Hot baths improved cardiorespiratory fitness in endurance runners by boosting red blood cell mass and heart function. Digest
Endurance athletes often look for ways to increase cardiorespiratory fitness beyond what exercise alone can provide, yet many proven methods are expensive and require major logistical demands. In a new study, researchers tested whether hot baths offer a simpler approach to stimulate cardiovascular adaptations.
The study enrolled 10 well-trained endurance runners with an average maximal oxygen consumption (VO₂max, the highest rate of oxygen use during intense exercise) of about 64 milliliters per kilogram per minute. Each runner completed two five-week blocks: one with hot water immersion and one control block with the same regular training. During the immersion block they sat in at least 40°C/104℉ water for 45 minutes, five times per week, usually after training. To maintain a consistent thermal load, the temperature was adjusted based on participants' heat perception and reached an average of about 41.7°C/107.1℉ by week five. The researchers focused on three key features of circulation: haemoglobin mass (the total amount of oxygen-carrying protein in red blood cells), total blood volume (the combined liquid and cell volume of blood), and left-ventricular end-diastolic volume (LVEDV, how much blood fills the main pumping chamber of the heart just before it contracts).
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The runners already had high cardiorespiratory fitness, yet heat immersion still changed their blood, heart, and performance in measurable ways:
- VO₂max rose by 2.7 milliliters per kilogram per minute, roughly a 4% increase, after the heat block, while it stayed essentially unchanged during the control block.
- Haemoglobin mass increased by about 4% (from about 857 to 891 grams) in the heat condition, whereas it slightly declined in the control period. This was accompanied by red blood cell volume rising by about 7% (from about 2.45 to 2.62 liters).
- Plasma volume, the liquid part of blood, expanded by roughly 8% (about 250 milliliters) in the first week of hot baths, then began to trend back toward baseline after the second week.
- LVEDV increased by about 6% (from 170 to 180 milliliters). Resting stroke volume, the amount of blood pumped with each heartbeat, rose by around 7% (from 94 to 101 milliliters), which resulted in the heart pumping about 13% more blood per minute (from 4.5 to 5.1 liters). Comparable shifts did not appear during the control phase.
- Across the five weeks, participants acclimated to the heat. They showed a smaller rise in body temperature and an increase in sweat rate of roughly 38% (from about 1.3 to 1.8 liters per hour), while their ratings of heat and discomfort remained similar.
The sequence of changes points to a typical heat acclimation pattern. Plasma volume expanded in the early weeks. This early expansion is a well-known response to repeated heat exposure and reflects fluid shifting into the bloodstream and adjustments that help the body conserve more fluid during heat stress. As plasma expansion diluted the blood, it likely signaled the need for additional red blood cell production. In the later weeks, red blood cell volume and haemoglobin mass rose, which increased the blood's oxygen-carrying capacity. The larger circulating volume also increased how much blood returned to the heart, allowing the left ventricle to fill more completely and pump more with each beat. Further analysis showed that haemoglobin mass and LVEDV accounted for most of the heat-related rise in VO₂max.
Because this trial involved only ten runners, lacked a control condition where participants sat in normal-temperature water, and scheduled most baths immediately after exercise, the findings point to a promising but still preliminary approach that needs to be further tested. In this clip, Dr. Martin Gibala explains VO₂max's role in health and how non-athletes can optimize workouts to boost their cardiorespiratory fitness.