Forecasting the effects of obesity and smoking on U.S. life expectancy

Abstract

Background: Although increases in obesity over the past 30 years have adversely affected the health of the U.S. population, there have been concomitant improvements in health because of reductions in smoking. Having a better understanding of the joint effects of these trends on longevity and quality of life will facilitate more efficient targeting of health care resources.

Methods: For each year from 2005 through 2020, we forecasted life expectancy and quality-adjusted life expectancy for a representative 18-year-old, assuming a continuation of past trends in smoking (based on data from the National Health Interview Survey for 1978 through 1979, 1990 through 1991, 1999 through 2001, and 2004 through 2006) and past trends in body-mass index (BMI) (based on data from the National Health and Nutrition Examination Survey for 1971 through 1975, 1988 through 1994, 1999 through 2002, and 2003 through 2006). The 2003 Medical Expenditure Panel Survey was used to examine the effects of smoking and BMI on health-related quality of life.

Results: The negative effects of increasing BMI overwhelmed the positive effects of declines in smoking in multiple scenarios. In the base case, increases in the remaining life expectancy of a typical 18-year-old are held back by 0.71 years or 0.91 quality-adjusted years between 2005 and 2020. If all U.S. adults became nonsmokers of normal weight by 2020, we forecast that the life expectancy of an 18-year-old would increase by 3.76 life-years or 5.16 quality-adjusted years.

Conclusions: If past obesity trends continue unchecked, the negative effects on the health of the U.S. population will increasingly outweigh the positive effects gained from declining smoking rates. Failure to address continued increases in obesity could result in an erosion of the pattern of steady gains in health observed since early in the 20th century.

Figure 1. Forecasted Distribution of Risk Factors, 2005 through 2020

The projected rates of change in the categorical distribution of smoking status (Panel A) and body-mass index (BMI) (Panel B) from 2005 to 2020 are shown, assuming a continuation of trends occurring over the 15 years preceding 2005. The categories for weight were defined according to the World Health Organization criteria: normal weight (BMI [the weight in kilograms divided by the square of the height in meters], 18.5 to 24.9), overweight (BMI, 25.0 to 29.9), obese (BMI, 30.0 to 34.9; obesity class I), and morbidly obese (BMI, ≥35.0; obesity classes II and III). Historical trends for smoking were measured with the use of the National Health Interview Survey (1990 through 1991 and 2004 through 2006) and historical trends for BMI with the use of the National Health and Nutrition Examination Survey (1988 through 1994 and 1999 through 2002).

Figure 2. Forecasted Life Expectancy and Quality-Adjusted Life Expectancy at 18 Years of Age from 2005 through 2020, Considering Trends in Smoking Alone, Body-Mass Index Alone, and Smoking and Body-Mass Index Combined

The life expectancy (Panel A) and quality-adjusted life expectancy (Panel B) for a typical 18-year-old are depicted in accordance with the following simulation scenarios: entire population classified as having normal weight and all current and former smokers classified as former smokers who have not smoked for 10 years or more by 2020 (blue line); declines in smoking set to the rate of the past 15 years with body-mass index (BMI) remaining at 2005 levels (green line); continuation of the rates of increase in life expectancy from 1990 through 2004, independent of changes in smoking and BMI over that time period (black line); trends in smoking and BMI continued at the rate of the past 15 years (brown line); and increases in BMI continued at the rate of the past 15 years (purple line). Panel C shows life expectancy for a typical 18-year-old, forecasted on the basis of relative risks of death from different studies. Two of the forecasts shown here are from Panel A (based on the relative risks of death calculated in this study with the use of the National Health and Nutrition Examination Surveys [NHANES] I, II, and III), with one forecast continuing the rates of increase in life expectancy from 1990 through 2004, independent of changes in smoking and BMI over that period (black line) and the other forecast a continuation of the trends in smoking and BMI at the rates of the past 15 years (brown line). For the sake of comparison with these forecasts, we also included forecasts based on relative risks of death as reported in prospective studies by Freedman and colleagues in 2006 (green line, hidden beneath brown line) and by van Dam and colleagues in 2008 (women only, blue line), assuming a continuation of the trends in smoking and BMI at the rates of the past 15 years. To obtain relative risks for the joint categories of smoking and obesity from the study by van Dam and colleagues, a multiplicative relationship between smoking and obesity was assumed. Since, unlike our study, the van Dam study combined the categories of obese and morbidly obese when calculating risks of death, we performed an alternate forecast for the sake of comparison in which these categories were combined, using mortality data from NHANES (purple line).