The Effects of Caffeine Ingestion Before Passive Heat Loading on Serum Leptin Levels in Humans

The effect of dietary supplements on core temperature and sweating responses in hot environmental conditions: a meta-analysis and meta-regression

Dietary supplements are widely used among individuals exposed to hot environments, but whether their consumption confers any thermoregulatory effect is unclear. Therefore, we systematically evaluated the effect of dietary supplementation on key aspects of thermoregulation [core temperature (Tcore) and sweating responses] in the heat. Three databases were searched in April 2024. After screening, 124 peer-reviewed articles were identified for inclusion within three separate meta-analyses: 1) peak Tcore; 2) whole body sweat rate (WBSR); 3) local sweat rate (LSR). The moderating effect of several variables (e.g., training and heat acclimation status), known to influence thermoregulatory function, were assessed via subanalysis and meta-regression. There was no overall effect of the differing supplement types on WBSR (P = 0.405) and LSR (P = 0.769), despite taurine significantly increasing WBSR (n = 3, Hedges' g = 0.79, P = 0.006). Peak Tcore was significantly affected by supplement type (P = 0.011), primarily due to caffeine's "small" significant positive effect (n = 30; Hedges' g = 0.44, P < 0.001) and taurine's (n = 3, Hedges' g = -0.66, P = 0.043) and oligonol's (n = 3; Hedges' g = -0.50, P = 0.014) "medium" significant negative effects. Dietary supplements, such as amino acids (e.g., taurine), some antioxidants and anti-inflammatories (e.g., oligonol) conferred the greatest thermoregulatory benefits during heat exposure. Taurine ingestion in such conditions may lower heat strain, which is likely through its augmentation of thermal sweating. Conversely, caffeine intake may potentially pose the greatest risk in the heat due to its effect on Tcore.NEW & NOTEWORTHY The effects of dietary supplements on core temperature and sweating responses when ingested in the heat varied greatly. Some supplements demonstrated the potential to improve thermoregulatory capacity (e.g., select amino acids, anti-oxidants and anti-inflammatories), while others had no or even deleterious effects on thermal balance (e.g., caffeine). These findings have implications for those ingesting dietary supplements for their health and/or performance effects during exposure to hot environmental conditions. Certain supplements should possibly be avoided in the heat, while others may elicit a thermoregulatory benefit.

Thermotherapy as an alternative to exercise for metabolic health in obese postmenopausal women: focus on circulating irisin level

Irisin is a myokine caused by exercise that improves insulin resistance and weight loss. However, under unfavorable conditions such as air pollution, and during the pandemic, outdoor activities are uncomfortable. Therefore, in this study, the effect of heat therapy (half bath 42 ± 0.5°C for 30 min) on irisin circulation levels as an exercise alternative for middle-aged obese women after menopause was investigated. Subjects were 33 women aged 49.54 ± 6.04 years, with parameters of height, 160.12 ± 4.33 cm, weight, 69.71 ± 7.52 kg, body surface area 1.73 ± 0.13 m², body mass index, 27.19 ± 3.40 kg/m². The results suggest that circulating irisin levels showed a significant increase after one-time thermotherapy (TH-1). However, the increase in circulating irisin levels after 15 treatments (TH-15, 5 days/week, 3 weeks) was significantly varied. The level of adiponectin, which increases fatty oxidation to reduce fatty deposition, increased significantly at TH-1, but further increased at TH-15, which was significantly different from the level of TH-1. In addition, the basic serum free fatty acid (FFA) level was significantly increased at TH-15 compared to TH-1. Significant differences were also found in the lipid profile (body mass index, waist circumference, and % body fat). Thermotherapy can significantly increase the tympanic temperature and induce changes in circulating irisin and adiponectin levels. Thus, it resulted in positive changes in FFA and lipid profiles. Therefore, repeated thermotherapy is effective in increasing circulating irisin levels in postmenopausal obese women.

Effects of caffeine ingestion and thermotherapy on blood orexin circulation in humans

Caffeine and orexin can affect awakening, neuroendocrine, and sympathetic nerve function. Our previous study has reported that caffeine intake can increase human body temperature. However, little is known about the combined effects of thermotherapy and caffeine intake on human serum orexin concentrations. Forty-two healthy male subjects with age of 26.72 ± 5.05 years, height of 174.10 ± 7.09 cm, and body weight of 74.68 ± 8.91 kg participated in this study. They were randomly assigned to a control (CON) group (n = 21) and a caffeine (CAFF) group (n = 21). After thermotherapy (half-body immersion in a hot water bath at 42 ± 0.5 °C, circulating orexin level increased more (p < 0.05) in the CAFF group than in the CON group. Positive relationships between mean body temperature and orexin were observed before and after heat stimulation (p < 0.001). Caffeine intake boosted the upregulation of serum orexin concentrations in subjects undergoing thermotherapy.

Low caffeine dose improves intermittent sprint performance in hot and humid environments

While the effects of caffeine have been evaluated in relation to endurance exercise, few studies have assessed the ergogenic effects of low caffeine doses on intermittent exercise performance in hot and humid environments. Thus, we aimed to determine the effects of low-dose caffeine supplementation on intermittent exercise performance under these conditions. Eight male soccer players (age, 19.9 ± 0.3 years; height, 173.7 ± 6.3 cm; body mass, 65.1 ± 5.5 kg; V˙O₂max, 50.0 ± 3.1 mL ⋅ kg^-1⋅ min^-1) participated in this double-blind, randomized, cross-over study. Caffeine was orally administered at 60 min before exercise (dosage, 3 mg ⋅ kg^-1). The participants completed a 90-min intermittent sprint cycling protocol under two conditions (after receiving caffeine and placebo) at 32 °C and at 70% relative humidity. A significant improvement in the total amount of work was observed in the caffeine condition compared to the placebo condition (155.0 ± 15.8 vs 150.8 ± 14.5 kJ, respectively; p < 0.05, d = 0.28). In contrast, the rectal temperature measured at the end of exercise showed no significant difference between the conditions (38.9 ± 0.4 °C and 38.7 ± 0.5 °C in the caffeine and placebo conditions, respectively; p > 0.05, d = 0.57). Other thermal responses, such as the mean skin temperature, heart rate, or sweat volume, were not significantly different between these conditions. These results suggested that a low caffeine dose improved the intermittent sprint performance and the reasons could be explained by the fact that a low caffeine dose ingestion did not affect the thermoregulatory responses compared to the placebo condition and, thus, did not attenuate its ergogenic effect on exercise in hot and humid environments.

