Influence of Aerobic Exercise Combined with Forest Bathing on Immunocytes, Stress Hormones, VO2peak, and Body Composition in Elderly Men: A Randomized Controlled Trial

Background: Forest bathing and aerobic exercise are known to be factors that increase natural killer (NK) cell, but it is considered to provide a greater effect when the two factors are combined. To date, it has not been studied whether aerobic exercise combined with forest bathing can further increase innate immunocytes, including NK cell. Therefore, this study investigated the effect of aerobic exercise + forest bathing on NK cell and whether stress hormones (epinephrine and cortisol) are involved in this physiological process. In addition, this study tried to confirm whether the mixed effect of the two had a more positive effect on cardiorespiratory endurance as well as body composition in the elderly men. Methods: Thirty-two participants were randomly assigned to one of four groups: control group (CON, n = 8) which was not provided with any intervention, exercise group (EXE, n = 8) which performed treadmill exercises without phytoncide, phytoncide group (PHYT, n = 8) which was provided with phytoncide intervention, and exercise + phytoncide group (EXE + PHYT, n = 8) which performed treadmill exercises and was provided with phytoncide. Treadmill exercises and phytoncide exposures were performed for 45 similar to 60 min a day, 3 days a week for 12 weeks. Results: Compared with CON, (1) the leucocytes and lympocytes of EXE, PHYT, and EXE + PHYT showed positive changes. Specifically, total NK cells, NKT cells, and NKG2D + NK of the EXE + PHYT increased after 12 weeks of intervention. (2) The cortisol concentrations of EXE, PHYT, and EXE + PHYT showed decreasing changes, whereas the epinephrine concentration were significantly increased. (3) Although there were no changes in the muscle mass of EXE, PHYT, and EXE + PHYT, cardiorespiratory endurance was significantly increased in those groups. In addition, the body weight, fat mass, and fat percentage significantly decreased only in the EXE + PHYT. Conclusions: This study confirmed that forest bathing and aerobic exercise positively affected immunocyte function in elderly men. It also found that the improved results from forest bathing + treadmill walking were caused by an increase in cardiorespiratory endurance by increased epinephrine concentrations. In addition, this increased cardiorespiratory endurance can be interpreted as significantly reducing the body weight and fat in the group that participated in the exercise combined with phytoncide exposure.