As noted, the two-dimensional perspective can account for variation in health status across age groups and periods of observation. The three-dimensional perspective incorporates these factors, but it also accounts for variation in health status across different birth cohorts—groups of people born during a particular period or year. As we will show, this is a crucial difference when considering the future health implications of the US obesity epidemic.
In one analysis that adopted a three-dimensional perspective by accounting for the latent health effects of the US obesity epidemic, the current life-shortening effect of adult obesity was shown to be about three-quarters of a year—an amount larger than from all sources of accidental death combined. But the life-shortening effect of obesity could increase to two to five years, perhaps more, once the cohort effects of childhood obesity on subsequently observed adult mortality are taken into account.15
Although that study accounted for the third dimension, it was criticized by scholars who argued that the obesity epidemic is unlikely to counter the complex array of medical, social, and behavioral factors that have resulted in improvements in US life expectancy in recent decades.16,17
Nevertheless, a number of recent studies show that the US obesity epidemic, which began in the early 1980s,18,19
could erode hard-won gains in health and longevity and is already doing so. For instance, data from the National Health and Nutrition Examination Surveys show that cardiovascular risk-profiles have worsened since the late 1980s as the result of unfavorable trends in type 2 diabetes, blood pressure, and body mass index.20
Autopsy findings for people ages 16–64 who died of external causes show that declines in the severity of coronary heart disease among younger people ended in the mid-1990s and may have reversed after 2000.21,22
Consistent with this evidence, trends in coronary heart disease mortality between 1980 and 2002 showed substantial improvement among Americans age fifty-five or older, but much less improvement among Americans ages 35–54.23
In fact, the most recent period of observation (2000–02) showed a decline of only 0.5 percent in coronary heart disease mortality among younger adult males and an increase of 1.5 percent among females in this age range.
An analysis of nearly one million participants in fifty-seven prospective studies in North America and Western Europe have shown that elevated body mass index increases the risk of premature mortality.24
This study found that the optimal body mass index is 22.5–25.0 and that median survival was two to four years less among participants with an index of 30–35 and eight to ten years less among those with an index of 40–45. Similarly, an analysis of 1.46 million white participants in nineteen prospective studies from the National Cancer Institute Cohort Consortium has shown that overweight and obesity are associated with mortality from all causes.25
In accord with findings from these prospective studies, statistically significant declines in life expectancy occurred between 1983 and 1999 for a substantial number of US counties.26
These declines were most pronounced among females who lived in disadvantaged counties in the Southeast—precisely where the obesity epidemic has struck the hardest. In the period of observation just before the onset of the obesity epidemic (1961–83), no significant declines in life expectancy at birth were observed in any US county. The authors attribute worsening life expectancy in these counties to chronic diseases related to smoking, hypertension, and obesity—risk factors that tend to be concentrated in those areas.
Studies published within the past two years have shown that recent cohorts of younger Americans (born since 1960) are at increased risk of obesity, compared to cohorts born prior to 1960.19,27,28
For instance, Joyce Lee and colleagues used data from the National Health and Nutrition Examination Surveys to show that although at least 20 percent of Americans born after 1965 were obese by ages 20–29, this threshold was not reached until ages 30–39 for Americans born between 1946 and 1965, until ages 40–49 for Americans born between 1936 and 1945, and until ages 50–59 for Americans born between 1926 and 1935.28
Using data from twenty-seven National Health Interview Surveys, Eric Reither and colleagues found that the predicted probability of obesity at age twenty-five increased by 30 percent for cohorts born between 1955 and 1975.19
These studies indicate that recent cohorts of Americans are heavier than their predecessors and have become obese earlier in life. This implies that an increasing share of young Americans are about to carry excess weight—and the elevated morbidity and mortality risks associated with it—for a larger proportion of their lives.
Although these trends were observed in the general US population, they were consistently stronger among blacks, particularly black females.19,27,28
John Komlos and colleagues found that body mass index increased by 5.6 units among black females ages 2–19 between 1941 and 2004, compared to just 1.5 units among white females.27
This finding was reinforced by evidence demonstrating that the predicted probability of obesity for a twenty-five-year-old black female from the 1980–84 birth cohort was 0.27—more than twice the probability for a white female from the same cohort.19
Substantial racial/ethnic disparities in obesity have been documented elsewhere.29
This emerging body of three-dimensional research goes further by suggesting that the observed disparities are deeply rooted in the past and therefore likely to persist into the future.