Using a relative measure of excess body fat derived from the BMI distribution by subpopulations, we find a strong effect of excess weight on the risk of mortality among individuals aged 60 and older in Mexico and in the United States. Our findings indicate significantly higher mortality among elderly persons with a BMI of 30.3 and over in the United States and among those with a BMI of 27.3 and over in Mexico compared with elderly persons in the middle of the distribution of BMI (25.08–27.37 for the United States and 24.7–27.3 for Mexico).
The effect of excess weight is larger in Mexico than in the United States for the entire elderly population, irrespective of age. For instance, the relative difference in the probability of dying at age 60 between individuals in the fifth quintile and those in the third quintile of BMI is 20% in the United States. In Mexico, the difference is about three times larger. These relative differences tend to diminish slightly by age, but they are largely age invariant, and the differences of differences between the United States and Mexico are virtually unchanged as age increases (see ).
In contrast, the differences in mortality associated with excess weight tend to vary by educational level, a coarse indicator of SES. Our findings suggest that at least part of the higher effects in Mexico could be due to differences in educational composition between the cohorts aged 60 and over in the two countries. To divide the samples into low-educated and high-educated individuals, we used only one cutoff point (six years of formal education) in both countries, which led to sufficient number of cases for meaningful comparisons. This is a pragmatically driven decision, and it does not carry with it the belief that these subgroups are strictly comparable across countries. Among less-educated individuals aged 60–69, the relative differences in the probability of dying by weight status in the United States are very close to those observed in Mexico (more than 60% in both cases). Among people with higher levels of formal education, the differences among countries are substantial. Thus, for example, the relative difference between normal and overweight individuals in the conditional probability of dying at age 60 is about 15% in the United Sates and about 60% in Mexico.
Are our results comparable to others obtained by other researchers? Recent findings in the literature show significant effects of obesity on losses of life expectancy for the United States. Thus, Olshansky et al. (2005)
estimated losses in life expectancy at birth by calculating the reduction in the probability of death that would occur if all overweight or obese respondents were to attain their “optimal” BMI. According to their results, the loss of life expectancy at birth in the United States due to obesity falls in a range between one-third to three-fourths of a year. On the other hand, Fontaine et al. (2003)
estimated the differences in life expectancy between non-obese and obese people. According to their estimates, the difference in life expectancy at age 60 between normal weight (BMI = 24) and severely obese people (BMI ≥ 45) is five years for white women and six years for white men.
We first use cutoff points identical to those used by Fontaine et al. (BMI = 25 and BMI = 45) and estimate age-specific conditional probabilities of dying for individuals aged 60 and older in the United States by weight status (at baseline). We then estimate the life expectancies associated with each of the two sets of conditional probabilities of dying by age and calculate differences in life expectancy between individuals with BMI = 24 and severely obese people (BMI ≥ 45). Our estimates show differences in total (men and women) life expectancy at age 60 of about 6.7 years for the United States and 7.7 for Mexico. The results for the United States are very close to those obtained by Fontaine and colleagues.
We then follow a counterfactual analysis identical to that used by Olshansky et al. (2005)
and estimate losses in life expectancy in the population at age 60 by assuming that a given set of mortality risks are “optimal.” To do so, we simply set the conditional probabilities of dying at age 60 and older among individuals in the highest quintiles of BMI (the fifth quintile for the United States and the fourth and fifth quintiles for Mexico) at levels observed among individuals with “optimal” BMI (the third quintile). The results of removing excess mortality risks among people in the higher quintiles of BMI indicates that the total life expectancy at age 60 would be almost 2 years higher for Mexico and just about 0.55 years higher for the United States. Thus, whatever metric we use yields the same results: excess mortality among those classified as obese or overweight is higher in Mexico than in the United States.
Note that the estimates of losses of life expectancy estimated by Olshansky et al., those from Fontaine et al., and those calculated from our data were produced by applying a sequence of age-specific conditional probabilities of dying associated with each BMI group to obtain life expectancies at specific ages. These estimates apply to individuals who are assumed to remain in the initial state (obese or non-obese) for the rest of their lives. It is a strong counterfactual, but one that provides a sense of magnitude for excess mortality risks even though it does not really represent the length of life that a typical obese individual at age x is expected to live thereafter.
