Hot baths could mimic altitude training for marathon runners

Elite runners have been travelling to high-altitude locations for decades as part of their preparation.

When the air contains less oxygen, the body adapts by making additional red blood cells - the cells that transport oxygen around the body.

Back at sea level, that increased ability to carry oxygen can translate into better endurance.

The drawback is that altitude camps are expensive and time-consuming, and they usually involve long-distance travel. For most of the runners on the start line of this year’s London Marathon, that kind of training simply isn’t realistic.

With that in mind, our research set out to find a more practical alternative. We focused on a different environmental challenge: heat.

Many athletes already use brief blocks of heat exposure - usually seven to 14 days - to get ready for races in hot conditions. What we wanted to test was whether a longer period of heat exposure, lasting four to five weeks, might prompt physiological adaptations similar to those linked with altitude.

The heat exposure protocol: five hot baths a week

We enlisted a group of well-trained endurance runners and told them to keep their usual training exactly as it was. The only change was simple: five hot baths each week for five weeks.

There was nothing specialised about the set-up. The baths took place in ordinary household bathtubs.

Using a low-cost thermometer, we kept the water at 40°C, topping up with hot water when required. Each bath lasted 45 minutes and was taken soon after training.

Before and after the five-week block, we assessed several indicators of endurance physiology, including red blood cell volume, heart structure and maximal oxygen uptake (VO₂max) - commonly considered the gold-standard measure of aerobic fitness.

What we found

After five weeks of consistent hot baths, the runners showed a significant rise in red blood cell volume - put simply, more oxygen-carrying cells circulating in the bloodstream.

On the face of it, that can look unexpected. At altitude, red blood cell production rises because oxygen in the air is scarce. Heat does not reduce the oxygen available to breathe, but it changes the blood through another pathway.

Even after a single heat session, plasma - the fluid portion of blood - increases.

As plasma expands, it temporarily dilutes the concentration of red blood cells, which briefly reduces how much oxygen the blood can carry.

The body detects this shift and compensates by generating additional red blood cells to re-establish balance.

Over time, both plasma volume and red blood cell volume increase - meaning a larger total blood volume and a greater overall capacity to transport oxygen.

We also saw the heart adapt. Endurance training typically enlarges the left ventricle, the heart’s main pumping chamber, enabling more blood to be pushed out with each heartbeat.

After the heat intervention, this chamber’s volume increased again. The extra blood volume created by repeated heat exposure probably helped drive that enlargement.

Taken together, these adaptations boosted aerobic capacity. On average, VO₂max rose by around 4%, and the runners managed higher speeds during maximal treadmill testing.

Although laboratory outcomes are not the same as finishing times in a race, gains of this size matter for trained athletes - especially as they occurred without any increase in training intensity or weekly mileage.

Why this matters for marathon training

For runners and coaches, these findings raise some interesting possibilities.

First, heat exposure may provide a low-impact method for encouraging useful adaptations without adding more running stress. Increasing intensity or distance always increases the risk of injury.

Hot baths, by comparison, challenge the cardiovascular system while avoiding extra impact on muscles and joints.

Second, it is comparatively easy to access. Many people can use a bath, and compared with altitude camps the financial outlay and environmental footprint are far smaller.

That creates the potential for wider, more equitable access to performance-enhancing (and entirely legal) training methods.

Limitations and safety considerations

As with any study, there are constraints. We tested one specific approach: 40°C water, 45 minutes per session, five times per week, across five weeks. It remains unclear whether shorter exposures, cooler water, or other forms of heat - such as steam rooms or saunas - would deliver the same effects.

Safety also matters. Extended heat exposure can raise the risk of dehydration, light-headedness and heat illness.

Anyone trying something similar should stay well hydrated, prevent overheating and carry out sessions with appropriate supervision.

People with underlying health conditions should seek medical advice before attempting this kind of protocol.

Lastly, we measured physiological markers and treadmill performance rather than actual marathon finishing times. While VO₂max improvements are closely associated with better endurance performance, future research will need to establish how these changes translate into real competition.

Even so, our results indicate that meaningful improvements do not always depend on running more miles or travelling overseas. Sometimes the body can be prompted to adapt in unexpectedly simple ways.

For marathon runners seeking a practical addition to their preparation, passive heat exposure could be a remarkably straightforward tool to explore.

Mike Stembridge, Professor of Cardiovascular and Environmental Physiology Cardiff School of Sport & Health Sciences, Cardiff Metropolitan University and Elliott Jenkins, PhD Candidate in Exercise and Environmental Physiology, Cardiff Metropolitan University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.