Reexamining the declining effect of age on mortality differentials associated with excess body mass: evidence of cohort distortions in the United States.

OBJECTIVES: I examined age patterns of mortality differentials associated with body mass because the declining age effect observed in previous comparisons of cross-sectional age groups is susceptible to cohort and period distortions and because previous studies used time since baseline as time at risk, making the evaluation of age-specific mortality impossible.
METHODS: I conducted a parametric survival analysis of data from the 1988-1994 National Health and Nutrition Examination Survey for 3 cohorts of American men and women born from 1901 through 1957 and observed from 1988 through 2006 under an age-period-cohort framework.
RESULTS: Mortality differentials strengthened across cohorts but did not decline with age or change over the study period. Because excess overweight and obesity mortality increased from earlier cohorts to more recent ones, ignoring cohort differences led to a declining age pattern of excess mortality.
CONCLUSIONS: Cross-sectional age patterns of mortality differentials appear to be distorted by cohort differences. Age should be used as risk time to study age variations in associations between risk factors and time to event.