Data from the US Life Tables (1999), NHANES III (1988-1994), First National Health and Nutrition Epidemiologic Follow-up Study (NHANES I and II, 1971-1992) and NHANES II Mortality Study (1976-1992) were used to derive years of life lost estimates for adults aged 18-85 years.
Statistical Analysis (method of calculation)
The YLL due to obesity was estimated by combining three types of information: an estimate of the distribution of BMI for each year of adult life; an estimate of the hazard ratio for death given various BMI levels in each year of adult life; and the probability of death during each year of adult life.
BMI distribution was derived from NHANES III. BMI was divided into ordered, non-overlapping categories for staring with less than 17, 17 to <18, 18 to <19 and so on to the highest category, ≥45kg/m2. For each race and gender group, the proportion of individuals within each BMI category was estimated. The proportions were calculated using a smoothing procedure based on moving averages.
To estimate the hazard ratio for death at a given BMI level in each year of adult life, data was combined from the First National Health and Nutrition Epidemiologic Follow-up Study and The NHANES II Mortality Study. Cox regression models were used with mortality as the outcome and BMI, BMI2, age, age2, and their interaction terms as covariates. Potential age, race and gender interactions were accounted for by fitting separate models to each race and gender group. Interactions of BMI and BMI2 with age2 were not used because prior research indicated that change in the BMI parameters appears to be roughly linear in age. Smoking status was controlled for in all analyses (including former smoking status).
The life table for the total population of the United States in 1999 was used to derive information on the probability of death during each year of life.
Using the above mentioned data, an estimation was made for each age interval, in each of four race/gender groups, of the probability of death within the interval, which was conditional on having lived to the start of that interval and being in the first BMI category. For a person of a given age in a given BMI category, an estimation of the expected age of death was made. BMI=24 was used as a reference category to calculate the YLL.