Can stable isotope analysis reveal dietary differences among groups with distinct income levels in the city of Piracicaba (southeast region, Brazil)?

BACKGROUND: The present study aimed to determine whether the δ¹³C and δ¹⁵N of fingernails (δ¹³CF and δ¹⁵NF, respectively) would exhibit differences between groups from different income levels in the city of Piracicaba, Brazil. METHODS: In 2010, the fingernails of 273 participants belonging to six income groups were analysed to determine isotopic composition. δ¹³CF and δ¹⁵NF were compared with the stable isotope of a putative diet (δ¹³CD and δ¹⁵ND, respectively), which was estimated via an isotopic mass balance using, as a weighting factor, macronutrient intake by the main food items, as obtained by the 2008–2009 household food purchases conducted by the Brazilian Institute of Geography and Statistics. RESULTS: δ¹³CF values showed a decreasing trend towards the lower income level groups, in accordance with the δ¹³CD values that showed the same trend. This isotopic change was mainly a result of the higher consumption of beef, bread, soft drinks and dairy by the highest income group; and also as a result of the higher consumption of soybean oil, rice and sugar by the lowest income group. The δ¹⁵NF values failed to capture differences in groups between income levels. This outcome was not expected because individuals in a higher income group tend to consume more animal protein and, as a result of the trophic fractionation, have higher δ¹⁵N values. CONCLUSIONS: The combination of household purchase surveys and stable isotopic composition in modern humans is found to be a valuable tool, especially with respect to determining the role of C₃ and C₄ plants through the complex modern food chain.