Why Sitting in a Hot Tub Might Boost VO₂max Like an Altitude Camp
Estimated read time: 4.2 minutes (about as long as it takes to explain to your roommate why you need a hot tub "for training purposes" 🤔)
Hey Performance Nerds! Jonah here. 🤓
New research shows hot tubs might actually be better than altitude for elite runners.
Jenkins and colleagues showed a 5-week hot water protocol produced VO₂max and hemoglobin gains similar to altitude camps, without changing training.
4.4 percent higher VO₂max. 3.9 percent more hemoglobin mass. Achieved by sitting in hot water after runs.
Here's what you'll learn today:
Why "better than altitude" needs a reality check (and what it actually means)
The exact protocol these well-trained runners used
How heat tricks your kidneys into making more red blood cells
Who should try this, and when it fits in your season
(Augie, my dog, thinks any excuse for my parents to buy me a hot tub sounds great. He’s not wrong.)
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🧪 Why This Study Is Legit
The athletes were well-trained. Average VO₂max of 64.5 mL/kg/min. These aren't beginners seeing newbie gains.
The protocol was passive. No heat chambers or sauna workouts. Just hot water immersion after normal training.
Training stayed matched. No differences in volume, intensity, or well-being scores between conditions. The gains came from the heat, not extra work.
They measured what matters. Hemoglobin mass via carbon monoxide rebreathing, cardiac structure via 4D echocardiography, and treadmill VO₂max testing.
🔥 The Hot Tub Protocol
Parameter | Details |
|---|---|
Duration | 5 weeks |
Frequency | 5 sessions per week |
Session length | 45 minutes |
Water temperature | ≥40°C (104°F), increased over time |
Immersion depth | Neck-deep (could adjust to chest if needed) |
Hydration | Up to 500 mL water allowed during session |
Key detail: About 75 percent of sessions happened immediately after a training run.
Post-run, your core temperature is already elevated. Adding heat on top likely increases thermal stress without needing hotter water or longer sessions.
Practical implication: Post-run timing may preserve workout quality while maximizing the heat dose.
📊 The Results That Matter
VO₂max increased by 2.7 mL/kg/min (4.4 percent).
For a 3-hour marathoner, that kind of aerobic bump could support meaningful pace improvements. Not guaranteed, but the engine got bigger.
Hemoglobin mass increased by 33 grams (3.9 percent).
Comparable to classic "Live High, Train Low" altitude interventions. More hemoglobin means more oxygen-carrying capacity.
Left ventricular end-diastolic volume increased by about 10 mL.
Your heart holds more blood before each beat. More blood in, more blood out. This is an oxygen delivery story.
Treadmill speed at VO₂max improved by 0.8 km/h.
They ran faster at their max.
🧬 Why Hot Water Builds Blood
Heat doesn't work the same way altitude does.
Week 1-2: Plasma volume expands.
Heat stress causes your body to hold onto more fluid. Blood volume goes up, but red blood cell concentration drops. Your blood gets "diluted."
The kidney signal.
Your kidneys detect lower oxygen-carrying capacity. They respond by ramping up erythropoietin (EPO), the hormone that stimulates red blood cell production.
Week 4-5: Red blood cells and hemoglobin mass rise.
The delayed increase in red blood cell volume restores hematocrit while total blood volume stays elevated. You end up with more total oxygen-carrying capacity.
The cardiac angle.
Expanded blood volume stretches your heart's chambers during filling (preload), increasing stroke volume via the Frank-Starling mechanism. More blood per beat.
Important caveat: EPO was not directly measured. The kidney signaling explanation fits the time course but is not fully confirmed.
⚖️ Heat vs. Altitude
Altitude's strengths:
Hypoxia directly stimulates erythropoiesis (red blood cell production). Decades of research and a proven track record with elite athletes.
Altitude's limitations:
Not everyone responds. Some athletes see minimal gains or even maladaptation. And altitude training often forces reduced intensities, which can compromise key sessions.
