Overtraining syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities
Key Findings — Core Insight
OTS / training-overload and RED-S share a hypothalamic-pituitary origin and a large overlapping symptom space, and both can be driven by low carbohydrate (CHO) and low energy availability (EA).
The authors' thesis: many negative outcomes attributed to training overload (with or without an OTS diagnosis) may be misdiagnosed under-fuelling — RED-S via low EA and/or low CHO availability.
Re-analysis of the overload/OTS studies that also measured intake: 18 of the 21 eligible studies (86%) showed an EA decrease or a between-cohort EA difference (14 studies) or a CHO-availability decrease (4 studies) during the overload period — with both OTS-type and RED-S-type symptoms appearing relative to control conditions.
Symptom overlap demonstrated across the OTS literature (57 studies) and the RED-S / Female Athlete Triad literature (88 studies).
Conclusion: OTS and RED-S have many shared pathways, symptoms and diagnostic complexities; clinicians need to exclude LEA/RED-S before diagnosing OTS.
Practical Application
Stellingwerff et al. re-read the overtraining literature and found that in 18 of 21 overload studies with dietary data (86%) the "overtrained" athletes were also under-fuelled (lower energy or carbohydrate availability) — so an athlete presenting as overtrained should be screened for low energy availability before rest alone is prescribed.
Screen EA, menstrual function (female) and testosterone-related symptoms (male) in every "overtrained" athlete; if fuelling is low, restore fuelling first and reassess.
If EA and CHO availability are adequate and underperformance persists, proceed to the classical OTS work-up (REF-206).
Inference, not tested: prevention of under-recovery is multi-factorial, but the authors place EA and CHO availability at the centre of it.
Limitations
Narrative review; the 18/21 count is a re-analysis of existing studies, not new data
Clinical decision rules for the differential diagnosis are not standardized
Depends on accurate EA measurement, which remains problematic (see REF-343)
How this entry was checked and changed
Content correction 2026-09-27
"Proposed unified model: Unexplained Underperformance Syndrome (UUPS) with four axes" -> not in the abstract; removed.
Added the abstract's quantitative findings: 18 of 21 overload/OTS studies (86%) with an EA or CHO decrease (14 and 4 studies), and symptom overlap across 57 OTS and 88 RED-S/Triad studies.
Practical bullets reworded from "fuelling before rest" as a rule to the authors' actual conclusion: exclude LEA/RED-S before an OTS diagnosis.
2026-10-01: cross-reference repointed from a merged framework to the one that replaced it (SYNTH-01→31, 05→34, 09→70, 12/15→32, 19→26).