Human thermoregulation : individual differences in cold induced thermogenesis

This thesis reports on the variation in human metabolic and insulative responses to mild cold. Whether a significant amount of cold-induced non-shivering thermogenesis exists in adult humans, is still under debate. The existence and variation of cold-induced nonshivering thermogenesis was shown by exposing healthy volunteers to mild cold (Chapter 2). Measurements were performed in summer and repeated in winter (Chapter 3) to investigate seasonal changes in cold response. The magnitude of the cold response was a function of cold acclimatization. On average, non-shivering thermogenesis was higher in winter compared to summer. Interestingly, the relative contribution of metabolic and temperature response was subject specific and consistent throughout the seasons. This means that a person with a relatively large response in summer does so again in winter although the size of the response is not necessarily the same. In addition, subjects that show a metabolic response, i.e. a high metabolism during cold, show a small insulative response, i.e. a decreased skin temperature during cold, and vice versa. Once this was established, the magnitude of the metabolic increase prior to the initiation of shivering was studied (Chapter 4). Subjects with a comparatively large heat production during cold exposure maintained a relatively high skin temperature but started shivering earlier, independent of differences in body composition. This combination of responses is in line with a metabolic defense mechanism of the body to a mild cold stimulus. In the previous experiments, the relation between thermogenesis during cold and body composition was studied in lean subjects. Subsequently, lean and overweight subjects were compared with respect to thermogenesis and insulation in response to mild cold and rewarming (Chapter 5). The increase in heat production corrected for body surface area was relatively low in overweight subjects during cold exposure and rewarming. It is argued that not body temperature triggers heat production, but that differences in heat balance may explain the differences between lean and overweight subjects. The energy efficient response of the overweight subjects can have consequences for energy balance in the long term. If heat balance leans towards an insulative response, the body conserves energy instead of expending it, possibly resulting in a positive energy balance that causes weigh gain. In search of factors related to cold-induced non-shivering thermogenesis, sympathetic nervous system activity was studied (Chapter 6). Both whole body and local cooling showed that both groups respond very different to the same environmental temperature change. During both conditions the overweight subjects seemed to conserve more heat and showed lower cold-induced thermogenesis compared to their lean controls. The observed larger increase in cold-induced norepinephrine levels in overweight subjects indicates a blunted response of the sympathetic nervous system that can contribute to body weight gain and resistance to weight loss. Thus, the sympathetic nervous system plays a role in cold-induced thermogenesis differences between lean and overweight subjects. Differences in thermoregulation between individuals may lead to inaccuracies in body temperature predictions from thermal models. The data that was collected during the research for this thesis was used to validate a model for the predictions of body temperature (Chapter 7). The successful incorporation of individual data in this model showed that research is needed to define subject-specific characteristics that predict the metabolic response or to identify tests for characterization of individuals. In conclusion, the considerable variation in cold-induced non-shivering thermogenesis has potential implications for energy balance. Weight status and the sympathetic nervous system are determinants. Future research should focus on further explanation of variation in cold-induced thermogenesis, relation to diet-induced energy expenditure and skin perfusion, implications for energy balance and the potential of pharmacological interventions.