Effects of lipid extraction on human bone collagen: Comparing stable carbon and nitrogen isotope values with and without lipid extraction

This study analyzes 24 human bone fragments to assess how lipid extraction affects stable carbon and nitrogen isotope ratios. Eighteen samples were archaeological bone and six were modern bone. Each bone was split into two pieces, producing 48 bone samples, and half underwent lipid extraction and half did not. This was done to assess whether the two techniques yielded statistically significant differences in 813C and 815N. The results are an important contribution for establishing standard stable isotope preparation protocols, which affect time and cost of laboratory procedures. Further, given that many dietary stable isotope studies compare original results to published material, this study addresses whether those comparisons are suitable if some samples undergo lipid extraction while others do not. We also examine 144 articles that report stable carbon and nitrogen isotope results to evaluate the proportion of studies that conduct lipid extraction versus those that do not. We explore those outcomes by archaeological geographic region and by stable isotope laboratories. Findings show that 15% of the published studies do lipid extraction, and they tend to be grouped by researchers and laboratories who conduct research in specific global regions. The meta-analysis of the published articles and the findings that some laboratories conduct lipid extraction, while others do not, prompted the experimental portion of this study. Results of the experimental study show no significant differences in 813C between samples prepared with and without lipid extraction and no significant differences in 815N between samples prepared with and without lipid extraction. These results suggest that stable carbon and nitrogen isotope analysis of human bone collagen may not require lipid extraction for certain dietary reconstruction studies, specifically for non-coastal contexts (i.e., non-marine based diets). The isotope data was analyzed overall and among subgroups (e.g., archaeological bone, modern bone, mid-valley Peruvian ecological zones, and highland Peruvian ecological zones). In all cases, stable isotope ratios from samples prepared with and without lipid extraction were not significantly different.