Objectives To provide a scientific rationale for lower water temperature and wetsuit rules for elite and subelite triathletes.

Methods 11 lean, competitive triathletes completed a 20 min flume swim, technical transition including bike control and psychomotor testing and a cycle across five different wetsuit and water temperature conditions: with wetsuit: 10°C, 12°C and 14°C; without wetsuit (skins): 14°C and 16°C. Deep body (rectal) temperature (T_re), psychomotor performance and the ability to complete a technical bike course after the swim were measured, as well as swimming and cycling performance.

Results In skins conditions, only 4 out of 11 athletes could complete the condition in 14°C water, with two becoming hypothermic (T_re<35°C) after a 20 min swim. All 11 athletes completed the condition in 16°C. T_re fell further following 14°C (mean 1.12°C) than 16°C (mean 0.59°C) skins swim (p=0.01). In wetsuit conditions, cold shock prevented most athletes (4 out of 7) from completing the swim in 10°C. In 12°C and 14°C almost all athletes completed the condition (17 out of 18). There was no difference in temperature or performance variables between conditions following wetsuit swims at 12°C and 14°C.

Conclusion The minimum recommended water temperature for racing is 12°C in wetsuits and 16°C without wetsuits. International Triathlon Union rules for racing were changed accordingly (January 2017).