A 2015 study suggests that cold-water immersion can enhance the formation of new mitochondria, one of the key adaptations to endurance training.
The study was published in the American Journal of Physiology, Regulatory, Integrative and Comparative Physiology and carried out by researchers from Australia, New Zealand and Singapore. This study investigated the effect of regularly using ice baths on how muscles adapt to endurance training.
Eight males performed three sessions per week of endurance training for a four week period. Following each session, the subjects immersed one leg in a 10°C water bath for 15 minutes, while the opposite leg served as a control. The researchers took muscle biopsies from the athletes’ vastus lateralis (muscle on the outside of the leg) of both treated and untreated legs prior to training and 48 hours after the last training session. The samples were then analyzed for signaling enzymes which would indicate mitochondrial biogenesis (i.e. production of new mitochondria), and protein subunits which would represent activity across the mitochondrial membrane. See the link above for more details on the specific enzymes and protein subunits measured in the study.
The researchers found that the repeated cold-water immersion resulted in higher totals of certain enzymes and protein subunits while no differences were detected in other enzymes and protein subunits. They concluded that regular cold-water immersion possibly enhances mitochondrial biogenesis.