Percutaneously absorbed carbon dioxide enhances blood flow. The mechanism by which it does so is unclear, we hypothesized that it involves bicarbonate ions. Balb/c mice were bathed in neutral bicarbonate ionized water (NBIW) and showed increased blood bicarbonate levels and blood flow via phosphorylation of peripheral vascular endothelial nitric oxide synthase (eNOS) and production of nitric oxide (NO). Phosphorylation of eNOS and NO production were also increased in human umbilical vein endothelial cells cultured in medium containing NBIW, and NBIW showed reactive oxygen species scavenging activity. In a double-blind, randomized study in men and women aged 30 to 59 years with subjective cold intolerance, bathing in NBIW elevated body temperature faster than bathing in a control solution and improved chills and sleep quality. Taken together, our results show that percutaneously absorbed bicarbonate ions act directly on endothelial cells, phosphorylation of eNOS increases NO production and thus improves blood flow.

We performed a randomized, open-label, crossover trial to examine the effects of neutral bicarbonate ionized water (NBIW) bathing on mental stress, sleep, and immune function. Participants who regularly felt stressed were randomly assigned to NBIW or regular bathing for 4 weeks. Mental stress was assessed with the Brief Job Stress Questionnaire (BJSQ) and the Profile of Mood States Second Edition (Japanese version); sleep quality, with the Pittsburgh Sleep Quality Index (PSQI-J; Japanese version) and actigraphy; and immune function, with laboratory tests. PSQI-J scores and actigraphy sleep latency and bed out latency improved significantly more with NBIW bathing than with regular bathing (p < 0.05). Furthermore, NBIW bathing reduced both stress-induced fluctuations in CD4+ and CD8+ T-cell counts and fluctuations in the naïve to memory T cell ratio and neutrophil phagocytosis, indicating improved immune function. These findings suggest that NBIW bathing improves mental stress, sleep quality, and immune function.