Carbon and Nitrogen Isotopic Composition of Duplicate Diet of the Japanese

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, isotopic information of whole diet is required.

δ¹³C and δ¹⁵N of 150 duplicate diet samples collected in Japan during 2016-2017 were measured. Sixty-five males and 85 females (mean age: 45 years) donated duplicate diet sample of which δ¹³C and δ¹⁵N were measured by element analyzer-isotope ratio mass spectrometry.

Mean δ¹³C and δ¹⁵N of the 150 duplicate diets were -24.3 (1.1) ‰ and 3.58 (0.93) ‰, respectively, with no gender- and age-dependent variation. δ¹⁵N of diet containing seafood (median: 3.60‰, n = 111) was significantly more elevated than that not containing seafood (3.01‰, n = 39). δ¹⁵N of Japanese diet is decreasing from 1990s to the present, which is consistent with the national statistics showing decreasing trend of seafood consumption of the Japanese. Contradictory to these observation, dietary δ¹⁵N was not elevated in diet samples from the elderlies though the diet of elderlies contained seafood more frequently than those of younger study participants.

There were some uncertainties as to whether seafood is a major determinant of dietary δ¹⁵N of the Japanese. To further characterize dietary components that determine δ values, isotope ratio analysis of diet of known quantitative dietary components is warranted.