Book-embedded hair reveals mineral-rich diets among urban commoners in early modern to modern Japan.

Urban populations in early modern to modern Japan depended strongly on rice and marine fish, raising the question of whether such apparently simple diets provided sufficient mineral availability in everyday life. Here, we analyze human hair preserved in historical book covers to reconstruct elemental exposure among urban commoners from the seventeenth to nineteenth centuries. Screening approximately 70,000 volumes allowed individual hair samples to be linked to specific cities and years through colophon records, enabling a rare population-scale reconstruction from abundant and well-dated materials. Multi-element analyses showed that several elements, including essential ones, were consistently higher in historical hair than in contemporary reference populations, indicating greater mineral availability in past diets. Carbon and nitrogen isotope data confirmed strong reliance on C3 staples, particularly rice, with marine fish providing a major and increasing nitrogen source. Interpreted together, the elemental and isotopic patterns suggest effects of rice polishing, marine resources, salt preservation, cookware, and other material practices. These findings show that historical books can preserve bioarchives for reconstructing everyday diet and material culture, and they indicate that urban commoners in early modern to modern Japan had substantially greater dietary mineral availability than contemporary populations, despite apparently simple diets centered on rice and marine fish.