Light-Element Isotopes (H, C, N, and O) as Tracers of Human Diet: A Case Study on Fast Food Meals

Abstract

Tracking the production and movement of foods from farm to fork has become increasingly complex in recent years due to changes in modern food markets, including the globalization of food sources and their distribution. Dietary habits have changed as well, with meals eaten outside of the home representing an ever-increasing fraction of the modern human diet. Stable isotope analysis has proven to be a reliable technique for characterizing biological substances, such as food, because stable isotopes record aspects of photosynthesis, nutrition, and source water. Here we present stable isotope ratios (δ13C, δ15N, δ2H, and δ18O) measured for one ubiquitous example of modern convenience food, the fast food meal. In a series of paired observations among cities in the U.S., we compared the isotopic composition of components of a fast food meal to similar foods purchased from grocery stores. Isotope analyses revealed several interesting patterns in both food production and food origin within the modern American food market. Carbon isotope ratio analysis of proteins highlighted the impact of consumer choice: patrons in a fast food restaurant consumed beef containing a lower proportion of C4 plants than beef purchased from a grocery store. The relative trophic-level position of beef available in both fast food restaurants and grocery stores was similar based on δ15N values. The δ2H and δ18O values of the beef, bread, and milk samples were independently and positively correlated, suggesting that both plants and animals recorded the isotopic composition of local environmental water and, therefore, geographic source. Overall, these results demonstrated the promise and potential of food stable isotope analyses to broad areas of societal interest: food safety, food authenticity, and food traceability.