Boris R. M. Kingma

Cold exposure – an approach to increasing energy expenditure in humans

A little cold a day keeps the doctor away Obesity is a consequence of positive energy balance, which can be counterbalanced by eating less, increasing physical activity, or pharmacological approaches. However, weight maintenance is generally disappointing, and long-term use of pharmaceuticals has been limited because of lack of efficacy, poor long-term adherence rates, and serious adverse effects. These limitations indicate that, given our current knowledge and available technologies, insights from other fields of research will be necessary to permit exploration of new ideas and develop effective applications. We suggest that regular exposure to mild cold may provide a healthy and sustainable alternative strategy for increasing energy expenditure. Obesogenic thermal environment In the past century several dramatic changes in the daily living circumstances in Western civilization have occurred, affecting health. For example, we are much better able to control our ambient temperature. Consequently, we cool and heat our dwellings for maximal comfort while minimizing our body energy expenditure necessary to control body temperature. Although people are generally becoming older, and many life-threatening diseases have decreased in prevalence, other diseases and syndromes have increased immensely. Most eye-catching are overweight and obesity, now affecting over 500 million people (http://www.who.int). Obesity increases the risk of developing type 2 diabetes (T2D), cardiovascular diseases (CVD), and some forms of cancer. Obesity is linked not only to excessive food intake (energy intake) but also to physical inactivity (reduced energy expenditure). In the past two decades major scientific and financial interests have focused on counteracting excessive energy intake to tackle ‘diabesity’. This approach now appears to be rather disappointing in terms of Public Health and, until now, effective treatment strategies against obesity and T2D are still lacking. This holds true, with respect to long-term effects on weight loss and weight maintenance, for both lifestyle intervention programs and pharmacological therapies. Because 90% of the time we are exposed to indoor conditions, health aspects of ambient temperatures warrant exploration. What would it mean if we let our bodies work again to control body temperature? It is hypothesized in this paper that the thermal environment affects human health ‐ and more specifically that frequent exposure to mild cold can affect our energy expenditure significantly over sustained time-periods.

Influence of thermophysiology on thermal behavior: the essentials of categorization

Predicted energy use of dwellings often deviates from the actual energy use. Thermoregulatory behavior of the occupant might explain this difference. Such behavior is influenced by thermal sensation and thermal comfort. These subjective ratings in turn are linked to physiological parameters such as core and skin temperatures. However, it is unclear which physiological parameters best predict thermoregulatory behavior. The objective of this research was to study physiological parameters that potentially can be used to predict thermoregulatory behavior. Sixteen healthy females (18-30 years) were exposed to two dynamic temperature protocols: a gradual increase (+4 K/h, ranging from 24 °C to 32 °C) and a gradual decrease in ambient temperature (-4 K/h, ranging from 24 °C to 16 °C). During the experiments physiological responses, thermal sensation, thermal preference and the intention of thermoregulatory behavior were measured. Thermal sensation is highly correlated with thermal preference (r=-0.933, P<0.001). The skin temperature of the wrist best predicts thermal sensation (R(2)=0.558, P<0.001) and therefore seems useful as a physiological parameter to predict the intention of thermoregulatory behavior. When the subjects are categorized based on their thermal sensation votes, more precise predictions of thermal sensation can be made. This categorization therefore can be of value for the determination of the actual energy use of occupant in dwellings.

Building and occupant energetics: a physiological hypothesis

Recent studies indicate that the basic premise to provide a tightly controlled thermal indoor climate is unfounded. On the contrary, research suggests that health and comfort may benefit from thermal stimulation outside that established by the current thermal environmental standards. Such an approach may cut two ways: any control strategy that allows temperatures to vary more than allowed by the current standards can (1) improve health and (2) reduce energy consumption by the built environment substantially. So-called adaptive comfort models provide a flexible framework for dynamic indoor-climate-design with ample opportunities for individual fine-tuning. The permissible temperature ranges can extend beyond the physiological thermal neutral zone, ensuring a mild form of temperature training (acclimatization). The hypothesis of this article therefore, is that a dynamic, periodic and spatial variation in indoor temperature (within and beyond the thermoneutral zone) will positively influence long-term comfort and health.

