The effect of hot water immersion on glucose tolerance: Differences between acute and chronic exposure

Comment on: Maley MJ, Hunt AP, Stewart IB, et al. Hot water immersion acutely reduces peripheral glucose uptake in young healthy males: An exploratory crossover randomized controlled trial. Temperature, 2023.

We read with great interest the recent work of Maley and colleagues [1], examining the impact of an acute bout of hot water immersion on an oral glucose tolerance test. This study, coupled with the work of James and colleagues [2] employing a similar design in people with Type 2 diabetes, indicate that hot water immersion does not provide an acute benefit in glucose tolerance. However, for readers of the paper by Maley et al. [1], it is important to discuss the discrepancy between acute and chronic responses to passive heat exposure as displayed in Figure 1. In contrast to this acute study, chronic hot water immersion (30 sessions over 10 weeks) led to substantial improvements in a 75 g 2-hour oral glucose tolerance test in women with impaired metabolic health [3]. Similar improvements in metabolic health and glucose metabolism have been observed with 10-day passive heat exposures in overweight men [4,5]. While these acute [1,2] and chronic [3–5] interventions employed different heating protocols, studied subject populations ranging from young healthy men to men and women with impaired metabolic health (obesity, Type 2 Diabetes, and Polycystic Ovary Syndrome), and used various assessments of glucose tolerance and metabolic health, the composite results clearly show a chronic benefit to metabolic health in contrast to a lack of benefit or possible impairment with an acute exposure. We encourage the readers and researchers to examine the full scope of literature on this topic, as several relevant studies were not included in the discussion [1], and could lead to an erroneous conclusion regarding the impact of passive heating on glucose regulation.

Figure 1. Mean change in glucose area under the curve (AUC) with exposure to chronic (30 1-hr hot tubs over 10 weeks) [3] or acute heat, representing either a 2-hr hot tub during an OGTT [1], a 1-hr hot tub 30 min before an OGTT [2], and a 1-hr hot tub from minutes 30–90 of an OGTT [2]. * Indicates a significant (p<0.05) change as reported in the original papers.

We also implore future researchers to include women in their study design. This seems to be particularly common in any research involving thermal stress, due to basal temperature fluctuations and changes in thermoregulatory function throughout the menstrual cycle. However, it does not add substantial time or cost to either schedule acute studies in the same phase of the menstrual cycle, or simply track and report the phases during which women were studied. Moreover, relevant to research examining glucose tolerance or insulin sensitivity, healthy women do not experience changes in fasting glucose, glucose tolerance, or insulin secretion in luteal vs follicular phases [6].

Heat exposure, whether acute or chronic, is a fascinating topic with many remaining mechanisms to be explored. Currently, our knowledge of the physiology, mechanisms, and best practices of heat therapy are at a place similar to the early days of studying the benefits of exercise in the 1940”s and 1950”s. Additionally, heat therapy appears to have some similarities to exercise in that a single bout may cause no change in a health parameter or even an apparent negative response (increased inflammation, elevated systolic blood pressure, decreased muscular strength) but a chronic intervention allows benefits to be realized (reduced systemic inflammation, lower systolic pressure, increased muscular strength). Conversely, both acute and chronic exercise are associated with improvements in insulin sensitivity and glucose tolerance. Understanding the physiology behind acute and chronic exposures is critical to our exploration, as in some cases acute responses may not predict or correlate with longer term adaptations, whereas in other cases they may be predictive. Acute studies should, however, be placed in the context of more chronic exposures, particularly when related to using the stress as a health benefit or lifestyle. Regardless, perhaps we can avoid some of the mistakes in those early exercise studies by including women in the study design, or at a minimum include studies in the discussion that were focused on or included women.