Deadweight? — The Influence of Obesity on Longevity
http://www.100md.com
《新英格兰医药杂志》
Obesity has clearly become a major personal and public health problem for Americans; it affects many aspects of our society. In this issue of the Journal, Olshansky et al.1 make an important contribution to national discussions of the future of longevity by calling attention to the very substantial increase in the prevalence and severity of obesity since 1980 and its consequences on health and mortality. They estimate that the current life expectancy at birth in the United States would be one third to three quarters of a year higher if all overweight adults were to attain their ideal weight.
Although Olshansky et al. put obesity in the foreground of their vision of the future, the background for their vision is at least as bleak. They argue that past gains in life expectancy were largely a product of saving the young, which is unrepeatable. They claim that advances in life expectancy at older ages will be much smaller than in previous decades and that demographers and actuaries fail to recognize the disjunction and blindly continue to extrapolate the past into the future. They add to this concern that AIDS, antibiotic-resistant pathogens, and influenza pandemics represent additional threats to health. In their scenario, our children may have lives shorter than our own.
I believe that these background elements are excessively gloomy. Decreases in the rate of death at older ages have been the principal force driving American longevity for at least half a century, and they show no signs of abating. Sixty percent of the 9.23-year increase in life expectancy at birth between 1950 and 2002 is attributable to decreases in mortality among persons above 50 years of age.2 Although improvements in life expectancy among women have slowed in the past decade, improvements among men have accelerated. The mean of male and female life expectancies at 65 years of age grew by 0.081 year per calendar year between 1950 and 1990, and by an identical 0.081 year per year between 1990 and 2002, the last year for which official U.S. life tables have been prepared.3
Demographers and actuaries use extrapolation to project the future of life expectancy because it seems to work better than any alternatives.4,5,6 The biggest mistake, which has been made repeatedly in projections of mortality in the past, is to assume that life expectancy is close to a biologic maximum.7 Confidence in the use of extrapolation is increased by the very steady behavior of mortality trends themselves. The mean of life expectancies at birth in 21 high-income countries shows a nearly perfect fit (a coefficient of determination, R2, of 0.994) to a linear time trend during the period from 1955 to 1996.8
The effect of an increase in the prevalence and severity of obesity on the longevity of U.S. citizens is already embedded in extrapolated forecasts made in recent periods. In fact, these forecasts implicitly assume that the severity of obesity will continue to worsen, and the prevalence will rise, since it is the rate of change in the determinants of mortality, rather than the level, that drives projected changes in life expectancy. Hundreds of factors affect a population's rate of death in any particular period, and it is their combined effect that establishes the trend.
Although Olshansky et al. cite threats to future improvements in life expectancy, it is important to recognize that many factors are at work to maintain a steady pace of advance. These include medical research organizations whose products have, for example, been responsible for much of the massive decrease in the rates of death from cardiovascular causes during the past four decades.9 Public support for the National Institutes of Health remains very strong, and private companies will continue to have incentives to develop new products that enhance health and longevity. Longevity seems to have a strong genetic component,10 which holds out the possibility that genetic engineering may, sometime within the 75-year projection of the Social Security Administration, begin to enhance longevity.
