A VEGETARIAN DIET MAKES YOU "ISOTOPICALLY LIGHTER"

With the goal of investigating the relationship between the diet and the isotopic composition of human fingernails, we analyzed the bulk carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope composition of 199 fingernail clippings from seven students attending the Isotope Geochemistry course at Utah Valley University. We collected the fingernail clippings in January, February, and March 2019. During the same period, the students also kept a detailed record of their daily diet. In terms of controlling factors, the position in the trophic chain of the consumer primarily affects his/her δ¹⁵N value. Every level up from the primary producers adds ~4‰. The plants consumed directly by the individual or by the animals he/she eats mainly control his/her δ¹³C value. C₃ plants (e.g., rice and vegetables) discriminate less against ¹³C than C₄ plants (e.g., corn and millet) and therefore have a lower δ¹³C (~-27‰ vs. ~-13‰). In addition, there is an increase of ~1‰ in the δ¹³C value for each level in the trophic chain. Additional minor factors that may affect the δ¹³C and δ¹⁵N of human keratin are health, age, sex, metabolic/physiological processes (e.g. urea excretion rate), agricultural practices (e.g., use of synthetic fertilizers vs. manure), climate, and food origins (e.g., corn from the Midwest U.S. vs corn from Mexico). Average (±1 sd) δ¹³C (vs. PDB) and δ¹⁵N (vs. air) of all students are -17.82±0.70‰ and 9.10±0.32‰, respectively. The small isotopic variability in fingernails likely results from globalization in food production and distribution. We also found higher average δ-values in omnivores (δ¹³C=-17.68±0.57‰; δ¹⁵N=9.13±0.33‰; n=179) than in the vegetarian student (δ¹³C=-19.03±0.49‰; δ¹⁵N=8.90±0.12‰; n=20). Higher δ-values in omnivores vs. vegetarians likely reflect the higher position in the trophic chain and the dominant use of corn for livestock feeding. Results from this study can be useful for the geographical allocation of individuals in forensic cases and for investigating the paleodiets of mummies in archeological studies.