Mohamed Saat

Cutaneous Warm and Cool Sensation Thresholds and the Inter-threshold Zone in Malaysian and Japanese Males

The purpose of this study was to investigate ethnic differences in cutaneous thermal sensation thresholds and the inter-threshold sensory zone between tropical (Malaysians) and temperate natives (Japanese). The results showed that (1) Malaysian males perceived warmth on the forehead at a higher skin temperature (Tsk) than Japanese males (p<0.05), whereas cool sensations on the hand and foot were perceived at a lower Tsk in Malaysians (p<0.05); (2) Overall, the sensitivity to detect warmth was greater in Japanese than in Malaysian males; (3) The most thermally sensitive body region of Japanese was the forehead for both warming and cooling, while the regional thermal sensitivity of Malaysians had a smaller differential than that of Japanese; (4) The ethnic difference in the inter-threshold sensory zone was particularly noticeable on the forehead (1.9±1.2C for Japanese, 3.2±1.6°C for Malaysians, p<0.05). In conclusion, tropical natives had a tendency to perceive warmth at a higher Tsk and slower at an identical speed of warming, and had a wider range of the inter-threshold sensory zone than temperate natives.

Ethnic differences in thermoregulatory responses during resting, passive and active heating: application of Werner’s adaptation model

For the coherent understanding of heat acclimatization in tropical natives, we compared ethnic differences between tropical and temperate natives during resting, passive and active heating conditions. Experimental protocols included: (1) a resting condition (an air temperature of 28°C with 50% RH), (2) a passive heating condition (28°C with 50% RH; leg immersion in a hot tub at a water temperature of 42°C), and (3) an active heating condition (32°C with 70% RH; a bicycle exercise). Morphologically and physically matched tropical natives (ten Malaysian males, MY) and temperate natives (ten Japanese males, JP) participated in all three trials. The results saw that: tropical natives had a higher resting rectal temperature and lower hand and foot temperatures at rest, smaller rise of rectal temperature and greater temperature rise in bodily extremities, and a lower sensation of thirst during passive and active heating than the matched temperate natives. It is suggested that tropical natives’ homeostasis during heating is effectively controlled with the improved stability in internal body temperature and the increased capability of vascular circulation in extremities, with a lower thirst sensation. The enhanced stability of internal body temperature and the extended thermoregulatory capability of vascular circulation in the extremities of tropical natives can be interpreted as an interactive change to accomplish a thermal dynamic equilibrium in hot environments. These heat adaptive traits were explained by Wilder’s law of initial value and Werner’s process and controller adaptation model.

Authors’ response to A. Flouris’s correspondence ‘The advantage of using differential data in thermal biology’

We are grateful for the correspondence from Dr. Flouris (Flouris 2010) suggesting “differential data analysis” for a comparison of time course data of body temperature during 60 min of submaximal exercise between Japanese and Malaysians, in our recent original paper (Wakabayashi et al. 2010). Following the suggestion, we additionally conducted data analysis for time course of a relative change of rectal (ΔTre) and hand skin temperature (ΔThand) from their resting baseline, instead of actual level data. As a result, a significant difference was observed in both ΔTre and ΔThand between groups from 10 min after onset of exercise to the end of the experiment (P<0.05, Fig. 1); ΔTre was greater in Japanese and ΔThand was greater in Malaysians. The greater increase of Thand in Malaysian participants suggested a greater hand blood flow as a heat dissipation response during exercise in humid heat stress, which partly explained the smaller increase of Tre in Malaysians. This differential data analysis is pertinent to the discussion in our manuscript (Wakabayashi et al. 2010). As Dr. Flouris suggests, the differential data analysis has the advantage of focusing on the magnitude of change in thermoregulatory response to thermal stress, while minimizing variation of data due to individual differences in the resting baseline. However, in some cases, the actual level data have a significant meaning for the thermoregulation. In our series of studies, tropical indigene Malaysians showed a significantly higher resting Tre than Japanese (Lee