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•Intra-cellular volume fraction (ICVF), a proxy of axonal content, shows region-specific linear and quadratic associations with age.•ICVF and myelin water fraction were positively correlated in all examined regions.•Men showed greater ICVF than women, which was accounted for by intracranial volume.•Neither metabolic syndrome risk score nor adiposity scores were related to ICVF.•The adiposity- or metabolic syndrome-myelin associations were independent of ICVF.

•Greater metabolic syndrome risk score in adults correlate with lower brain myelin.•Higher adiposity (BMI and waist circumference) correlate with lower brain myelin.•These associations were present only in late-myelinating white matter regions.•These associations were independent of age, sex, race, blood pressure and SES.•Metabolic syndrome and adiposity: key modifiable risk factors for decreased myelin.