Five days exercising in the heat can transform an athlete’s cardiovascular system. As summer temperatures climb across Saudi Arabia, this is critical. A 2026 study published in PLOS ONE provides a precise answer. Researcher Daniel Snape and colleagues at Leeds Beckett University subjected 14 endurance-trained male athletes to five consecutive days of isothermic heat acclimation at 32°C and 70% humidity. The results were striking: resting heart rate dropped by 8 beats per minute, plasma volume expanded by 7.3%, and sweat loss increased by 500 mL. These cardiovascular adaptations occurred rapidly, but the study revealed an important caveat about kidney function that every athlete training in Gulf conditions should understand.
How Heat Acclimation Transforms Your Cardiovascular System
When you repeatedly exercise in hot conditions, your body initiates a cascade of physiological adaptations. The first and most significant change is plasma volume expansion. Plasma, the liquid component of blood, increases in volume, allowing the heart to pump more blood with each beat. This improved stroke volume means the heart doesn’t need to beat as frequently to maintain adequate blood flow to working muscles and the skin, where heat is dissipated. This mechanism explains the pronounced reduction in resting heart rate observed in the study.
The second adaptation is enhanced sweating capacity. The body learns to cool itself more efficiently by producing sweat earlier and in greater quantities. In Snape’s study, whole-body sweat loss increased by 500 mL after just five days of heat acclimation. This 37% increase means the body became more effective at dissipating the heat generated during exercise, reducing overall thermal strain. For more on how heat adaptation works, see our guide to passive heat acclimation.
These adaptations align with findings from a 2024 meta-analysis by Tyler and colleagues, which confirmed that heat acclimation improves performance in hot conditions through multiple mechanisms including cardiovascular and thermoregulatory changes. The consistency across studies strengthens confidence in heat acclimation as a practical preparation strategy for athletes competing in hot environments.
Five Days Exercising in the Heat: The Numbers
The quantitative results from Snape’s study paint a clear picture of how rapidly short-term heat acclimation works. Resting heart rate decreased by 8 ± 5 bpm in the heat acclimation group (p=0.007, effect size d=1.0), a statistically significant and large reduction. This decline reflects improved cardiac efficiency, where the heart can pump the same volume of blood with fewer beats.
Plasma volume increased by 7.3 ± 5.1% (p=0.022), which exceeded the mean increase of 4.3% reported in earlier studies. This expansion in blood volume improves the body’s capacity to maintain blood pressure and deliver blood to active muscles and the skin for cooling. The magnitude of this change suggests the isothermic protocol, which maintained core temperature at or above 38.5°C for 60-90 minutes per session, provided a potent stimulus.
Sweat loss increased by 500 ± 539 mL (p=0.018, d=1.1), indicating enhanced sweat gland responsiveness. This 37% increase was more pronounced than the 5% increase seen in previous short-term heat acclimation studies, likely due to the high humidity (70% relative humidity) reducing evaporative cooling and promoting greater sweat gland activation. Perceived exertion also improved, with ratings dropping by 2 points (p=0.007) and thermal comfort increasing (p=0.016). These perceptual changes mean exercise felt less difficult after acclimation. For comprehensive hydration strategies during heat training, explore our complete guide to training and hydration in extreme heat.
The Kidney Surprise: What Heat Acclimation Did Not Change
Despite the clear cardiovascular improvements, the study revealed an unexpected finding: short-term heat acclimation did not alter markers of kidney stress. The researchers measured neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1), both biomarkers of renal tubular stress, and found no significant changes after heat acclimation. Similarly, levels of copeptin (a marker of fluid regulation) and normetanephrine (reflecting sympathetic nervous system activity) remained unchanged.
This result aligns with previous research showing that short-term heat acclimation may not fully protect kidney function during exercise in the heat. A 2025 study by Connor and colleagues found that 10 days of heat acclimation improved heat tolerance but did not attenuate the rise in serum creatinine during exercise-heat stress. This suggests that renal adaptation requires longer protocols, perhaps 10 days or more.
A likely explanation is that cardiovascular adaptations occur rapidly, but deeper kidney adaptations take longer. The body first adapts to heat stress by improving heart efficiency and sweating capacity, but kidney protection may require additional adaptations including better regulation of renal blood flow and reduced sympathetic vasoconstriction. The authors concluded that extended heat acclimation protocols (10+ days), combined with individualized hydration plans or cooling interventions, may be necessary to safeguard renal function during exercise in hot environments.
What This Means for Athletes in Saudi Arabia
In Saudi Arabia, where summer temperatures reach 45-50°C in some regions and humidity is high in coastal cities like Jeddah and Dammam, heat acclimation becomes essential rather than optional for endurance athletes. Snape’s results offer an encouraging message: just five days can produce significant cardiovascular improvements. But the important caveat is that these improvements don’t mean complete kidney protection.
An athlete planning to compete in hot conditions should begin heat acclimation at least 10-14 days beforehand, not just five days. This gives the body sufficient time to develop deeper adaptations including renal protection. The practical protocol involves exercising in a hot environment (32-35°C) for 60-90 minutes daily, either in a specialized heat chamber or a closed room with heating. If these conditions aren’t available, training during the hottest part of the day (with extreme caution) or wearing additional clothing to raise body temperature can simulate heat stress. Hydration remains critical even after acclimation, because the body loses more fluid through increased sweating.
Practical Takeaways
- Start heat acclimation 10-14 days before competing in heat, not just five days
- Train in a hot environment (32-35°C) for 60-90 minutes daily
- Monitor resting heart rate as an adaptation marker: an 8 bpm drop indicates good adaptation
- Don’t neglect hydration even after acclimation: your body loses more fluid through increased sweating
- Consider longer protocols (10+ days) to protect kidney function in extreme heat
- Consult a sports physician before starting a heat acclimation program, especially if you have pre-existing health conditions
Sources
- Snape D, Wainwright B, Parsons IT, et al. “Five days of heat acclimation improves cardiovascular and thermoregulatory responses without altering renal stress biomarkers in endurance athletes.” PLOS ONE. 2026;21(8):e0345346. doi:10.1371/journal.pone.0345346
- Tyler CJ, Reeve T, Sieh N, Cheung SS. “Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta-Analysis.” Journal of Science in Sport and Exercise. 2024;6(3):195-217. doi:10.1007/s42978-023-00263-8
- Connor KR, Brazelton SC, Greenfield AM, et al. “Heat acclimation alters the relation between hyperthermia and biomarkers of kidney function during exercise heat stress.” Am J Physiol Regul Integr Comp Physiol. 2025;329(6):R995-R1001. doi:10.1152/ajpregu.00215.2025
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