Hot baths may boost endurance runners' performance by 5%, study finds

A Small Study Shows: Five Weeks of Regular Hot Baths Can Measurably Boost Aerobic Performance—Without Extra Training. What's Behind the Effect?

Anyone aiming to run a marathon needs one thing above all: well-trained endurance. Elite athletes often turn to altitude training, which exposes the body to lower oxygen levels, stimulating the production of additional red blood cells. More red blood cells mean greater oxygen transport in the blood—and thus higher performance. But for most amateur runners, altitude training isn't a realistic option: it's too expensive and logistically demanding.

Could a hot bath achieve the same effect? Researchers at Cardiff Metropolitan University set out to answer this question—and their findings were surprising.

How the Study Worked

The study by Jenkins et al. (2025), published in the prestigious Journal of Physiology, examined ten well-trained endurance runners (nine men, one woman; average age 25) using a crossover design. This meant each participant experienced both conditions—once in the intervention group with hot baths, once in a control phase without them—with at least five weeks in between.

The intervention itself was straightforward: participants bathed five times a week for 45 minutes each in water heated to at least 40°C. The temperature was adjusted individually over the five weeks, rising from an average of 40.6°C in week one to 41.7°C in week five. All runners continued their usual training unchanged.

Before and after each five-week phase, researchers conducted comprehensive measurements: hemoglobin mass (a key indicator of the blood's oxygen-carrying capacity), blood volume, heart structure via 4D echocardiography, and maximum oxygen uptake (VO₂ max) on a treadmill.

The Results: More Oxygen in the Blood, Stronger Heart Function

After five weeks of regular hot baths, the runners showed clear physiological changes—changes that did not occur in the control group without baths.

• Increased oxygen in the blood: The heat expanded blood plasma, signaling the body to produce more red blood cells. The result? Greater blood volume and a significantly higher oxygen-carrying capacity.
• A more efficient heart: The heart adapted as well. The left ventricle enlarged slightly, allowing it to pump more blood with each beat. The heart became more effective—without participants running a single extra kilometer.
• Noticeably better performance: Combined, these adaptations led to a roughly 4.5% improvement in aerobic capacity. On the treadmill, runners could push their limits at higher speeds after the intervention—a meaningful gain for well-trained athletes. The control group saw no comparable improvement.

Remarkably, the baths imposed no additional strain. Sleep quality, muscle soreness, mood, and stress levels showed no differences between the bath and control groups at any point.

Significance of the Findings and Critical Considerations

It's worth noting that the study included only ten participants, and performance gains were measured on a treadmill—whether this directly translates to faster marathon times remains to be confirmed in further research. Whether shorter baths, lower temperatures, or sauna sessions could produce the same effect is also still unclear. (For more on how infrared saunas affect cardiovascular health, read here.)

Despite these limitations, the findings could be relevant for ambitious recreational runners. According to the researchers, the performance gains achieved through hot baths are comparable to those from traditional altitude training—but without the travel, expense of training camps, or disruption to regular workouts.

How Could a Hobby Marathon Runner Put This Into Practice? The study's protocol is fundamentally straightforward: 45 minutes, five times a week, in water at least 40°C—ideally right after training. A standard bathtub and a thermometer are all you need.

Safety Note: Prolonged hot baths can increase the risk of dehydration, dizziness, and heat-related illness, the researchers emphasize. Anyone with pre-existing health conditions—particularly circulatory disorders—should consult a doctor before trying this approach. Staying well-hydrated during and after the bath is essential.

Marathon Training Tips—and a Sample Plan

As intriguing as the hot-bath method may be, it's no substitute for solid foundational training. Whether you're tackling your first marathon or aiming for a personal best as a seasoned runner, stick to proven training principles:

• Endurance Before Speed: The bulk of your training should take place at an easy, aerobic pace. Speedwork is important, but it only makes sense once you've built a strong base. Learn more about heart rate zone training [here].
• Progressive Overload: Never increase your weekly mileage too abruptly—this helps prevent overuse injuries.
• Don't Skip the Long Run: The weekly long run is the cornerstone of marathon preparation. It boosts fat metabolism, strengthens muscles, and builds mental resilience.
• Prioritize Recovery: Training nonstop won't make you faster—your body needs rest to adapt. After intense sessions, schedule rest days or active recovery (walking, easy cycling, stretching, or mobility work). And don't underestimate the power of quality sleep.
• Nutrition and Hydration: Balanced carbohydrate intake before long runs and proper hydration are non-negotiable for endurance athletes. Alcohol, meanwhile, undermines both performance and recovery.