Other positive influences on longevity are embodied in cohorts of young persons who are approaching the ages at which death occurs most commonly and who will presumably enjoy greater protection from many diseases than will the people who have already reached those ages. Younger cohorts are better educated than older cohorts, and mortality is profoundly influenced by education. In 1998, life expectancy at age 25 was 7.1 years higher for men with some college education than for men with only a high-school education. For women, the discrepancy was 4.2 years.11
Younger cohorts have had lives less scarred by infectious diseases, which influence the development of many chronic diseases of adulthood.12,13 Younger cohorts have consumed fewer cigarettes at a given age than older cohorts, and the effect of smoking is clearly manifested in the rates of death of the general population. In fact, a large fraction of the decrease in the rate of the decline in mortality among older women in recent years is a result of the rising rate of death from lung cancer in this group, which is a reflection of the delayed uptake of smoking among women in comparison with men.14,15 Significant "cohort effects" have been demonstrated in the prevalence of cardiovascular disease, emphysema, and arthritis, suggesting that younger cohorts will have lower morbidity from these conditions as they age.16
Another reason to expect the longevity of U.S. citizens to continue to increase is that some populations have achieved life spans far longer than those of people in the United States, thus demonstrating what is possible even with no further technological advances. Japan has achieved a life expectancy of nearly 82 years, 4.5 years higher than that achieved by the United States and higher than that projected by the Social Security Administration for the United States for 2055.17,18 Some researchers have used a wide variety of data to suggest that within the United States, subgroups with the healthiest lifestyles may have already achieved life expectancies of 90 years or more.19
But let me be clear. The rising prevalence and severity of obesity are capable of offsetting the array of positive influences on longevity. How likely is that to happen? One promising observation is that the recent increase in the levels of obesity was produced by relatively few excess calories in the typical daily diet. The consumption of a median of 30 excess calories a day produced the observed increase in weight during an eight-year period for Americans 20 to 40 years of age.20 At the 90th percentile of weight gain, the excess consumed was about 100 calories a day. Reversing the increase in body mass might be accomplished through small behavioral changes that fit relatively easily into most people's lifestyles. The food and restaurant industries would be valuable allies in this effort, and there are recent indications of their willingness to cooperate.21
The fact that the U.S. population has already shown the ability to shift to healthier lifestyles is encouraging. Forty-two percent of U.S. adults were smokers in 1965, as compared with 23 percent in 2001.14 The percentage of Americans 20 to 74 years of age with high levels of serum cholesterol fell from 33 percent in 1961 to 18 percent in 1999 and 2000.14 Primarily because of behavioral changes, the incidence of AIDS has fallen by nearly 50 percent since 1992.22 The percentage of fatal crashes involving drunk drivers declined from 30 percent in 1982 to 17 percent in 1999.23 Each of these improvements in risk factors was facilitated by national campaigns that warned of the hazards of particular behaviors.23
The time has come to consider another major campaign. Even though the requisite behavioral changes may be small, they may be difficult to accomplish. The fact that most health-related behaviors have improved while obesity has worsened may be an indication of just how daunting the prospect of reducing levels of obesity may be. The rising prevalence and severity of obesity are already reducing life expectancy among the U.S. population. A failure to address the problem could impede the improvements in longevity that are otherwise in store.
I am indebted to John Wilmoth and Mitch Lazar for suggestions and assistance.
Source Information
From the Population Studies Center, University of Pennsylvania, Philadelphia.
References
Olshansky SJ, Passaro D, Hershow R, et al. A potential decline in life expectancy in the United States in the 21st century. N Engl J Med 2005;352:1138-1145.
Arias E. United States life tables, 2002. National vital statistics reports. Vol. 53. No. 6. Hyattsville, Md.: National Center for Health Statistics, 2002:25, 29. (DHHS publication no. (PHS) 2005-1120 PRS 04-0554.)
Arias E. United States life tables, 2002. Vol. 53. No. 6. Hyattsville, Md.: National Center for Health Statistics, 2002:29. (PHS) 2005-1120 PRS 04-0554.)
Lee R, Miller T. Evaluating the performance of the Lee-Carter method for forecasting mortality. Demography 2001;38:537-549.
Rosenberg M, Luckner W. Summary of results of survey of seminar attendees. North Am Actuarial J 1998;2:64-82.
Tuljaparkar S, Boe C. Mortality change and forecasting: how much and how little do we know? North Am Actuarial J 1998;2:13-47.
Oeppen J, Vaupel JW. Broken limits to life expectancy. Science 2002;296:1029-1031.
White K. Longevity advances in high income countries, 1955-96. Popul Dev Rev 2002;28:59-76.
