["American Journal of Biological Anthropology, Volume 190, Issue 3, July 2026. ", "\nABSTRACT\n\nObjectives\nIn this study, we analyzed δ13C and δ15N values in dentine collagen (reflecting early‐life diet) and fingernail keratin (reflecting recent diet) from 92 living adult individuals, having 75 individuals having paired dentine and fingernail samples available, across Brazilian regions with contrasting socioeconomic conditions. While the sample size was necessarily limited by the availability of paired tissues, it provides a unique dataset for assessing dietary change within the lifespan and its relationship to socioeconomic context.\n\n\nMaterials and Methods\nUsing Bayesian linear models and self‐reported dietary intake questionnaires, we investigated the influence of age, geographic region, and socioeconomic status on isotopic composition.\n\n\nResults\nWe found significant regional and socioeconomic effects: individuals from high‐Human Development Index (HDI) municipalities and those reporting greater sugar intake had elevated δ13C values, while frequent fish and seafood consumption was associated with higher δ15N. Notably, δ15N in dentine collagen declined with tooth age, suggesting a historical decrease in the δ15N of food sources, likely linked to increased reliance on synthetic fertilizers and nitrogen‐fixing crops.\n\n\nDiscussion\nBoth tissues show a consistent regional contrast: participants from the Southeast exhibit higher δ13C and lower δ15N than those from the Northeast, consistent with greater intake of C4‐derived, corn‐ and sugar‐based processed foods (“supermarket diet”’) and proportionally lower trophic‐level protein in the Southeast, versus lower δ13C in the Northeast consistent with diets centered on C3 staples (rice, beans, manioc) and more limited access to ultra‐processed foods.\n\n"]