
  • Bringing blood pressure down to optimal level would add 1.5 years of life expectancy to men and 1.6 years to women.
  • Bringing obesity down would result in 1.3 more years for men and women.
  • Bringing blood glucose down would result in 0.5 more years for men and 0.3 years for women.
  • And stopping smoking would give men 2.5 more years and women another 1.8 years.
Lead author Dr Goodarz Danaei, a postdoctoral research fellow at HSPH, emphasized the importance of public health measures to address the situation, saying it can't all be done by relying on personal choice and doctors:

"To improve the nation's overall health and reduce health disparities, both population-based and personal interventions that reduce these preventable risk factors must be identified, implemented, and rigorously evaluated," said Danaei, stressing how important it was for public health policy makers to understand this.

As an example, the authors discussed the role of salt intake, an important predictor of population blood pressure. Regulation and reduction of salt in prepared and packaged food has been shown to be an effective population-level intervention, they wrote, as is screening for high blood pressure and using antihypertensives or combination therapy to reduce blood pressure and cardiovascular risk. These are cost-effective measures that should be "scaled up as a part of expanding and improving primary care in the context of US health reform", wrote the authors.

A cooperative agreement from the US Centers for Disease Control and Prevention through the Association of Schools of Public Health paid for the study.

"The Promise of Prevention: The Effects of Four Preventable Risk Factors on National Life Expectancy and Life Expectancy Disparities by Race and County in the United States."
Goodarz Danaei, Eric B. Rimm, Shefali Oza, Sandeep C. Kulkarni, Christopher J. L. Murray, and Majid Ezzati.
PLoS Medicine, March 2010, vol. 7, issue 3: e1000248.

Source: Harvard School of Public Health.

Written by: Catharine Paddock, PhD