To the Editor: Dr Fontaine and colleagues1 estimated that a body mass index (BMI) of 33 at age 40 years was associated with a loss of life expectancy of 2 to 3 years. In contrast, we recently reported that a BMI of 30 or more (mean BMI of 33) at age 40 years was associated with a loss of 6 to 7 years of life.2 We believe that the method used by Fontaine et al underestimates the health effects of lifelong obesity. While we used a cohort life table to answer a cohort question, Fontaine et al used a period life table to answer a cohort question.
The cohort life table describes survival of a true cohort, using the age-specific mortality rates from a single cohort across many periods. This requires a population with a narrow age range at baseline and a long period of follow-up. By contrast, the period life table of Fontaine et al used the age-specific mortality rates from a single period across many cohorts. This approach requires a population with a wide age range at baseline and a short period of follow-up. Our analysis aimed to identify the loss of life expectancy associated with obesity at age 40 years, regardless of future changes in BMI. This is a question about a cohort and so the mortality rates throughout follow-up of those individuals who are obese or normal weight at age 40 years, incorporated into a cohort life table, give the correct result. The BMI was defined at age 40 years and the true cohort mortality was followed up over the next decades. Using this approach, the relative risks of mortality of the obese cohort remain constant with age.1 - 3
Fontaine et al attempted to estimate the expected loss of life if a given BMI were maintained throughout life. This is also a question regarding a cohort (although such individuals are unlikely to exist). However, they used period life tables, based on the age-specific and BMI-specific relative risks of mortality from the National Health and Nutrition Studies. In doing so, they assumed that these relative risks of mortality represent those of cohorts with one lifelong BMI, ignoring that the excess mortality risk associated with a given BMI at a given baseline age is derived from many different cohorts. These cohorts vary from those who have previously gained weight from a lower BMI to those who have previously lost weight from a higher BMI. Because individuals generally increase their weight with age, many of those obese individuals at baseline would have had lower BMIs when they were younger. Such individuals probably have lower mortality risks than those who have lifelong obesity, because the risk of death in those with a given BMI at age 60 years is higher in those with a higher BMI 20 years earlier.2 Furthermore, with increasing age, individuals with lifelong obesity are selected out of the population by their higher mortality. Consequently, the relative mortality risks associated with BMI reported by Fontaine et al decreased with age, as is seen when BMI and age are measured cross-sectionally.1 ,4
Period life tables for obesity, such as those constructed by Fontaine et al, are useful for answering population questions.5 They can be constructed using readily available, short-term prospective data from a single period, but they are not informative about the life course of a true cohort. To assess the impact of a risk factor on the future life course of a cohort, long-term prospective data, implicitly taking into account risk factor history and mortality selection are needed.
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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