Serum carbon and nitrogen stable isotopes as potential biomarkers of dietary intake and their relation with incident type 2 diabetes: the EPIC-Norfolk study

Background: Stable-isotope ratios of carbon (C-13/C-12, expressed as delta C-13) and nitrogen (N-15/N-14, or delta N-15) have been proposed as potential nutritional biomarkers to distinguish between meat, fish, and plant-based foods. Objective: The objective was to investigate dietary correlates of delta C-13 and delta N-15 and examine the association of these biomarkers with incident type 2 diabetes in a prospective study. Design: Serum delta C-13 and delta N-15 (parts per thousand) were measured by using isotope ratio mass spectrometry in a case-cohort study (n = 476 diabetes cases; n = 718 subcohort) nested within the European Prospective Investigation into Cancer and Nutrition (EPIC)-Norfolk population-based cohort. We examined dietary (food-frequency questionnaire) correlates of delta C-13 and delta N-15 in the subcohort. HRs and 95% CIs were estimated by using Prentice-weighted Cox regression. Results: Mean (+/- SD) delta C-13 and delta N-15 were -22.8 +/- 0.4 parts per thousand and 10.2 +/- 0.4 parts per thousand, respectively, and delta C-13 (r = 0.22) and delta N-15 (r = 0.20) were positively correlated (P < 0.001) with fish protein intake Animal protein was not correlated with delta C-13 but was significantly correlated with delta N-15 (dairy protein: r = 0.11; meat protein: r = 0.09; terrestrial animal protein: r = 0.12, P <= 0.013). delta C-13 was inversely associated with diabetes in adjusted analyses (HR per tertile: 0.74; 95% CI: 0.65, 0.83; P-trend < 0.001], whereas delta N-15 was positively associated (HR: 1.23; 95% CI: 1.09, 1.38; P-trend = 0.001). Conclusions: The isotope ratios delta C-13 and delta N-15 may both serve as potential biomarkers of fish protein intake, whereas only delta N-15 may reflect broader animal-source protein intake in a European population. The inverse association of delta C-13 but a positive association of delta N-15 with incident diabetes should be interpreted in the light of knowledge of dietary intake and may assist in identifying dietary components that are associated with health risks and benefits.