In the present study, we found the potential dose-dependent and sex-dependent negative association of alcohol intake on the risk of RA in the meta-analysis and prospective cohort study. Moderate alcohol consumption (17.90 g/day to 69.59 g/day) was inversely associated with the risk of RA, and high intake (≥ 111 g/ day) was positively associated with the risk of RA. Besides, AUD was found to be positively associated with RA risk in prospective UK Biobank cohort. Genetic correlation and two-sample MR study yielded similar results.
Recent years, accumulating evidence has supported that moderate alcohol intake may have potential health benefits including reduced risk of RA. For example, a recent prospective UKB study found low to moderate alcohol consumption was preventive to RA onset (adjusted HR: (0.69, 0.83) [38]. In our study, in addition to multiple indicators of alcohol intake, such as alcohol intake frequency, drink groups, and whether drink over limits, which have been included in previous studies, we transformed the categorical variable, alcohol intake frequency, into continuous variables, and performed a dose-response analysis, whose results suggested a U-shape relationship. A meta-analysis in 2014 that included 195,029 participants and 1,878 incident RA cases also found a dose-response relationship between alcohol consumption and the risk of RA, with the low to moderate alcohol consumption associated with a reduced risk of RA [10]. Besides, it was found that females who had low to moderate alcohol consumption had 19% decreased risk of RA [10]. In our sex-stratified analysis, we found that the protective effect was only observed in women.
We converted the alcohol consumption frequency to alcohol intake per day, and developed a RCS model. Our study detected a potential inverse association of alcohol consumption on RA in UK Biobank cohort with the narrow 95% CIs, suggesting that such protective pattern was limited for RA, even though it was found statistically significant possibly due to the large sample size. Besides, we found that the effect of alcohol consumption may differ by age, suggesting that individuals in different age groups may have different susceptibility to alcohol intake on RA risk. A recent Global Burden of Disease Study found that alcoholic drinks may be harmful for adults younger than 40 years compared with those over 40 years [39]. Since UK Biobank only included participants aged over 40 years, it was possible that the health benefit of alcohol was more likely to be detected in UKB Biobank participants. Besides, harmful use of alcohol was particularly concentrated in males aged 15–39 years, whose population was lacking in UK Biobank.
A possible mechanism of the J-shaped relationship between alcohol intake and the risk of RA was that alcohol could dose-dependently affect immune system in the aspects of innate and adaption[40]. Low to moderate alcohol might downregulate immune response and decrease the production of pro-inflammatory cytokines, such as IL-1, IL-6 and TNF[12, 40]. In a Nurses’ Health Study cohort study, a J-shaped association of daily alcohol consumption (up to 20g) with IL-6 levels was found in patients of RA prior to symptoms[41]. Besides, results from a large cross-sectional study in the UK found that moderate alcohol intake up to 40g per day was associated with lower concentrations of CRP compared to non-drinking and heavy drinking [42], suggesting that excessive drinking could increase inflammation.
While low to moderate alcohol intake might have a protective effect on disease incidence and development, persistent heavy drinking could affect immune systems, decrease white blood cells, platelets, and granulocyte mobility, as well as shorten various disease onsets [43]. AUD was considered as a brain disorder caused by long-term alcohol misuse, encompassing both alcohol dependence and alcohol abuse. A National Inpatient Sample cohort study found that AUD hospitalizations increased over the 19-year study period from 1998 to 2014 to 3.5-fold in RA [44], suggesting a potential relationship between AUD and RA.
The present evidence from genetic study was largely consistent with observational evidence. By using two-sample MR, we found that genetically predicted alcohol intake was not associated with the increased risk of RA, while AUD was positively associated with RA. Results from LDSC also found that alcohol intake was not correlated with RA, while AUD was positively associated with the risk of RA. Consistent with previous studies, Bae et al. did not found evidence of a causal association between alcohol intake and RA (beta = 0.218, SE = 0.213, P = 0.306) using IVW method [45]. Besides, Jiang et al. did not detect a potential association between alcohol intake and RA, based on two-sample MR (OR: 0.80; 95% CI: 0.54, 1.19; P = 0.27) and genetic correlation analysis (rg = -0.07; P = 0.21)[46].
Though the underlying biological mechanism between AUD and RA is still unclear, there are some possible explanations. First, regular heavy drinking can promote pro-inflammatory immune responses and affects the normal functioning of the adaptive immune response which may increase the risk of RA [47]. In addition, binge drinking may reduce telomere length that could increase secretion of inflammatory cytokines such as IL-6, which could contribute to an increased inflammatory load, increasing the risk of RA [48, 49]. Furthermore, patients of AUD often have depression, which may increase the risk of RA through increasing immune cytokines like PGE2 [50, 51]. Although these explanations are biologically plausible, more studies are needed to explore the exact mechanism.
The present study has the following strength. First, our study was based on large prospective biomedical dataset, which increased the power and enabled our study to detect significant associations even though the effect was small. Second, our study confirmed a potential U-shaped relationship between alcohol intake and the risk of RA as suggested by previous studies. Besides, we also revealed a sex-specific pattern of alcohol intake in RA risk. However, several weaknesses worth the consideration. First, although the efforts to reduce pleiotropic effect across genetic variants, we cannot exclude the possibility of a shared genetic basis for alcohol consumption or AUD with RA. In that case, the association detected in our genetic study may not represent a direct causal relationship.