Hot baths could offer marathon runners an alternative to altitude training

For many decades, top-level runners have gone around the world to train at altitude.

At high altitude, where the air contains less oxygen, the body reacts by making additional red blood cells – the cells that transport oxygen throughout the body.

Once athletes return to sea level, this increased ability to carry oxygen can improve endurance performance.

Yet altitude training has drawbacks. It demands time away from home, considerable expense and long-haul flights. For most runners taking part in this year's London Marathon, it is not a realistic possibility.

In our research, we therefore looked for a more attainable option. We focused on heat, another form of environmental stress.

Athletes often use brief heat-exposure blocks – generally lasting seven to 14 days – when preparing to compete in hot conditions. But we set out to establish whether sustained heat exposure over four to five weeks might bring about physiological adaptations resembling those produced by altitude.

Hot baths as a heat-exposure protocol

We enlisted well-trained endurance runners, who continued with their usual training programme. The sole extra element was five hot baths each week for five weeks.

No advanced laboratory equipment was involved: the baths were ordinary household bathtubs.

Using an inexpensive thermometer, participants kept the water at 40°C, topping it up with warm water where necessary. They spent 45 minutes in the bath after training.

We assessed a range of endurance-physiology indicators before and after the five-week programme. These included red blood cell volume, heart structure and maximal oxygen uptake (VO₂max) – widely considered the benchmark measure of aerobic fitness.

What we found

Five weeks of regular hot baths produced a significant rise in the runners' red blood cell volume. Put simply, their bloodstreams contained more cells capable of carrying oxygen.

This may initially appear unexpected. At altitude, the body increases red blood cell production because less oxygen is available in the air. Heat does not restrict oxygen availability, but it changes the blood differently.

Even one heat session causes plasma, the liquid portion of blood, to expand.

As this increase dilutes red blood cells, the amount of oxygen carried in the blood falls temporarily.

The body detects the shift and makes more red blood cells to re-establish balance.

Eventually, both plasma volume and the total number of red blood cells rise. This means a greater overall blood volume and a larger capacity to transport oxygen.

We identified heart changes as well. Endurance training is already known to increase the size of the left ventricle, the heart's principal pumping chamber, enabling it to pump more blood on every beat.

After the heat intervention, the volume of this chamber rose further. The extra blood produced in response to heat exposure probably helped drive this enlargement.

Collectively, these adaptations raised aerobic capacity. The runners' VO₂max increased by approximately 4% on average, while they could also attain faster speeds in maximal treadmill tests.

Laboratory measurements are not equivalent to race outcomes, but changes of this size matter for trained athletes – especially as they were achieved without adding training intensity or mileage.

Why heat exposure matters for marathon training

The findings have potentially interesting implications for runners and coaches.

Firstly, heat exposure may provide a low-impact means of prompting helpful bodily adaptations without the extra load of further exercise. Raising mileage or intensity invariably brings some injury risk.

By comparison, hot baths challenge the cardiovascular system without imposing extra impact on muscles and joints.

Secondly, the method is comparatively accessible. As most people can use a bath, its financial and environmental demands are small compared with altitude camps.

This could make performance-enhancing – and completely legal – training strategies available more equitably.

Limitations and safety of hot baths

As with any research, the study has limitations. We examined one particular protocol: water at 40°C, sessions of 45 minutes, five times a week for five weeks. It remains unknown whether shorter baths, cooler water or alternative heat sources – including saunas and steam rooms – would have the same effects.

Safety must also be considered. Extended exposure to heat may raise the likelihood of dehydration, dizziness and heat illness.

Anybody trying a comparable method should stay properly hydrated, prevent overheating and complete sessions under suitable supervision.

Those with existing health conditions should obtain medical advice before trying this sort of protocol.

Finally, we assessed physiological markers and treadmill performance rather than actual marathon finishing times. Although VO₂max improvements are closely associated with endurance performance, further research must establish how these adaptations carry over into real-world racing.

Nevertheless, our results indicate that better performance does not invariably depend on running more miles or travelling internationally. Adaptation can sometimes be prompted through unexpectedly simple methods.

For marathon runners seeking a practical way to complement their training, passive heat exposure could be a remarkably simple option to investigate.

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.