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BACKGROUND: The natural abundance nitrogen stable isotope ratio (NIR) of whole tissue correlates with animal protein intakes, including meat and fish. Amino acid (AA) NIRs (NIR(AA)s) are more variable than the whole-tissue NIRs and may thus better differentiate among foods. OBJECTIVES: We evaluated whether NIR(AA)s were associated with intakes of fish and meat and whether these dietary associations were larger than with whole-tissue NIRs. METHODS: Men were recruited at the National Institute of Diabetes and Digestive and Kidney Diseases in Phoenix, Arizona, and randomly assigned to one of eight 12-wk inpatient dietary interventions, which varied the presence/absence of fish, meat, and sugar-sweetened beverages (SSBs) in all possible combinations. Fasting blood was drawn pre- and postintervention and plasma and RBC NIR(AA)s (free and protein-bound) were measured as secondary outcomes in 32 participants. Multivariable regression was used to determine responses of postintervention NIR(AA)s to dietary variables, and logistic regression was used to calculate receiver operating characteristic AUCs. RESULTS: Most plasma NIR(AA)s increased with fish and meat intakes, but to a greater extent with fish intake. The largest increase in response to fish intake was plasma NIR(Leucine) (β = 2.19, SE = 0.26). The NIR(Threonine) decreased with both fish and meat intakes. Fewer RBC NIR(AA)s increased with fish intake, and only RBC NIR(Proline) increased with meat intake. No plasma or RBC NIR(AA) responded to SSB intake. We identified fish intake with a high degree of accuracy using plasma NIR(Leucine) (corrected AUC, cAUC = 0.96) and NIR(Glutamic acid/glutamine) (cAUC = 0.93), and meat intake to a lower degree using plasma NIR(Proline) (cAUC = 0.80) and RBC NIR(Proline) (cAUC = 0.85). CONCLUSIONS: Plasma and RBC NIR(AA)s were associated with fish and meat intakes but were not superior to whole-tissue stable isotope biomarkers in identifying these intakes in a US diet. This trial is registered at www.clinicaltrials.gov as NCT01237093. |
