Every sauna post in this series so far has made some version of the same argument: heat helps you recover. That’s true, and it’s worth revisiting once more — but it undersells what the research actually shows. Used consistently, sauna doesn’t just help you bounce back from training. It can make you measurably better at it, independent of a single extra workout.
A Different Mechanism Than What We’ve Covered
The heat shock protein pathway — HSP70, cellular repair, reduced inflammation — has come up repeatedly in this series, and for good reason. But it isn’t the mechanism behind this post’s claim. This one runs through something else entirely: plasma volume.
Plasma is the liquid component of blood, and its volume isn’t fixed. Repeated heat exposure triggers the body to retain more of it — a fluid shift driven partly by changes in circulating proteins like albumin, which pull and hold extra fluid in the bloodstream. More plasma means more total blood volume, which means more blood delivered per heartbeat, which means more oxygen and nutrients reaching working muscle without the heart having to work harder to do it.
This is worth distinguishing clearly from what we covered in last week’s altitude post. Altitude adaptation runs through EPO and new red blood cell production — an entirely different pathway, adding more oxygen-carrying cells to the blood. Sauna’s plasma volume effect adds more of the fluid those cells travel through. Different mechanisms, arriving at a related outcome: a cardiovascular system that delivers oxygen more efficiently per beat.
What the Evidence Shows
The reference study here is Scoon et al., published in the Journal of Science and Medicine in Sport in 2007. Competitive male runners added a sauna session after training, three to four times a week for three weeks. The results: a 7.1% increase in plasma volume, a 32% increase in time to exhaustion, and roughly a 1.9% improvement in 5K time — genuinely significant for anyone racing competitively, where seconds separate finishing positions.
That result held up on replication, which matters more than the original finding itself. Stanley et al. in 2015 found similar plasma volume gains using the same post-exercise sauna protocol. Kirby et al., in 2021, found comparable improvements in trained middle-distance runners. A study on elite cross-country skiers found the same pattern. A 2018 review pooling multiple heat acclimation studies — sauna, hot water immersion, and heat chamber protocols combined — found an average plasma volume increase of roughly 11% across the pooled data. This isn’t one small study standing alone. It’s a consistent finding showing up across different athlete populations and different research groups.
What This Doesn’t Claim
Worth being precise here, because it’s easy to overstate this one. This is not the same mechanism as altitude training, and sauna does not produce new red blood cells the way sustained altitude exposure does — that claim shows up in casual wellness writing, and it isn’t what the evidence actually supports. Plasma volume expansion and erythropoietin-driven red cell production are different physiological events, even though both eventually improve oxygen delivery.
It’s also worth noting that most of this research was conducted on trained endurance athletes — runners, cyclists, skiers — not the general population, and one study specifically noted the effect may be harder to detect in athletes who are already highly trained and already carrying elevated plasma volume from years of adaptation. Someone starting from a less-trained baseline may see this effect more readily than someone already at the top of their sport.
The Practical Piece
The protocol used across most of this research is fairly consistent: a sauna session immediately after training, roughly 25–30 minutes, three to four times a week, sustained for at least two to three weeks before the plasma volume effect becomes measurable. This isn’t a same-day trick. It’s a cumulative adaptation, closer in spirit to a training block than a recovery hack.
Why This Belongs in the Conversation
Colorado Springs already sits at an elevation that puts real cardiovascular demand on anyone training here, a topic we covered last week. Layering a second, independent adaptation pathway on top of that — one that doesn’t require weeks of altitude exposure to access, just a sauna and consistency — is a genuinely practical tool for anyone training seriously in this city. Not a replacement for the work. A second lever, pulling in the same direction.
Sources: Scoon GS, Hopkins WG, Mayhew S, Cotter JD. “Effect of post-exercise sauna bathing on the endurance performance of competitive male runners.” Journal of Science and Medicine in Sport, 2007;10(4):259-262. Stanley J, Halliday A, D’Auria S, Buchheit M, Leicht AS. “Effect of sauna-based heat acclimation on plasma volume and heart rate variability.” European Journal of Applied Physiology, 2015;115(4):785-794. Kirby NV, et al. “Intermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runners.” European Journal of Applied Physiology, 2021;121:621-635.
