We first estimated the proportion at each BMI at approximately age 50 for both sexes using data from NHANES II (1976-1980) and NHANES (2001-2004). In order to generate a relatively smooth BMI distribution, we estimated the BMI distribution for the population ages 48-52 at each BMI in both surveys.
We then used the following formula to calculate the effect of obesity on life expectancy:
A BMI of 24 was defined as the reference category. Therefore, E[YLL | BMI = 24] was defined to be 0, and E[YLL | BMI = b] is interpreted as expected YLL at age 50, comparing those with BMI b with those with BMI 24.
This approach assumes that age-specific mortality and the association between BMI and mortality were constant.
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"6 Can Obesity Account for Cross-National Differences in Life-Expectancy Trends?--Dawn E. Alley, Jennifer Lloyd, and Michelle Shardell."
International Differences in Mortality at Older Ages: Dimensions and Sources.
Washington, DC: The National Academies Press, 2011.
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