Relationship between body mass index reference and all-cause mortality: evidence from a large cohort of Thai adults

We investigate variation in body mass index (BMI) reference and 5-year all-cause mortality using data from 87151 adult Open University students nationwide. Analyses focused on BMI reference bands: "normal" (≥18.5 to <23), "lower normal" (≥18.5 to <20.75), "upper normal" (≥20.75 to <23), and "narrow Western normal" (≥23 to <25). We report hazard ratios (HR) and 95% Confidence Intervals adjusting for covariates. Compared to lower normal, adults aged 35-65 years who were obese (BMI ≥ 30) were twice as likely to die during the follow-up (HR 2.37; 1.01-5.70). For the same group, when using narrow Western normal as the reference, the results were similar (HR 3.02; 1.26-7.22). However, different combinations of BMI exposure and reference band produce quite different results. Older age persons belonging to Asian overweight BMI category (≥23 to <25) were relatively protected from mortality (HR 0.57; 0.34-0.96 and HR 0.49; 0.28-0.84) when assessed using normal (≥18.5 to <23) and upper normal (≥20.75 to <23) as reference bands. Use of different "normal" reference produced varying mortality relationships in a large cohort of Thai adults. Caution is needed when interpreting BMI-mortality data.