Cutler D. Your money or your life: strong medicine for America's health care system. New York: Oxford University Press, 2004.
Perls TT, Wilmoth J, Levenson R, et al. Life-long sustained mortality advantage of siblings of centenarians. Proc Natl Acad Sci U S A 2002;99:8442-8447.
Molla MT, Madans JH, Wagener DK. Differentials in adult mortality and activity limitation by education in the United States at the end of the 1990s. Popul Dev Rev 2004;30:625-646.
Costa DL. Understanding the twentieth-century decline in chronic conditions among older men. Demography 2000;37:53-72.
Zimmer C. Do chronic diseases have an infectious root? Science 2001;293:1974-1977.
Freid VM, Prager K, MacKay AP, Xia H. Health, United States, 2003: with chartbook on trends in the health of Americans. Washington, D.C.: Government Printing Office, 2003:169, 212, 228. (DHHS publication no. 2003-1232.)
Pampel FC. Declining sex differences in mortality from lung cancer in high-income nations. Demography 2003;40:45-66.
Reynolds SL, Crimmins EM, Saito Y. Cohort differences in disability and disease presence. Gerontologist 1998;38:578-590.
OECD health data 2004. Paris: Organisation for Economic Co-operation and Development, 2004.
Board of Trustees. Federal old-age and survivors insurance and disability insurance trust funds: 2004 annual report. Baltimore, Md.: U.S. Social Security Administration, 2004.
Manton KG, Stallard E, Tolley DH. Limits to human life expectancy: evidence, prospects, and implications. Popul Dev Rev 1991;17:603-637.
Hill JO, Wyatt HR, Reed GW, Peters JC. Obesity and the environment: where do we go from here? Science 2003;299:853-858.
Carpenter D. Food industry push: cater to health needs. Press release of the Associated Press, New York, January 18, 2005.
Jaffe H. Whatever happened to the U.S. AIDS epidemic? Science 2004;305:1243-1244.
Cutler DM. Behavioral health interventions: what works and why? In: Anderson NB, Bulatao RA, Cohen B, eds. Critical perspectives on racial and ethnic differences in health in late life. Washington, D.C.: National Academies Press, 2004:643-74.(Samuel H. Preston, Ph.D.)
Although Olshansky et al. put obesity in the foreground of their vision of the future, the background for their vision is at least as bleak. They argue that past gains in life expectancy were largely a product of saving the young, which is unrepeatable. They claim that advances in life expectancy at older ages will be much smaller than in previous decades and that demographers and actuaries fail to recognize the disjunction and blindly continue to extrapolate the past into the future. They add to this concern that AIDS, antibiotic-resistant pathogens, and influenza pandemics represent additional threats to health. In their scenario, our children may have lives shorter than our own.
I believe that these background elements are excessively gloomy. Decreases in the rate of death at older ages have been the principal force driving American longevity for at least half a century, and they show no signs of abating. Sixty percent of the 9.23-year increase in life expectancy at birth between 1950 and 2002 is attributable to decreases in mortality among persons above 50 years of age.2 Although improvements in life expectancy among women have slowed in the past decade, improvements among men have accelerated. The mean of male and female life expectancies at 65 years of age grew by 0.081 year per calendar year between 1950 and 1990, and by an identical 0.081 year per year between 1990 and 2002, the last year for which official U.S. life tables have been prepared.3
Demographers and actuaries use extrapolation to project the future of life expectancy because it seems to work better than any alternatives.4,5,6 The biggest mistake, which has been made repeatedly in projections of mortality in the past, is to assume that life expectancy is close to a biologic maximum.7 Confidence in the use of extrapolation is increased by the very steady behavior of mortality trends themselves. The mean of life expectancies at birth in 21 high-income countries shows a nearly perfect fit (a coefficient of determination, R2, of 0.994) to a linear time trend during the period from 1955 to 1996.8
The effect of an increase in the prevalence and severity of obesity on the longevity of U.S. citizens is already embedded in extrapolated forecasts made in recent periods. In fact, these forecasts implicitly assume that the severity of obesity will continue to worsen, and the prevalence will rise, since it is the rate of change in the determinants of mortality, rather than the level, that drives projected changes in life expectancy. Hundreds of factors affect a population's rate of death in any particular period, and it is their combined effect that establishes the trend.
