Maintaining an adequate quantity and quality of lean tissue (including skeletal muscle) is important for health and performance across the lifespan. Lean tissues are constantly ‘turning over’ through the process of protein breakdown and synthesis with the algebraic difference determining net protein balance. Exercise and dietary amino acid ingestion are primary stimulators of synthesis whereas aging and/or inactivity may induce a relative ‘anabolic resistance’ as characterized by an attenuated utilization of dietary amino acids for tissue protein synthesis. Our foundational understanding of protein turnover in humans is based on the application of stable isotopes traditionally applied (e.g. intravenously infused) in controlled lab settings and involve invasive blood and tissue (e.g. muscle biopsy) sampling. However, development of techniques that can be used non-invasively and outside of traditional controlled laboratory settings are an advantage to the research and clinical communities given their ability to be deployed in remote settings and in vulnerable populations (e.g. children, older adults, clinical populations). We therefore adapted ¹³C-labeled amino acid methodologies to be utilized noninvasively and with minimal investigator oversight outside the lab to determine the ‘anabolic sensitivity’ of lean tissues and to define optimal daily protein requirements. These methods are based on the differential primary fate of dietary essential amino acids as substrates for: i) protein synthesis (i.e. retained in the body), or; ii) energy utilization (i.e. generating ¹³CO₂). Ingestion of 0.25g/kg of ‘protein’ (crystalline amino acids modeled on the composition of egg protein) enriched to 5% (~100mg) with [¹³C]leucine can detect exercise-induced anabolic sensitivity through a ~11% reduction (P<0.01) in ¹³CO₂ production (~10% greater leucine retention; P>0.01) after resistance exercise compared to non-exercise control (Mazzulla et al., 2022). Preliminary findings suggest this leucine ‘breath test’ can also detect ‘anabolic resistance’ of aging through a reduced leucine retention in older (~70y) compared to younger (~24y) men and women. To determine protein requirements remotely, we modified the indicator amino acid oxidation (IAAO) technique, which is based on the partitioning of [¹³C]phenylalanine between body protein synthesis and oxidation (OX), to be self-administered in male and female endurance athletes with only two suboptimal (0.2 and 1.2g/kg/d) and one excess (2.2g/kg/d) test protein intake. There was a difference (P>0.01) in OX between all test protein intakes with the modeled breakpoint indicating an estimated average requirement for protein of ~1.6g/kg/d and a safe intake (upper 95%CI) of ~1.8g/kg/d to maximize whole body protein synthesis during exercise recovery for both sexes. This recommended daily intake was identical to our previously determined requirement within a controlled lab setting using multiple test intakes (n=7 per participant), highlighting the utility of the IAAO to be adapted to remote research environments at a significantly lower participant burden. These noninvasive tracer methods have the potential for wide deployment (population and location) to advance our understanding of human protein turnover across the healthspan.