New findings: What is the central question to the study? Are primary indices of heat adaptation (e.g., expansion of plasma volume and reduction in resting core temperature) differentially affected by the three major modes of short-term heat acclimation, that is, exercise in the heat, hot water immersion and sauna? What it the main finding and its importance? The three modes elicited typical adaptations expected with short-term heat acclimation, but these were not significantly different between modes. This comparison has not previously been made and highlights that individuals can expect similar adaptation to heat regardless of the mode used.
Heat acclimation (HA) is a crucial process for enhancing heat tolerance and boosting cardiovascular health. While the method of heat acclimation could potentially influence the degree and speed of adaptation, there’s a notable lack of comparative research in this area. A recent study investigated and compared the adaptive responses from three common HA methods, with a particular focus on plasma volume changes.
Thirteen physically active individuals (including five women) participated in a crossover study involving four 5-day heat acclimation regimes, each lasting 60 minutes daily. These regimes, conducted in a randomized order and separated by at least four weeks, included: thermoneutral water immersion (TWI) at 36.6°C as a control; heat stress via 40°C water immersion; sauna exposure at 55°C with 52% relative humidity; and exercise in humid heat (ExH) at 40°C with 52% relative humidity. The magnitude of adaptation was evaluated by pooling data from days 4-6, and the adaptation timeline was assessed using the 6-day series.
The study revealed that plasma volume expansion occurred similarly across all heat-based conditions. However, when compared to the control condition (TWI), plasma volume was significantly higher only in the exercise-in-heat group, showing a 4% increase (P = 0.036). Intriguingly, about two-thirds of this expansion occurred within the first 24 hours and showed a moderate correlation with the volume present on day 6, irrespective of the heat acclimation method (r = 0.560-0.887). This expansion was primarily facilitated by the body’s retention of sodium and albumin, with no significant differences in the roles of these components across the different methods (P = 0.706 and 0.320, respectively).
Furthermore, resting rectal temperature decreased by 0.1-0.3°C in all heat-exposed conditions, and systolic blood pressure saw a reduction of 4 mmHg. These changes were consistent across all heat acclimation modes, with no significant differences observed between them (P ≥ 0.137).
In conclusion, the study demonstrates that the method of heat acclimation does not significantly alter the extent or pace of adaptation in key resting indicators of short-term heat acclimation. This suggests that individuals can achieve similar benefits from heat acclimation regardless of whether they choose exercise in heat, sauna, or hot water immersion, providing flexibility in how individuals approach heat acclimation for improved heat tolerance and cardiovascular health.
