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 ofThai adults. Caution is needed when interpreting BMI-mortality data.