A Hot Bath Instead of Mountains: How Heat Raises VO₂max

Altitude camps have long been considered the gold standard for building endurance: thin air forces the body to produce more red blood cells and carry more oxygen. But mountains are far away, expensive and not for everyone. What if you could get part of the same effect lying in a hot bath at home? A new study in The Journal of Physiology shows that regular heat exposure isn't a spa ritual but a working tool that raises VO₂max even in already well-trained runners.

What was studied

A team led by Elliott Jenkins took 10 well-trained runners (9 men, 1 woman) with a baseline VO₂max of about 64,5 ml/kg/min — the level of strong amateurs and semi-professionals. The design was crossover, with a control condition: each athlete completed both the "heat" and the control phase, which sharply increases the reliability of the conclusions.

The passive heat acclimation protocol: a 45-minute hot bath, 5 times a week, for 5 weeks. Water temperature started at 40°C and was gradually raised to about 42°C. The key word is "passive": no exercise, just immersion up to the chest. Most participants took the bath right after their usual run, without changing the training program itself.

Results

Over five weeks without a single extra workout, VO₂max rose on average by +2,7 ml/kg/min — about 4,4%. For an athlete at this level that's a noticeable gain: speed at VO₂max increased by +0,8 km/h.

Where did it come from? Blood and heart:

Haemoglobin mass: +33 g (about +4%) — more oxygen-carrying molecules.
Blood volume: +284 ml — the body literally "topped up" its working fluid.
Left-ventricular end-diastolic volume: +10 ml — the heart chamber filled more; stroke volume (about +7 ml) and resting cardiac output (about +0,6 l/min) both rose.

The dynamics are interesting. In the first 1–2 weeks plasma volume expanded sharply (haematocrit even dipped slightly), and by weeks 4–5 plasma returned to normal — but now against a backdrop of increased red-cell production. This is exactly how altitude works: first "dilution," then a real increase in red cells. Together, haemoglobin mass and left-ventricular volume explained about 82% of the variation in VO₂max gains. Meanwhile the heart walls did not thicken — it was chamber capacity that grew. Running economy and lactate threshold, on the other hand, did not change: heat affects the "engine and oxygen transport," not technique.

How to apply it in practice

Heat is a supplement to training, not a replacement for it. A sensible scheme for an amateur:

Start gentler than the protocol. Don't jump straight into 42°C for 45 minutes. Begin at 38–39°C and 15–20 minutes, adding a couple of minutes and half a degree per session.
Put the heat after key workouts. Heat exposure acts as a "multiplier" of load you've already accumulated — so it makes sense to build it into high-volume periods.
Sauna as a backup option. Chest-deep immersion warms the body more effectively than a dry sauna, but if you don't have a bath a sauna will do too; go by how you feel, not by a record.
Drink. You lose a lot of fluid in 45 minutes of hot water. Weigh yourself before and after, and make up losses with water plus electrolytes.
Listen to your body. Dizziness, nausea, a pounding heartbeat are a signal to get out. Get up from the bath slowly: your vessels are dilated and you could faint.

Who should be more careful: people with cardiovascular disease, pregnant women, and anyone with a cold or fever — only after consulting a doctor.

Let's bust a myth: "I sweated it out, so I burned fat and flushed out toxins." No. The value of heat isn't in sweating as such, but in the mild heat stress that triggers blood formation and cardiac adaptation.

Limitations

Small sample — just 10 people, almost all men.
Short duration — 5 weeks; the long-term dynamics and how well the effect is "retained" are unknown.
Only trained runners — how a beginner or a veteran would respond remains to be seen.
A single modality — a hot bath; you can't directly transfer the figures to a sauna or contrast treatments.
The VO₂max gain is comparable to what altitude provides, but that doesn't mean heat fully replaces it for every purpose.

The bottom line

Passive heat exposure (45 min, 5 times a week, 5 weeks, 40–42°C) raised trained runners' VO₂max by ~4,4% with no extra training.
The mechanism is haematological and cardiac: +33 g of haemoglobin, +284 ml of blood, +10 ml to left-ventricular volume.
Heat mimics part of the altitude effect and is available to almost anyone: a bath or sauna at home.
Introduce it gradually, watch your hydration and how you feel; heat enhances training but doesn't replace it.

Source: Jenkins E.J. et al. Long-term passive heat acclimation enhances maximal oxygen consumption via haematological and cardiac adaptation in endurance runners. The Journal of Physiology, 2026. https://doi.org/10.1113/JP289874