Comparison of density and output of sweat gland in tropical Africans and temperate Koreans

Modification of sweating could be due to changes in activated sweat gland density (ASGD) and/or activated sweat gland output (ASGO). The present study determined regional and inter-ethnic differences in ASGD and ASGO during passive heating between tropical natives (African, n=22) and temperate natives (Republic of Korean, n=25). Heat load was carried out by immersing the half body into a hot water bath for 30min. Tympanic temperature (Tty) and skin temperature (Ts) were measured. Mean body temperature (mTb) was calculated. Sudomotor activities including sweat onset time, sweat rate, ASGD, and ASGO were examined in eight regions of the skin. Africans had smaller increase in mTb during passive heating than Koreans. The onset time of sweating was much more delayed in Africans compared to Koreans. In response to thermal load, ASGD and ASGO differed between body regions in Africans and Koreans. In most skin regions, ASGD and ASGO were lower in tropical Africans compared to those in temperate Koreans. Trunk portion including chest, upper back, lower back, abdomen had greater swear rate, ASGD, and ASGO compared to peripheral segments including upper arm, forearm, leg, and thigh in both ethnic groups. Distribution patterns of ASGD over the body appeared to be similar in both Africans and Koreans at the peak of thermal loading. In conclusion, the present study demonstrates that sudomotor activity in tropical Africans is reduced with lower ASGD and ASGO over the body surface compared to temperate Koreans.

Increased levels of FFA during passive heat loading after a 2-week repeated heat load in Koreans

The purpose of this study was to determine whether repeated heat load is closely related to circulating levels of free fatty acids (FFA) during repeated passive heat loading (PHL), defined as immersion of the lower body up to an umbilical level in hot water, 42 ± 0.5 °C (three times/week, 30 min/day) for 2 weeks. There were significant correlations between mean body temperature and FFA before and after repeated heat load (p < 0.001, respectively), and the level of FFA was significantly higher after repeated heat load during PHL (p < 0.01). The threshold of mean body temperature for lipolysis was lowered by repeated heat load and enhanced lipolysis during PHL. However, caution is needed for diabetic individuals.

Oligonol supplementation modulates plasma volume and osmolality and sweating after heat load in humans

Oligonol is a low-molecular-weight polyphenol that possesses antioxidant and anti-inflammatory properties. This study investigated the effects of Oligonol supplementation on sweating response, plasma volume (PV), and osmolality (Osm) after heat load in human volunteers. We conducted a placebo-controlled crossover trial. Participants took a daily dose of 200 mg Oligonol or placebo for 1 week. After a 2-week washout period, the subjects were switched to the other study arm. As a heat load, half-body immersion into hot water (42°C±0.5°C for 30 min) was performed in an automated climate chamber. Tympanic and mean body temperature (Tty, mTb) and whole-body sweat loss volume (WBSLV) were measured. Changes in PV, Osm, and serum levels of aldosterone and sodium were analyzed. Oligonol intake attenuated increases in Tty, mTb, and WBSLV after heat load compared with the placebo (P<.01, P<.05, and P<.01, respectively). In addition, serum aldosterone was maintained at a relatively low degree and serum sodium was maintained at a relatively high degree with Oligonol compared to the placebo (P<.01 and P<.05, respectively). However, PV decreased and Osm increased significantly with Oligonol compared to the placebo (P<.05 and P<.05, respectively). This study demonstrates that Oligonol supplementation for 1 week can attenuate elevation of body temperature and excessive sweating under heat load in healthy humans, but interpretation of the results requires caution due to the potent diuretic effect of Oligonol.

Beneficial effect of Oligonol supplementation on sweating response under heat stress in humans

Oligonol is a low-molecular weight polyphenol that possesses antioxidant and anti-inflammatory properties. However, nothing is known regarding the impact of Oligonol on sudomotor activity. This study investigated the effects of Oligonol supplementation on sudomotor activity during heat load in humans. Initially, we conducted a placebo-controlled, cross-over trial where participants took a daily dose of Oligonol 200 mg or placebo for one week. After a 2 week washout period, the subjects were switched to the other study arm. As a heat load, half-body immersion into hot water (42 ± 0.5 °C for 30 min) was performed in an automated climate chamber. Tympanic and skin temperatures were measured. Sudomotor activity, including onset time, sweat rate (SR) and volume (SV), active sweat gland density (ASGD), and sweat gland output (SGO), was tested in four or eight areas of skin. When compared with placebo, Oligonol attenuated increases in tympanic and skin temperatures after the heat load. There was an increasing trend in local sweat onset time, but there was a decrease in local SR, SV, ASGD, and SGO for Oligonol compared to placebo. The mean ASGD was significantly higher in the Oligonol group than in the placebo group for 10, 20, and 30 min. This study demonstrates that Oligonol appears to be worthy of consideration as a natural supplement to support more economical use of body fluids against heat stress.