The purpose of this study was to investigate the genetic factors responsible for individual differences in cold tolerance. Focusing on mitochondrial haplogroup D (D4) in Japanese subjects, differences in cold tolerance between haplogroup D subjects and other haplogroup subjects were investigated. The experiment was conducted in summer (August-September). Room temperature was decreased from 27 to 10°C in 30 min, and cold exposure was then maintained at 10°C for 60 min. During that time, measurements were made of physiological responses, including rectal temperature, skin temperature, oxygen consumption, blood pressure, and heart rate. A subjective evaluation was also made. It was found that, under cold exposure, the decrease in rectal temperature of haplogroup D subjects was significantly less than that in other groups (P < 0.001). There was no significant main group effect for oxygen consumption or mean skin temperature. In haplogroup D subjects, the thermal comfort sensation was judged to be significant discomfort at 20 and 30 min after the start of cold stimulation (P < 0.05). From the relation of oxygen consumption to rectal temperature, it is conjectured that the rectal temperature at which increased thermogenesis starts is higher in haplogroup D subjects. This study suggests that differences in mitochondrial haplogroup are one factor in individual differences in cold tolerance in Japanese.
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