Background Water submersion challenges human thermoregulatory (Treg) homeostasis [typically a core temperature (TC) of ~37°C], accelerating rates of heat loss (cold water; hypothermia) and gain (warm/hot water; hyperthermia) when swimming, and increasing susceptibility to heat/cold illness. Particularly, during open water swimming (OWS) competition where race drop-out and fatalities occur within a broad mandated water temperature (Tw) range (16-31°C). Conversely, pool-based swimming (PBS) competition (performed within a narrow-mandated Tw range 25-28°C) schedules encompass circadian TC changes alongside multiple cycles of warm-ups, transition phases and races. Understanding Treg responses within OWS and PBS would: (i) enhance knowledge regarding swimmer`s Treg demands; (ii) inform competition health and safety policy from a heat/cold illness perspective; and (iii) underpin Treg orientated performance enhancement. Objective Systematically review available Treg response data in OWS and PBS. Methods An open science framework registered (https://osf.io/v4ags/) systematic literature search was performed using Web of Science, MEDLINE, and SPORTDiscus to identify studies up to September 2024 using a string of key words relating to body temperature and swimming. Data with body temperature measurement during OWS or PBS training and/or competition, in participants of all sporting levels and abilities, were reviewed. Results Sixty-two studies were included (43 PBS, 19 OWS, 1 both) with a total of 605 participants (72% male, 27% female, 5% Para-athletes). Twenty-one percent of PBS Treg responses and 63% of OWS responses were collected below the respective World Aquatics (WA) Tw thresholds (PBS: 25°C; OWS: 16°C). Forty-two percent of PBS and no OWS studies were above the WA Tw thresholds (PBS: 28°C; OWS: 31°C). Mild hypothermia (35-36°C; 19% PBS; 63% OWS studies) and hypothermia (<35°C; 9% PBS; 42% OWS studies) were evident within studies; 39 of 191 OWS participants suffered from mild and/or hypothermia, and no individual data were reported for PBS studies. Mild hyperthermia was evident in 19% of PBS and 16% of OWS studies, individual data was reported for 5 of 414 PBS participants. Conclusions Some reviewed evidence (21% PBS and 63% OWS: <WA Tw thresholds; 42% PBS and 0% OWS: >WA Tw thresholds) lacked external validity to competition mandated Tw`s. Prevalence of mild-hypothermia/hypothermia and mild hyperthermia was evident within competition mandated Tw`s. Characterising Treg responses (using continuous, ingestible methods) in competition-relevant Tw`s (with increased female and Para-athlete representation) are necessary towards mitigating in-race Treg illness/risk and drop-outs (OWS) and informing Treg performance optimisation strategies (PBS and OWS).
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