Is the higher relative risk of mortality observed in Mexico a result of the higher probability of suffering obesity-related diseases, or is it a consequence of the higher relative risk of mortality associated with these chronic diseases? According to the decomposition analysis, despite the fact that the probability of suffering obesity-related chronic diseases among individuals in the highest quintile of BMI (as compared with people in the third quintile of BMI) is much higher among the elderly in the United States than it is in Mexico, the relative risk of dying conditional on experiencing these diseases is higher in Mexico. Why should this be the case?
The higher prevalence of obesity-related chronic conditions in the United States and the lower level of lethality of these conditions may be the results of better diagnoses as well as better treatment and adherence to treatment. Indeed, Flegal et al. (2005)
showed evidence of substantial improvements in the treatment of obesity-related chronic diseases in the United States, which contribute to reductions in the lethality of these conditions.
But the higher probability of experiencing chronic conditions in the United States might also be due to an artifact produced by greater accuracy of self-reported conditions, itself a result of better access to and use of medical care. However, for this to explain our findings, the accuracy of self-reported conditions would have to vary by the groups being compared (obese versus normal weight individuals). If both groups underreport at the same rate, one cannot argue that the magnitude of the chronic disease component in Mexico is attributable to underreporting. Thus, the case for an artifact produced by self-reports is a weak one.
There is another, more intriguing explanation for the findings unearthed by the decomposition analysis. The excess prevalence of chronic conditions in the United States may reflect a naturally occurring phenomenon whereby obese people in high-income countries are more likely to experience a host of chronic conditions than those in low-income countries (Burke et al. 2001
) simply because of the different stages these countries occupy in the health transition. Higher mortality among obese Mexicans may be due to a combination of factors. First, inferior access to health services could disproportionally affect those who suffer chronic conditions more often (obese individuals). Second, other forces might induce increases in mortality for obese individuals in Mexico without necessarily increasing the incidence of chronic conditions. For instance, excess mortality may be the result of a double exposure to infectious and parasitic diseases that could aggravate the standard course of chronic diseases. The latter interpretation is consistent with the belief that the ecology of diseases (a combination of infections and chronic conditions) in Mexico, a feature that is quite unique to low-income countries (Monteverde, Noronha, and Palloni 2009
; Palloni et al. 2006
), is more likely to yield higher mortality among the obese without necessitating a pathway operating through chronic conditions.
Could potential future increases in obesity prevalence among the elderly affect mortality risks to the point of reversing past trends of gains in the life expectancy at older ages? Gregg et al. (2005)
showed that changes in behavioral patterns and improvements in medical technologies in the United States have led to substantial declines in the prevalence of obesity-related chronic diseases (such as hypercholesterolemia and high blood pressure). Thus, despite the increases in obesity in the United States, all evidence points to an attenuation of the effect of obesity on mortality over time (Flegal et al. 2005
). Preston (2005)
also noted the importance of cohort effects in the United States. Compared with older cohorts, younger cohorts are exposed to factors—such as higher educational levels, lower exposure to infectious diseases that affect the development of chronic diseases during adulthood, and lower consumption of cigarettes—that decrease morbidity and have a positive influence on longevity. However, it is by no means unthinkable that sufficiently high rates of increases in the age-specific prevalence of obesity could offset these positive influences unless changes toward healthier lifestyles are adopted by the U.S. population (Preston 2005
It is not clear that younger cohorts in Mexico (or in any other country in the LAC region for that matter) are exposed to such benign improvements of mortality regimes. It is more likely that it is in these countries where the increase in obesity will take a larger toll. Indeed, although educational levels have increased steadily over the past 50 years, these populations are still exposed to a higher share of infectious diseases that affect the unfolding of adult chronic diseases, have not achieved the same living standards (particularly nutritional standards) as their counterparts in high-income countries, and are just now being exposed to the consequences derived from increased uptake rates of smoking. According to projections from the WHO (2005
), the prevalence of obesity in Mexico is expected to increase by 52% among men and by 24% among women aged 30 and older between 2002 and 2010. Unless Mexico and LAC countries experience medical improvements, like those observed in the United States, that partially negate the deleterious effects of obesity, longevity among older individuals may be compromised in the future.