Testing the causal effects between subjective wellbeing and physical health using Mendelian randomisation

Objectives To investigate whether the association between subjective wellbeing (subjective happiness and life satisfaction) and physical health is causal. Design We conducted two-sample bidirectional Mendelian randomisation between subjective wellbeing and six measures of physical health: coronary artery disease, myocardial infarction, total cholesterol, HDL cholesterol, LDL cholesterol and body mass index (BMI). Participants We used summary data from four large genome-wide association study consortia: CARDIoGRAMplusC4D for coronary artery disease and myocardial infarction; the Global Lipids Genetics Consortium for cholesterol measures; the Genetic Investigation of Anthropometric Traits consortium for BMI; and the Social Science Genetics Association Consortium for subjective wellbeing. A replication analysis was conducted using 337,112 individuals from the UK Biobank (54% female, mean age =56.87, SD=8.00 years at recruitment). Main outcome measures Coronary artery disease, myocardial infarction, total cholesterol, HDL cholesterol, LDL cholesterol, BMI and subjective wellbeing. Results There was evidence of a causal effect of BMI on subjective wellbeing such that each 1 kg/m 2 increase in BMI caused a 0.045 (95%CI 0.006 to 0.084, p =0.023) SD reduction in subjective wellbeing. Replication analyses provided strong evidence of an effect of BMI on satisfaction with health (β=0.034 (95% CI: −0.042 to −0.026) unit decrease in health satisfaction per SD increase in BMI, p <2 -16 ). There was no clear evidence of a causal effect between subjective wellbeing and the other physical health measures in either direction. Conclusions Our results suggest that a higher BMI lowers subjective wellbeing. Our replication analysis confirmed this finding, suggesting the effect in middle-age is driven by satisfaction with health. BMI is a modifiable determinant and therefore, our study provides further motivation to tackle the obesity epidemic because of the knock-on effects of higher BMI on subjective wellbeing.