Seafood Abundance Correlates With Nitrogen Stable Isotope Ratio in Japanese Diet

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of whole diet have rarely been measured to date though the isotope ratios in human sample have been extensively used for diet and nutritional researches. In order to fully validate the isotope dietary analysis, association between quantitative information on diet composition and isotope ratios is required to be compared.

Duplicate diet samples collected from 30 adult Japanese participants of a human biomonitoring survey in 2012 and 2014 were measured for δ¹³C and δ¹⁵N by element analyzer-isotope ratio mass spectrometry. The correlation between δ values and the abundances of 12 food categories included in the diet samples, of which weight was separately measured at the time of diet sampling, was examined.

Mean ± standard deviation of δ¹³C and δ¹⁵N values of the 30 duplicate diets were -24.4 ± 0.7 and 4.64‰ ± 0.82‰, respectively. Correlation between δ¹⁵N value and % abundance of "fish and shellfish" ([weight of "fish and shellfish"] / [total weight of duplicate diet] (%)) in the diet was highly significant (r = 0.752, p < 0.001), indicating that seafood abundance is a major determinant of diet δ¹⁵N value. Abundance of pulse also had a negative association with δ¹⁵N value due to low δ¹⁵N value of pulse. The δ¹³C value of duplicate diet was significantly correlated with % abundance of "rice and rice products" (r = -0.603, p < 0.001) and of "meat and egg" (r = 0.489, p < 0.01). The reason of the latter was ascribed to less negative δ¹³C value of meat and egg from livestock raised on corn-based feed, but the reason(s) of the former was not known.

The highly significant positive correlation between the abundance of seafood and δ¹⁵N supports the utility of δ¹⁵N of biological samples for isotopic diet reconstruction/nutritional research.