The influence of bright and dim light on substrate metabolism, energy expenditure and thermoregulation in insulin-resistant individuals depends on time of day.

AIMS/HYPOTHESIS: In our modern society, artificial light is available around the clock and most people expose themselves to electrical light and light-emissive screens during the dark period of the natural light/dark cycle. Such suboptimal lighting conditions have been associated with adverse metabolic effects, and redesigning indoor lighting conditions to mimic the natural light/dark cycle more closely holds promise to improve metabolic health. Our objective was to compare metabolic responses to lighting conditions that resemble the natural light/dark cycle in contrast to suboptimal lighting in individuals at risk of developing metabolic diseases. METHODS: Therefore, we here performed a non-blinded, randomised, controlled, crossover trial in which overweight insulin-resistant volunteers (n = 14) were exposed to two 40 h laboratory sessions with different 24 h lighting protocols while staying in a metabolic chamber under real-life conditions. In the Bright day-Dim evening condition, volunteers were exposed to electric bright light (~1250 lx) during the daytime (08:00-18:00 h) and to dim light (~5 lx) during the evening (18:00-23:00 h). Vice versa, in the Dim day-Bright evening condition, volunteers were exposed to dim light during the daytime and bright light during the evening. Randomisation and allocation to light conditions were carried out by sequential numbering. During both lighting protocols, we performed 24 h indirect calorimetry, and continuous core body and skin temperature measurements, and took frequent blood samples. The primary outcome was plasma glucose focusing on the pre- and postprandial periods of the intervention. RESULTS: Spending the day in bright light resulted in a greater increase in postprandial triacylglycerol levels following breakfast, but lower glucose levels preceding the dinner meal at 18:00 h, compared with dim light (5.0 ± 0.2 vs 5.2 ± 0.2 mmol/l, n = 13, p=0.02). Dim day-Bright evening reduced the increase in postprandial glucose after dinner compared with Bright day-Dim evening (incremental AUC: 307 ± 55 vs 394 ± 66 mmol/l × min, n = 13, p=0.009). After the Bright day-Dim evening condition the sleeping metabolic rate was identical compared with the baseline night, whereas it dropped after Dim day-Bright evening. Melatonin secretion in the evening was strongly suppressed for Dim day-Bright evening but not for Bright day-Dim evening. Distal skin temperature for Bright day-Dim evening was lower at 18:00 h (28.8 ± 0.3°C vs 29.9 ± 0.4°C, n = 13, p=0.039) and higher at 23:00 h compared with Dim day-Bright evening (30.1 ± 0.3°C vs 28.8 ± 0.3°C, n = 13, p=0.006). Fasting and postprandial plasma insulin levels and the respiratory exchange ratio were not different between the two lighting protocols at any time. CONCLUSIONS/INTERPRETATION: Together, these findings suggest that the indoor light environment modulates postprandial substrate handling, energy expenditure and thermoregulation of insulin-resistant volunteers in a time-of-day-dependent manner. TRIAL REGISTRATION: ClinicalTrials.gov NCT03829982. FUNDING: We acknowledge the financial support from the Netherlands Cardiovascular Research Initiative: an initiative with support from the Dutch Heart Foundation (CVON2014-02 ENERGISE).

The effects of a novel personal comfort system on thermal comfort, physiology and perceived indoor environmental quality, and its health implications - Stimulating human thermoregulation without compromising thermal comfort.

The classical textbook interpretation of thermal comfort is that it occurs when the thermoregulatory effort is minimized. However, stimulating human thermoregulatory systems may benefit health and increase body thermal resilience. To address this gap, we tested a novel personal comfort system (PCS) that targets only the extremities and the head, leaving the rest of the body exposed to a moderately drifting temperature (17-25°C). A randomized, cross-over study was conducted under controlled laboratory conditions, mimicking an office setting. Eighteen participants completed two scenarios, one with a PCS and another one without a PCS in 17-25°C ambient conditions. The results indicate that the PCS improved thermal comfort in 17-23°C and retained active thermoregulatory control. The torso skin temperature, underarm-finger temperature gradients, energy expenditure, substrate oxidations and physical activity were not affected by the PCS in most cases. Only slight changes in cardiovascular responses were observed between the two scenarios. Moreover, the PCS boosted pleasure and arousal. At 25°C, the PCS did not improve thermal comfort, but significantly improved air quality perceptions and mitigated eye strain. These findings suggest that human physiological thermoregulation can be stimulated without compromising thermal comfort by using a PCS that only targets the extremities in cold conditions.

The effect of cold exposure with shivering on glucose tolerance in healthy men.

Muscle glycogen use and glucose uptake during cold exposure increases with shivering intensity. We hypothesized that cold exposure, with shivering, would subsequently increase glucose tolerance. Fifteen healthy men (age = 26 ± 5 yr, body mass index = 23.9 ± 2.5 kg·m-2 ) completed two experimental trials after an overnight fast. Cold exposure (10°C) was applied during the first trial, via a water-perfused suit, to induce at least 1 h of shivering in each participant. For comparison, a thermoneutral (32°C) condition was applied during the second trial, under identical conditions, for the same duration as determined during the cold exposure. After the thermal exposures, participants rested under a duvet for 90 min, which was followed by a 3-h oral glucose tolerance test. Skin temperature (means ± SE) decreased at the end of the cold exposure compared with that before (26.9 ± 0.3 vs. 33.7 ± 0.1°C, P < 0.001). Total energy expenditure during the 1 h of shivering was greater than that during the time-matched thermoneutral condition (619 ± 23 vs. 309 ± 7 kJ, P < 0.001). Cold exposure increased the areas under the glucose and insulin curves by 4.8% (P = 0.066) and 24% (P = 0.112), respectively. The Matsuda and insulin-glucose indices changed after cold exposure by -21% (P = 0.125) and 30% (P = 0.100), respectively. Cold exposure did not subsequently increase glucose tolerance. Instead, the Matsuda and insulin-glucose indices suggest insulin resistance post shivering.NEW & NOTEWORTHY This is the first study to examine the effect of cold-induced shivering on subsequent glucose tolerance determined under thermoneutral conditions. Plasma glucose and insulin concentrations increased during the oral glucose tolerance test post shivering. Additionally, insulin sensitivity indices suggest insulin resistance following cold exposure. These results provide evidence for an acute post-shivering response, whereby glucose metabolism has deteriorated, contrary to the results from earlier studies on cold acclimation.