This disclosure is drawn, inter alia, to methods, kits, products, devices, and/or apparatus generally related to monitoring a metabolism of an individual. The kits disclosed herein include a food item including at least one GRAS tracer. The GRAS tracer is composed to be absorbed by a body of the individual and excreted in sweat as at least one detectable tracer. The GRAS tracer includes the detectable tracer or precursor(s) of the detectable tracer. The kit includes a sensor device configured to be positioned adjacent to a surface of the body. The sensor device includes sensor(s) located to be fluidly coupleable to the surface of the body when the sensor device is positioned adjacent to the surface. The sensor(s) are configured to detect at least one of a presence of, a selected quantity of, or a selected concentration of the at least one detectable tracer.