Effect of Organic Food Intake on Nitrogen Stable Isotopes

Food choices affect the isotopic composition of the body with each food item leaving its distinct isotopic imprint. The common view is that the natural abundance of the stable isotopes of nitrogen (expressed as δ(15)N) is higher in animals than in plants that constitute our contemporary diets. Higher δ(15)N is thus increasingly viewed as a biomarker for meat and fish intake. Here we show that organic compared to conventional farming increases plant δ(15)N to an extent that can appreciably impact the performance of δ(15)N as a biomarker. The error that can arise when organic plants are consumed was modelled for the entire range of proportions of plant versus animal protein intake, and accounting for various intakes of organic and conventionally grown crops. This mass balance model allows the interpretation of differences in δ(15)N in light of organic food consumption. Our approach shows that the relationship between δ(15)N and meat and fish intake is highly contextual and susceptible to variation at the population, community or group level. We recommend that fertilization practices and organic plant consumption must not be overlooked when using δ(15)N as a biomarker for meat and fish intake or to assess compliance to nutritional interventions.