Although Olshansky et al. cite threats to future improvements in life expectancy, it is important to recognize that many factors are at work to maintain a steady pace of advance. These include medical research organizations whose products have, for example, been responsible for much of the massive decrease in the rates of death from cardiovascular causes during the past four decades.9 Public support for the National Institutes of Health remains very strong, and private companies will continue to have incentives to develop new products that enhance health and longevity. Longevity seems to have a strong genetic component,10 which holds out the possibility that genetic engineering may, sometime within the 75-year projection of the Social Security Administration, begin to enhance longevity.
Other positive influences on longevity are embodied in cohorts of young persons who are approaching the ages at which death occurs most commonly and who will presumably enjoy greater protection from many diseases than will the people who have already reached those ages. Younger cohorts are better educated than older cohorts, and mortality is profoundly influenced by education. In 1998, life expectancy at age 25 was 7.1 years higher for men with some college education than for men with only a high-school education. For women, the discrepancy was 4.2 years.11
Younger cohorts have had lives less scarred by infectious diseases, which influence the development of many chronic diseases of adulthood.12,13 Younger cohorts have consumed fewer cigarettes at a given age than older cohorts, and the effect of smoking is clearly manifested in the rates of death of the general population. In fact, a large fraction of the decrease in the rate of the decline in mortality among older women in recent years is a result of the rising rate of death from lung cancer in this group, which is a reflection of the delayed uptake of smoking among women in comparison with men.14,15 Significant "cohort effects" have been demonstrated in the prevalence of cardiovascular disease, emphysema, and arthritis, suggesting that younger cohorts will have lower morbidity from these conditions as they age.16
Another reason to expect the longevity of U.S. citizens to continue to increase is that some populations have achieved life spans far longer than those of people in the United States, thus demonstrating what is possible even with no further technological advances. Japan has achieved a life expectancy of nearly 82 years, 4.5 years higher than that achieved by the United States and higher than that projected by the Social Security Administration for the United States for 2055.17,18 Some researchers have used a wide variety of data to suggest that within the United States, subgroups with the healthiest lifestyles may have already achieved life expectancies of 90 years or more.19
But let me be clear. The rising prevalence and severity of obesity are capable of offsetting the array of positive influences on longevity. How likely is that to happen? One promising observation is that the recent increase in the levels of obesity was produced by relatively few excess calories in the typical daily diet. The consumption of a median of 30 excess calories a day produced the observed increase in weight during an eight-year period for Americans 20 to 40 years of age.20 At the 90th percentile of weight gain, the excess consumed was about 100 calories a day. Reversing the increase in body mass might be accomplished through small behavioral changes that fit relatively easily into most people's lifestyles. The food and restaurant industries would be valuable allies in this effort, and there are recent indications of their willingness to cooperate.21
The fact that the U.S. population has already shown the ability to shift to healthier lifestyles is encouraging. Forty-two percent of U.S. adults were smokers in 1965, as compared with 23 percent in 2001.14 The percentage of Americans 20 to 74 years of age with high levels of serum cholesterol fell from 33 percent in 1961 to 18 percent in 1999 and 2000.14 Primarily because of behavioral changes, the incidence of AIDS has fallen by nearly 50 percent since 1992.22 The percentage of fatal crashes involving drunk drivers declined from 30 percent in 1982 to 17 percent in 1999.23 Each of these improvements in risk factors was facilitated by national campaigns that warned of the hazards of particular behaviors.23
The time has come to consider another major campaign. Even though the requisite behavioral changes may be small, they may be difficult to accomplish. The fact that most health-related behaviors have improved while obesity has worsened may be an indication of just how daunting the prospect of reducing levels of obesity may be. The rising prevalence and severity of obesity are already reducing life expectancy among the U.S. population. A failure to address the problem could impede the improvements in longevity that are otherwise in store.
