Foraging is a key behaviour, and several aspects of foraging remain to be investigated in many wild species. Low energy gain or reduced protein, nutrient, and mineral intake may explain poor individual condition, low reproductive output, high mortality, and, in extreme cases, population declines. Our study explores how foraging ecology, diet composition, and nutritional status could influence individual performance in an endangered population of caribou (Rangifer tarandus caribou). We measured stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) as well as the concentration of 21 trace elements in hairs of 44 caribou (~ 50% of the population). Our results showed that the Atlantic-Gaspésie caribou population consumed mainly lichens, horsetails, deciduous shrubs, and deciduous trees. We found a correlation between diet (described using stable isotope ratios) and accumulation of trace elements in hair using a redundancy analysis, and we detected relationships between diet, trace elements and vital rates using logistic and Cox models. Pregnancy and calf survival probabilities were higher when the δ13C signature of female caribou was higher, and the adult survival probability was higher in caribou that had higher concentrations of zinc and sodium, and lower concentrations of cesium and manganese in their hair. Based on the correlation we found between nutrition and individual performance, our study suggests that nutrition may be an important factor to consider in the recovery of this endangered caribou population.