Objectives: Self-reported Body Mass Index (BMI) based on self-reported height and weight is a widely used measure of adiposity in epidemiological research. Knowledge about the accuracy of these measures in late life is scarce, and especially if there is intra-individual changes over time.
Methods: Seven hundred seventy-four men and women, aged 40 to 88 at baseline (mean age 63.9), in The Swedish Adoption/Twin Study of Aging self-reported and had their height and weight measured by experienced research nurses five times during a twenty year period. BMI was calculated as weight (kilos)/height (meter)2.
Results: There was significant correlation between self-reported and measured height (0.97-0.98), weight (0.97-0.98), and BMI (0.93-0.95) at each measurement occasion, and substantial agreement for BMI as a categorical variable (Kappa coefficient 0.72-0.81). Latent growth curve modeling showed an increase in the mean difference between self-reported and measured values over time for height (0.04 cm/year) and BMI (0.02 kg/m2/year), but not for weight.
Conclusions: There is a very small significant increase in the mean difference between self-reported and measured BMI with aging, mainly due to unawareness of changes in height, which probably would not affect the results when self-reported BMI is used as a continuous variable in longitudinal studies.