Frequent Extreme Cold Exposure and Brown Fat and Cold-Induced Thermogenesis: A Study in a Monozygotic Twin

Introduction Mild cold acclimation is known to increase brown adipose tissue (BAT) activity and cold-induced thermogenesis (CIT) in humans. We here tested the effect of a lifestyle with frequent exposure to extreme cold on BAT and CIT in a Dutch man known as ‘the Iceman’, who has multiple world records in withstanding extreme cold challenges. Furthermore, his monozygotic twin brother who has a ‘normal’ sedentary lifestyle without extreme cold exposures was measured. Methods The Iceman (subject A) and his brother (subject B) were studied during mild cold (13°C) and thermoneutral conditions (31°C). Measurements included BAT activity and respiratory muscle activity by [18F]FDG-PET/CT imaging and energy expenditure through indirect calorimetry. In addition, body temperatures, cardiovascular parameters, skin perfusion, and thermal sensation and comfort were measured. Finally, we determined polymorphisms for uncoupling protein-1 and β3-adrenergic receptor. Results Subjects had comparable BAT activity (A: 1144 SUVtotal and B: 1325 SUVtotal), within the range previously observed in young adult men. They were genotyped with the polymorphism for uncoupling protein-1 (G/G). CIT was relatively high (A: 40.1% and B: 41.9%), but unlike during our previous cold exposure tests in young adult men, here both subjects practiced a g-Tummo like breathing technique, which involves vigorous respiratory muscle activity. This was confirmed by high [18F]FDG-uptake in respiratory muscle. Conclusion No significant differences were found between the two subjects, indicating that a lifestyle with frequent exposures to extreme cold does not seem to affect BAT activity and CIT. In both subjects, BAT was not higher compared to earlier observations, whereas CIT was very high, suggesting that g-Tummo like breathing during cold exposure may cause additional heat production by vigorous isometric respiratory muscle contraction. The results must be interpreted with caution given the low subject number and the fact that both participants practised the g-Tummo like breathing technique.

Healthy excursions outside the thermal comfort zone

ABSTRACT The concepts of comfort and health may be related but are not synonyms. New knowledge has been gathered regarding metabolic health effects of temperature exposure outside the human thermal comfort zone. Mild cold and warm environments increase metabolism, thereby targeting obesity by counterbalancing excess energy intake. Furthermore, mild cold influences glucose metabolism. Ten days of intermittent mild cold exposure in type 2 diabetes patients increased insulin sensitivity, and thereby glucose handling by more than 40%. This is comparable with the best available pharmaceutical or physical activity therapies. Lastly, there are indications that cardiovascular parameters may be positively affected by regular exposure to heat and cold. Does this mean that we have to suffer from discomfort in order to become healthy? Probably not. Firstly, prolonged temporal excursions outside the thermal comfort zone result in acclimatization resulting in increased comfort ratings. Secondly, low or high temperatures in a dynamic thermal environment may be perceived as acceptable or even pleasant (evoking thermal alliesthesia). The study of dynamic thermal conditions is advocated: linking this to the adaptive comfort model, and monitoring these conditions in actual living conditions. This information is needed to support the design of healthy, comfortable and energy-friendly indoor environments.

Exploring internal body heat balance to understand thermal sensation

ABSTRACT A biological perspective is used to understand thermal sensation. The main premise is that thermal sensation serves an organism for the regulation of body temperature. A biological concept related to this premise is the physiological thermoneutral zone (TNZ). Within the TNZ the body can adjust body tissue insulation to maintain thermal balance and a stable core temperature. The approach presented here is based on the assumption that humans express neutral thermal sensation near the centre of their TNZ. To test this hypothesis, dTNZop is defined as the distance between measured operative temperature and the centre of the TNZ, and dTNZsk as the distance between measured mean skin temperature and the centre of the TNZ. The TNZ centre is calculated with a biophysical model using measured data from a climate chamber study with 16 female subjects. Regression between observed thermal sensation votes (TSV) and dTNZx revealed that the intercept corresponds with a slightly higher-than-neutral TSV and a strong linear relationship between TSV and dTNZop and dTNZsk. This approach shows great potential to improve the understanding of human thermal sensation in the context of physiology.

Thermophysiological adaptations to passive mild heat acclimation

ABSTRACT Passive mild heat acclimation (PMHA) reflects realistic temperature challenges encountered in everyday life. Active heat acclimation, combining heat exposure and exercise, influences several important thermophysiological parameters; for example, it decreases core temperature and enhances heat exchange via the skin. However, it is unclear whether PMHA elicits comparable adaptations. Therefore, this study investigated the effect of PMHA on thermophysiological parameters. Participants were exposed to slightly increased temperatures (∼33°C/22% RH) for 6 h/d over 7 consecutive days. To study physiologic responses before and after PMHA, participants underwent a temperature ramp (UP), where ambient temperature increased from a thermoneutral value (28.8 ± 0.3°C) to 37.5 ± 0.6°C. During UP, core and skin temperature, water loss, cardiovascular parameters, skin blood flow and energy expenditure were measured. Three intervals were selected to compare data before and after PMHA: baseline (minutes 30–55: 28.44 ± 0.21°C), T1 (minutes 105–115: 33.29 ± 0.4°C) and T2 (minutes 130–140: 35.68 ± 0.61°C). After 7 d of PMHA, core (T1: −0.13 ± 0.13°C, P = 0.011; T2: −0.14 ± 0.15°C, P = 0.026) and proximal skin temperature (T1: −0.22 ± 0.29°C, P = 0.029) were lower during UP, whereas distal skin temperature was higher in a thermoneutral state (baseline: +0.74 ± 0.77°C, P = 0.009) and during UP (T1: +0.49 ± 0.76°C, P = .057 (not significant), T2:+0.51 ± 0.63°C, P = .022). Moreover, water loss was reduced (−30.5 ± 33.3 ml, P = 0.012) and both systolic (−7.7 ± 7.7 mmHg, P = 0.015) and diastolic (−4.4 ± 4.8 mmHg, P = 0.001) blood pressures were lowered in a thermoneutral state. During UP, only systolic blood pressure was decreased (T2: −6.1 ± 4.4 mmHg, P = 0.003). Skin blood flow was significantly decreased at T1 (−28.35 ± 38.96%, P = 0.037), yet energy expenditure remained unchanged. In conclusion, despite the mild heat stimulus, we show that PMHA induces distinct thermophysiological adaptations leading to increased resilience to heat.

