The causal associations of smoking and alcohol and coffee intake with fracture and bone mineral density are unknown. We investigated the associations using Mendelian randomization (MR). Summary-level data from UK Biobank for bone fractures (main outcome) (53,184 cases; 373,611 non-cases) and estimated bone mineral density (eBMD) (n = 426,824 individuals) were used. Single-nucleotide polymorphisms associated with smoking initiation (n = 378) and alcohol (n = 99) and coffee (n = 15) intake at the genome-wide significance threshold (P = 5 × 10-8) were identified from published genome-wide association studies. Univariable and multivariable inverse-variance weighted, weighted median, MR-Egger, and MR-PRESSO methods were used for statistical analyses. Genetic predisposition to smoking initiation was associated with fracture but not eBMD. The odds ratio of fracture per one-unit increase in log odds of smoking was 1.09 (95% confidence interval 1.04, 1.15; P = 8.58 × 10-4) after adjustment for alcohol intake in the multivariable MR analysis. The association remained in complementary analyses. Genetically predicted alcohol and coffee intake was not associated with fracture or eBMD. Nevertheless, genetic liability to alcohol dependence, based on variants in the ALD1B gene, was associated with fracture and lower eBMD. The odds ratio was 1.06 (95% confidence interval 1.01, 1.12; P = 0.018) per genetically predicted one-unit higher log odds of liability to alcohol dependence. This MR study strengthens the causal inference on an association between smoking and higher fracture risk but found no linear association of modestly higher alcohol and coffee intake with fracture or BMD. However, alcohol dependence may increase fracture risk.
Keywords: Alcohol; Bone mineral density; Coffee; Fracture; Mendelian randomization; Smoking.