Heat training and VO2max: does training in the heat make you fitter?

Heat training has gone from a niche idea to a mainstream tool, with elite cyclists and triathletes using hot rooms, saunas and extra clothing to prepare for races. It is often called "the poor man's altitude camp". But does it actually raise VO2max, or does it simply make you better at coping with heat?

What happens when you train in the heat

Exercising in the heat creates competition for blood: the muscles need it for oxygen, and the skin needs it to lose heat. Cardiac output is shared between the two, so on a hot day heart rate climbs at any given pace and VO2max falls. This is why race times get slower when it is hot, and why heart rate drifts upward during long runs.

How the body adapts

Repeated exercise in the heat triggers heat acclimation. The review by Périard, Racinais and Sawka (2015) describes the main changes:

Adaptation	What it does	Timeline
Plasma volume expansion	More blood to share between skin and muscle; higher stroke volume	First days
Lower heart rate at a given workload	Less cardiovascular strain	Within about a week
Earlier and greater sweating	Better evaporative cooling	One to two weeks
Less salt lost in sweat	Better fluid and electrolyte balance	One to two weeks
Lower core and skin temperature	More room before overheating	One to two weeks
Better thermal comfort	Exercise feels easier	Throughout

Most adaptations are in place after about 10–14 days of daily exposure. They also fade over a few weeks without heat, though some re-acclimation happens faster afterwards.

The VO2max question

The headline study is Lorenzo et al., 2010. Trained cyclists completed 10 days of exercise in a hot room, while a control group did the same training in cool conditions. The heat group improved VO2max and time-trial performance in both hot and cool conditions, along with an expansion of plasma volume. The control group did not.

That result generated much excitement, but later studies were less clear. Keiser and colleagues (2015), in a carefully controlled cross-over study, found that heat training improved exercise capacity in the heat but not in temperate conditions, despite plasma volume expansion. Plasma volume tends to return towards normal after a while, and haemoglobin mass changes are small and inconsistent.

The fair summary: heat acclimation reliably improves performance in the heat. Effects on VO2max and performance in cool conditions are possible but small and debated — far less dependable than a block of interval training (see HIIT vs steady cardio). It is a similar story to altitude training: real physiology, modest and variable payoff.

Practical protocols

• Hot-room training: 60–90 minutes of easy to moderate exercise in heat, daily for 5–14 days, aiming for a raised core temperature. This is what most studies used.
• Overdressing: extra layers on an indoor trainer raise body temperature at home; it is crude but works.
• Post-exercise sauna or hot bath: sitting in a sauna or hot bath straight after training, building up over a couple of weeks. A small study by Scoon and colleagues reported improved running performance in runners using sauna after training.
• Maintenance: once acclimated, two or three heat exposures a week seem to slow decay.

Before a hot race, the consensus recommendations led by Racinais (2015) suggest acclimatising for about two weeks, ideally in conditions similar to the race.

Training in hot climates (like Bangkok)

If you live somewhere hot and humid, you are already partly acclimated. The challenge is keeping training quality:

• Run easy sessions by effort or heart rate, not pace. Humidity makes sweating less effective, so pace drops more than in dry heat.
• Schedule hard sessions early, before sunrise, or indoors with air conditioning or a fan.
• Your watch's VO2max estimate will often dip in the heat because pace falls at a given heart rate (see how accurate is your watch VO2max). Compare like with like.
• Drink to thirst plus a little more, and include salt on long or very sweaty sessions.
• Plan a cooler, easier week when you travel to a cold climate for a race: you may feel surprisingly strong.

Easy aerobic work in the heat still builds fitness; combine it with zone 2 training principles and protect recovery. Triathletes and marathoners racing in heat can find sport-specific advice under triathlon and running.

Safety: heat stroke is a medical emergency (see the NATA position statement on exertional heat illness). Stop immediately and cool down if you feel dizzy, confused, nauseous or stop sweating, and get urgent help if symptoms persist. Build heat exposure gradually, avoid alcohol beforehand, and get medical advice first if you have heart disease, are pregnant, or take medication that affects heat tolerance.

Sources

• Lorenzo S, et al. Heat acclimation improves exercise performance. J Appl Physiol, 2010.
• Périard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scand J Med Sci Sports, 2015.
• Keiser S, et al. Heat training increases exercise capacity in hot but not in temperate conditions: a mechanistic counter-balanced cross-over study. Am J Physiol Heart Circ Physiol, 2015.
• Racinais S, et al. Consensus recommendations on training and competing in the heat. Br J Sports Med, 2015.
• Scoon GSM, et al. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. J Sci Med Sport, 2007.
• Casa DJ, et al. National Athletic Trainers' Association position statement: exertional heat illnesses. J Athl Train, 2015.