I am indebted to John Wilmoth and Mitch Lazar for suggestions and assistance.
Source Information
From the Population Studies Center, University of Pennsylvania, Philadelphia.
References
Olshansky SJ, Passaro D, Hershow R, et al. A potential decline in life expectancy in the United States in the 21st century. N Engl J Med 2005;352:1138-1145.
Arias E. United States life tables, 2002. National vital statistics reports. Vol. 53. No. 6. Hyattsville, Md.: National Center for Health Statistics, 2002:25, 29. (DHHS publication no. (PHS) 2005-1120 PRS 04-0554.)
Arias E. United States life tables, 2002. Vol. 53. No. 6. Hyattsville, Md.: National Center for Health Statistics, 2002:29. (PHS) 2005-1120 PRS 04-0554.)
Lee R, Miller T. Evaluating the performance of the Lee-Carter method for forecasting mortality. Demography 2001;38:537-549.
Rosenberg M, Luckner W. Summary of results of survey of seminar attendees. North Am Actuarial J 1998;2:64-82.
Tuljaparkar S, Boe C. Mortality change and forecasting: how much and how little do we know? North Am Actuarial J 1998;2:13-47.
Oeppen J, Vaupel JW. Broken limits to life expectancy. Science 2002;296:1029-1031.
White K. Longevity advances in high income countries, 1955-96. Popul Dev Rev 2002;28:59-76.
Cutler D. Your money or your life: strong medicine for America's health care system. New York: Oxford University Press, 2004.
Perls TT, Wilmoth J, Levenson R, et al. Life-long sustained mortality advantage of siblings of centenarians. Proc Natl Acad Sci U S A 2002;99:8442-8447.
Molla MT, Madans JH, Wagener DK. Differentials in adult mortality and activity limitation by education in the United States at the end of the 1990s. Popul Dev Rev 2004;30:625-646.
Costa DL. Understanding the twentieth-century decline in chronic conditions among older men. Demography 2000;37:53-72.
Zimmer C. Do chronic diseases have an infectious root? Science 2001;293:1974-1977.
Freid VM, Prager K, MacKay AP, Xia H. Health, United States, 2003: with chartbook on trends in the health of Americans. Washington, D.C.: Government Printing Office, 2003:169, 212, 228. (DHHS publication no. 2003-1232.)
Pampel FC. Declining sex differences in mortality from lung cancer in high-income nations. Demography 2003;40:45-66.
Reynolds SL, Crimmins EM, Saito Y. Cohort differences in disability and disease presence. Gerontologist 1998;38:578-590.
OECD health data 2004. Paris: Organisation for Economic Co-operation and Development, 2004.
Board of Trustees. Federal old-age and survivors insurance and disability insurance trust funds: 2004 annual report. Baltimore, Md.: U.S. Social Security Administration, 2004.
Manton KG, Stallard E, Tolley DH. Limits to human life expectancy: evidence, prospects, and implications. Popul Dev Rev 1991;17:603-637.
Hill JO, Wyatt HR, Reed GW, Peters JC. Obesity and the environment: where do we go from here? Science 2003;299:853-858.
Carpenter D. Food industry push: cater to health needs. Press release of the Associated Press, New York, January 18, 2005.
Jaffe H. Whatever happened to the U.S. AIDS epidemic? Science 2004;305:1243-1244.
Cutler DM. Behavioral health interventions: what works and why? In: Anderson NB, Bulatao RA, Cohen B, eds. Critical perspectives on racial and ethnic differences in health in late life. Washington, D.C.: National Academies Press, 2004:643-74.(Samuel H. Preston, Ph.D.)