Regular cold water swimming during winter time affects resting hematological parameters and serum erythropoietin

Recreational winter swimming in cold sea water evokes body responses to regularly repeated cold water immersion. However, the understanding of adaptive changes is still limited and data regarding very short-term exposure to severe cold stress are scarce. The purpose of the study was to examine the effects of regular active cold water exposure on resting blood elements and erythropoietin in male and female cold water swimmers (CWSs). Thirty four healthy subjects (18 men and 16 women) aged 50.0 ± 12.2 years were swimming in cold sea water during winter season at least twice a week. The average water temperature was 9.5°C in October, 1.0°C in January and 4.4°C at the end of April. Fasting blood samples were taken within the first weeks of October, January and April. Serum erythropoietin (EPO), complete blood count (CBC) including evaluation of: red blood cells (RBC count, hemoglobin, hematocrit and RBC indices), white blood cells (WBC count with WBC differential), platelets (PLT count), serum folate and serum immunoglobulins (IgG, IgA, IgM) were determined. Between October and April an increase was observed in the following parameters: RBC (from 4.8 x 10¹²/L to 5.2 x 10¹²/L, P < 0.001), hemoglobin (from 8.6 mmol/L to 9.4 mmol/L, P < 0.001), MCH (from 1.8 fmol to 1.9 fmol, P = 0.003), MCHC (from 19.9 mmol/L to 20.6 mmol/L, P < 0.001), EPO (from 6.3 IU/L to 8.1 IU/L, P = 0.001). At the same time decreased concentrations of PLT (from 249.9 x 10⁹/L to 221.6 x 10⁹/L, P = 0.005), folate (from 10.5 ng/mL to 7.4 ng/mL, P < 0.001) and immunoglobulins (IgG: from 11.8 g/L to 10.9 g/L, P < 0.001; IgA: from 2.5 g/L to 2.2 g/L, P < 0.001; IgM: from 0.9 g/L to 0.8 g/L, P < 0.001). Statistically significant changes in EPO and PLT values were noted only in female CWSs. We conclude that regular cold water swimming induces adaptive changes in the resting blood elements and EPO concentrations which are more evident in female organism.