As the northern hemisphere tips into summer, the question of how to train and race in the heat returns to the front of every distance runner's mind. Heat is rarely a runner's friend on race day, yet a growing body of research suggests that deliberate, structured exposure to it during training can deliver some of the same physiological gains that draw athletes to altitude. Understanding that distinction, between heat as an obstacle and heat as a stimulus, is central to making the most of the warmer months.
The performance penalty of racing in the heat is well established. Both physiological mechanisms, such as glycogen depletion, raised oxygen uptake and the demands of thermoregulation, and biomechanical changes, including altered ground contact time and muscle activation, conspire to slow runners down. Notably, the research indicates the effect is not uniform: under identical conditions the fastest runners tend to suffer a larger decline than slower ones, a reminder that elite athletes have the most to gain from preparing their bodies for warm conditions rather than hoping to escape them.
This is where heat acclimation earns its place in a training plan. Recent work examining four weeks of heat acclimation in trained middle and long-distance runners found that the protocol lowered carbohydrate oxidation during submaximal exercise in the heat, pointing to a metabolic shift that spares precious glycogen stores. For marathon runners in particular, anything that delays the depletion of carbohydrate reserves has obvious appeal, and the finding adds weight to the case for a sustained block of heat work ahead of a goal race.
Perhaps the most intriguing development is the evidence for passive heat exposure. Research has reported that repeated hot-water immersion, the humble post-run bath, can increase haemoglobin mass, total blood volume and left-ventricular end-diastolic volume, all of which feed into an improved VO2 max. The implication is striking: a portion of the adaptation traditionally associated with hard training in the heat may be accessible simply by sitting in a hot bath, a low-cost option for runners who cannot tolerate or schedule repeated sessions in the sun.
None of this removes the need for caution. Heat acclimation is a stressor, and stacking it carelessly on top of an already demanding training load risks overreaching, dehydration and disrupted recovery. The prudent approach is to introduce heat work gradually, prioritise fluid and electrolyte replacement, and treat the bath or sauna as a deliberate training tool rather than an afterthought. Done sensibly, a summer of heat exposure can leave a runner not merely surviving the warmth but genuinely faster because of it.