The effect of warmth acclimation on behaviour, thermophysiology and perception

ABSTRACT Public and commercial buildings tend to overheat and considerable energy is consumed by air-conditioning and ventilation. However, many occupants remain unsatisfied and consequently exhibit thermoregulatory behaviour (TRB), e.g. opening windows or controlling the air-conditioning. This, in turn, might negatively influence the building energy use. This paper hypothesizes that warmth acclimation influences thermophysiology, perception and TRB in a warm environment. Therefore, the effect of warmth acclimation on TRB, physiology and perception is investigated. Twelve participants underwent a so-called SWITCH protocol before and after warmth acclimation (7 days, 6 h/day, about 33°C, about 22% RH). During SWITCH, the participants chose between a warm (37°C) and a cold (17°C) condition. TRB was determined by the number of switches and the time spent in a specific condition. Mean skin temperature was recorded to assess behavioural thresholds. Thermal comfort and sensation were indicated on visual analogue scales (VAS). After acclimation, the upper critical behavioural threshold significantly increased from 35.2 ± 0.6 to 35.5 ± 0.5°C (p ≤ 0.05) and the range of mean skin temperatures at which no behaviour occurred significantly widened (3.6 ± 0.7 to 4.2 ± 0.6; p < 0.05). The total number of switches tended to decrease (p = 0.075). The present study is the first to show that prolonged passive exposure to warmth extends TRB thresholds.

Improving rational thermal comfort prediction by using subpopulation characteristics: a case study at Hermitage Amsterdam

ABSTRACT This study aims to improve the prediction accuracy of the rational standard thermal comfort model, known as the Predicted Mean Vote (PMV) model, by (1) calibrating one of its input variables “metabolic rate,” and (2) extending it by explicitly incorporating the variable running mean outdoor temperature (RMOT) that relates to adaptive thermal comfort. The analysis was performed with survey data (n = 1121) and climate measurements of the indoor and outdoor environment from a one year-long case study undertaken at Hermitage Amsterdam museum in the Netherlands. The PMVs were calculated for 35 survey days using (1) an a priori assumed metabolic rate, (2) a calibrated metabolic rate found by fitting the PMVs to the thermal sensation votes (TSVs) of each respondent using an optimization routine, and (3) extending the PMV model by including the RMOT. The results show that the calibrated metabolic rate is estimated to be 1.5 Met for this case study that was predominantly visited by elderly females. However, significant differences in metabolic rates have been revealed between adults and elderly showing the importance of differentiating between subpopulations. Hence, the standard tabular values, which only differentiate between various activities, may be oversimplified for many cases. Moreover, extending the PMV model with the RMOT substantially improves the thermal sensation prediction, but thermal sensation toward extreme cool and warm sensations remains partly underestimated.

Evaluating the performance of thermal sensation prediction with a biophysical model

Neutral thermal sensation is expected for a human body in heat balance in near-steady-state thermal environments. The physiological thermoneutral zone (TNZ) is defined as the range of operative temperatures where the body can maintain such heat balance by actively adjusting body tissue insulation, but without regulatory increases in metabolic rate or sweating. These basic principles led to the hypothesis that thermal sensation relates to the operative temperature distance from the thermoneutral centroid (dTNZop ). This hypothesis was confirmed by data from respiratory climate chamber experiments. This paper explores the potential of such biophysical model for the prediction of thermal sensation under increased contextual variance. Data (798 votes, 47 participants) from a controlled office environment were used to analyze the predictive performance of the dTNZop model. The results showed a similar relationship between dTNZop and thermal sensation between the dataset used here and the previously used dataset. The predictive performance had the same magnitude as that of the PMV model; however, potential benefits of using a biophysical model are discussed. In conclusion, these findings confirm the potential of the biophysical model with regard to the understanding and prediction of human thermal sensation. Further work remains to make benefit of its full potential.