Fertilizer Type Affects Stable Isotope Ratios of Nitrogen in Human Blood Plasma—Results from Two-Year Controlled Agricultural Field Trials and a Randomized Crossover Dietary Intervention Study

[Image: see text] The stable nitrogen isotope ratio δ(15)N is used as a marker of dietary protein sources in blood. Crop fertilization strategies affect δ(15)N in plant foods. In a double-blinded randomized cross-over dietary intervention trial with 33 participants, we quantified the effect of fertilizer type (conventional: synthetic fertilizer and organic: animal or green manure) on δ(15)N in blood plasma. At study baseline, plasma δ(15)N was +9.34 ± 0.29‰ (mean ± standard deviation). After 12 days intervention with a diet based on crops fertilized with animal manure, plasma δ(15)N was shifted by +0.27 ± 0.04‰ (mean ± standard error) compared to synthetic fertilization and by +0.22 ± 0.04‰ compared to fertilization with green manure (both p < 0.0001). Accordingly, differences in the δ(15)N values between fertilizers are propagated to the blood plasma of human consumers. The results indicate a need to consider agricultural practices when using δ(15)N as a dietary biomarker.