Influence of Underlying Diseases and Age on the Association between Obesity and All-Cause Mortality in Post-Middle Age

Background: Studies on the association between obesity and all-cause mortality have found that the degree of obesity is directly proportional to all-cause mortality. In contrast, there have been studies indicating that obese people with underlying diseases have a higher survival rate. We hypothesized that age and underlying diseases lead to such contrasting results. Therefore, we conducted a study to clarify the influence of post-middle age obesity and underlying diseases on all-cause mortality. Methods: This study used data from longitudinal studies in the United States, which conducted follow-up for 19 years on 33,708 participants in different age groups: ≥45, 45 - 64, and ≥65 years. Hazard ratio (HR) was determined using the Cox proportional hazards model to analyze a group consisting of all participants, a group of those with underlying diseases, and a group of those without underlying diseases, considering age, gender, education history, marital status, household income, smoking history, and BMI category as covariates. Results: In the group aged ≥65 without underlying diseases, HR was almost 1 in those with BMI 25 - 35 kg/m2. Further, HR was higher in the 45 - 64 age group without underlying diseases if BMI was >35 kg/m2. However, HR was approximately 1 in the ≥65 age group. Conclusions: The study revealed that among individuals aged ≥65 years without underlying diseases, there was no association between obesity and all-cause mortality. Among individuals without underlying diseases, HR was higher in the 45 - 64 age group with BMI > 35 kg/m2 but was approximately 1 among those aged ≥65 years. Therefore, an interaction based on age was detected. These findings may lead to recommendations regarding the need to modify the advice and education provided to obese individuals in different age groups.