Mortality risk relationship using standard categorized BMI or knee-height based BMI – does the overweight/lower mortality paradox hold true?

The body mass index (BMI) is prone to misclassification of obesity due to age-related height loss and resulting measurement errors. Knee-height based BMI (KH-BMI) has not been previously studied in relation to mortality risk in older adults. To evaluate the age- and sex-specific mortality risk relationship using classic BMI and knee height predicted BMI (KH-BMI) overweight and obesity in a 15-year follow-up study including older Swedish adults aged 60–93 years. A 15-year follow-up study among 2,786 individuals aged ≥ 60 years. Height, weight and KH were measured. KH-predicted height was estimated using formulated gender-specific equations. Classic BMI and KH-BMI (kg/m2) were calculated. Mortality data was obtained from the Swedish death registry. Questionnaires were used to collect data on obesity-related lifestyle factors and comorbidities. Cox regression revealed that using the classic BMI, when comparing with the normal/underweight reference group, there was a mortality risk among overweight men (HR = 0.67, 0.52–0.87), overweight women (HR = 0.79, 0.65–0.97), and obese men (HR = 0.60, 0.41–0.89) aged ≥ 80 years old. Using the KH-BMI, only overweight men and overweight women aged ≥ 80 years had a lower mortality risk, men (HR = 0.71, 0.55–0.92); women (HR = 0.77, 0.62–0.95) after adjusting for obesity-related lifestyle factors and comorbidities. There is evidence that obesity is overestimated by the BMI, in comparison with the KH-BMI classification. In terms of mortality risk and after adjusting for height, there remains a paradoxical protective association between overweight and mortality. Regardless of classic BMI or KH-BMI estimation, overweight men and women aged ≥ 80 years had a lower mortality risk compared to normal/underweight men and women ≥ 80 years.