Modulation of endothelial progenitor cells by repeated ice-water swimming

Background Cold-water immersion is common in athletic and recreational settings and may influence vascular repair. We evaluated whether repeated ice-water swimming alters circulating endothelial progenitor cells (EPCs) and their functional capacity. Methods In this prospective study, healthy active participants completed one week of ice-water swimming at an Austrian lake and during the Molveno Ice Water Swimming World Competition (January 2025 and January 2026). Participants performed a mean 7.8 ± 5.8 swimming sessions, spending 24.2 ± 20.3 minutes in water averaging <4 °C. Peripheral blood was drawn the day before the first immersion and one day after the last immersion. EPCs were quantified by flow cytometry (CD34+VEGFR2, CD133+VEGFR2). In 15 of 35 participants, mononuclear cells were cultured for colony-forming unit (CFU) assays and assessed for viability by MTT (OD570 nm, AU). Results Forty-five participants (mean age 53 ± 10 years; 40% female) were included. Baseline CD34+VEGFR2 EPCs were 0.85% ± 0.43 and CD133+VEGFR2 EPCs 0.96% ± 0.52. Following the immersion week, CD34+VEGFR2 increased to 1.11% ± 0.46 (p = 0.027) and CD133+VEGFR2 to 1.26% ± 0.55 (p = 0.030). CFU/well rose from 1.60 ± 0.43 to 2.07 ± 0.60 (p = 0.007). MTT absorbance increased from 0.18 ± 0.06 to 0.22 ± 0.65 (p = 0.021) (Figure 1a–c). Conclusion Repeated ice-water swimming acutely increases circulating EPC counts and in vitro measures of EPC function. These findings suggest suggesting potential cardiovascular benefits. Further research is necessary to explore underlying mechanisms and assess long-term effects.