Carbon and nitrogen isotopic ratios of urine and faeces as novel nutritional biomarkers of meat and fish intake

PURPOSE Meat and fish consumption are associated with changes in the risk of chronic diseases. Intake is mainly assessed using self-reporting, as no true quantitative nutritional biomarker is available. The measurement of plasma fatty acids, often used as an alternative, is expensive and time-consuming. As meat and fish differ in their stable isotope ratios, δ(13)C and δ(15)N have been proposed as biomarkers. However, they have never been investigated in controlled human dietary intervention studies. OBJECTIVE In a short-term feeding study, we investigated the suitability of δ(13)C and δ(15)N in blood, urine and faeces as biomarkers of meat and fish intake. METHODS The dietary intervention study (n = 14) followed a randomised cross-over design with three eight-day dietary periods (meat, fish and half-meat-half-fish). In addition, 4 participants completed a vegetarian control period. At the end of each period, 24-h urine, fasting venous blood and faeces were collected and their δ(13)C and δ(15)N analysed. RESULTS There was a significant difference between diets in isotope ratios in faeces and urine samples, but not in blood samples (Kruskal-Wallis test, p < 0.0001). In pairwise comparisons, δ(13)C and δ(15)N were significantly higher in urine and faecal samples following a fish diet when compared with all other diets, and significantly lower following a vegetarian diet. There was no significant difference in isotope ratio between meat and half-meat-half-fish diets for blood, urine or faecal samples. CONCLUSIONS The results of this study show that urinary and faecal δ(13)C and δ(15)N are suitable candidate biomarkers for short-term meat and fish intake.