Luteal phase, viral illness, alcohol, heavy training blocks, travel, and sleep debt produce biomarker signatures almost identical to underfueling. Here's how to tell them apart.
The physiological signatures of low energy availability show up in HRV, resting heart rate, sleep, body mass, and menstrual function. Each signal is real and each has significant limits.
Energy availability, RED-S risk factors, biomarker signatures of underfueling, and the confounders that mimic them — plus where data stops and clinical assessment begins.
Lab reference ranges are built for the general population. If you train seriously, most of your results need a completely different interpretation.