Alcohol consumption can have significant deleterious consequences, including brain atrophy, neuronal loss, poorer white matter fiber integrity, and cognitive decline, but the effects of lightto-moderate alcohol consumption on brain structure remain unclear. Here we examine the associations between alcohol intake and brain structure using structural, diffusion tensor, and neurite orientation dispersion and density imaging data from 19,825 generally healthy middle-aged and older adults from the UK Biobank. Systematically controlling for potential confounds, we found that greater alcohol consumption was associated with lower global gray and white matter volume, regional gray matter volume in cortical and subcortical areas, and white matter fiber integrity and complexity. Post hoc analyses revealed that these associations were non-linear. Our findings extensively characterize the associations between alcohol intake and gray and white matter macrostructure and microstructure. Consuming two or more units of alcohol per day, equivalent to one drink in some establishments, could have negative effects on brain health, an important public health finding.Converging lines of research provide compelling evidence that chronic, excessive alcohol consumption is associated with global brain atrophy and regional brain changes. [1][2][3] Recent meta-analyses of magnetic resonance imaging (MRI) findings show that individuals with alcohol use disorder (AUD) have less global white matter volume (WMV) 4 and less gray matter volume (GMV) -both globally and locally in corticostriatal-limbic regions 5 -than healthy controls.Further, in a meta-analysis of pooled, multinational datasets from 33 imaging sites, individuals with AUD had lower local thickness and surface area of the hippocampus, thalamus, putamen, and amygdala than controls. 6In studies using diffusion-weighted MRI (dMRI), which allows a non-invasive investigation of white matter microstructure via measures of water molecule diffusion, individuals with AUD had lower fractional anisotropy (FA; the directional coherence of water molecule diffusion) and greater mean diffusivity (MD; the magnitude of water molecule diffusion) in the corpus callosum, frontal forceps, internal and external capsules, fornix, superior cingulate, and longitudinal fasciculi than controls. 1,7 However, because conventional dMRI measures (FA and MD) are based on a simplistic model of brain tissue microstructure, they fail to account for the complexities of neurite geometry. 8 For example, the lower FA observed in individuals with AUD 1,7 may reflect lower neurite density and/or greater orientation dispersion of neurites, which conventional dMRI measures do not differentiate. 9,10 A key question raised by prior findings in individuals with AUD that remains is whether, similar to heavy drinking, light-to-moderate alcohol consumption adversely affects brain structure. Further, is the relationship between alcohol intake and brain structure linear? In some studies of middle-aged and older adults, moderate alcoh...