Birthweight and Breastfeeding Shape Patterns of Biological Aging Across Adult Organs

Early‐life nutrition may influence long‐term health, but its impact on organ‐specific biological aging is unclear. We investigated associations of birthweight and breastfeeding with multi‐organ aging and potential mediating pathways in adulthood. Using UK Biobank data, we examined associations of retrospectively reported birthweight ( n  = 21,446) and breastfeeding history ( n  = 29,713) with 19 proteomics‐derived aging indices, comprising 18 organ‐specific indices and one composite index. Multivariable linear regression assessed individual and joint associations, effect modification by sex and socioeconomic factors, and mediation by allostatic load (AL) and C‐reactive protein (CRP). Higher birthweight was inversely associated with aging across 11 organ systems (all FDR‐adjusted p  < 0.05), with strongest associations in the intestine ( β  = −0.069), esophagus ( β  = −0.055), and stomach ( β  = −0.053). Breastfeeding exposure was associated with slower aging of the intestine ( β  = −0.047) and adrenal gland ( β  = −0.030), but modestly faster thyroid aging ( β  = 0.033). The combination of higher birthweight and breastfeeding was associated with favorable aging profiles, particularly in the intestine (β = −0.13), stomach (β = −0.09), and kidney (β = −0.08). Associations were stronger among females and socioeconomically advantaged or healthier‐behavior subgroups. AL mediated up to 35.3% of the birthweight–aging association, whereas CRP showed minimal mediation. Higher birthweight and breastfeeding, particularly in combination, produce the most favorable profiles of multi‐organ biological aging in adulthood, with AL partially mediating these effects. These findings provide novel evidence that prenatal and postnatal nutrition leave enduring, system‐wide imprints on adult aging trajectories.