Early altitude exposure also tends to shrink plasma volume. That limits the cardiac filling benefits you'd get from more blood volume.
Heat's strengths:
Training quality preserved. In this study, volume, intensity, and well-being scores stayed matched between conditions. The adaptation came on top of normal training, not at its expense.
Heat may also provide a dual stimulus that altitude can't match: a blood-building effect (increased hemoglobin mass) plus a cardiovascular effect (expanded blood volume, larger left ventricular filling).
You're building oxygen-carrying capacity and a stronger pump.
The verdict: Different tools, sometimes complementary.
Heat doesn't replace altitude. But for athletes who can't access camps, respond poorly to hypoxia, or want to maintain gains between blocks, heat offers a legitimate alternative with fewer tradeoffs.
🎯 Who This Is Actually For
Category | Details |
|---|---|
Likely high value | Hot-weather race prep, no altitude access, maintaining Hbmass after altitude, elite athletes chasing marginal gains. |
Moderate value | Well-trained athletes seeking an additional stimulus |
Lower value | Beginners (regular training adaptations will dominate), athletes already at altitude |
Red flags | Heat intolerance history, cardiovascular conditions, current illness, within 2 weeks of a key race |
📆 Where This Fits in a Season
Good timing:
Heat race prep: 4-6 weeks before a hot marathon or HYROX event
Pre-heat acclimation primer: Before a dedicated heat training block
Alternative to altitude: When logistics make a traditional camp impractical
Altitude maintenance: After returning from camp, to preserve Hbmass gains
Not ideal timing:
Final 2 weeks before race: You need freshness, not a new stressor
During illness or injury: Heat is a physiological load
Peak training blocks: If you're already at max sustainable load, adding 25 hot baths might tip you over
♨️ Would a Sauna Work?
Probably similar, since heat stress is the key ingredient, especially when layered on top of an elevated core temperature.
But there's uncertainty. Hot water immersion includes hydrostatic pressure (water pushing on your body), which affects circulation differently than dry heat. Whether that matters for Hbmass and cardiac outcomes, we don't know yet.
Practical stance: Sauna is a reasonable substitute if that's what you have access to. But we can't assume identical outcomes until someone studies it directly.
🎯 Practical Takeaways
Passive heat acclimation works. 5 weeks of post-run hot water immersion increased VO₂max by 4.4 percent and hemoglobin mass by 3.9 percent in well-trained runners.
It's an oxygen delivery story. Both blood-building and cardiac adaptations contributed to the gains.
Training quality was preserved. No reduction in volume, intensity, or well-being.
"Better than altitude" means: comparable gains, easier logistics, potential dual stimulus. Not a guarantee of faster race times.
Timing matters. Best during base building, heat prep, or as an altitude alternative.
📚 Want to go deeper?
This piece was inspired by excellent work from Elliot Jenkins and colleagues. Huge credit to Elliot and the full author team for a really well-designed and carefully executed study.
If you want all the methods, figures, and nuance straight from the source, you can read the full paper here:
💬 One last thing before you go.
After the NFL, I wasn't sure if obsessing over performance science still mattered, until I started helping this community.
Your training goals? They brought that fire back.
So if you're stuck, second-guessing something, or just want to share how training's going, hit reply. I read every message and I'm here to help however I can.
Jonah & Augie
Are You a True Running Nerd? Prove it.. 🧐
Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.
Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.
What actually sets the upper limit for how many carbs you can absorb and tolerate per hour during a long run?
- A. ~30 g/hr, because that’s all the gut can handle during running 🥨🚫
- B. ~60 g/hr, once glucose transporters are saturated 📈🔒
- C. ~90 g/hr, using multiple transportable carbs (glucose + fructose) 🍯🔬
- D. Individual-specific, sometimes >90 g/hr in well-trained athletes with gut training, high oxidation demands, and glucose + fructose mixes 🧠🚀
Last Week’s Results: Protein Does More Than Build Muscle 🧬
A lot of runners know protein matters, but this question exposed where most people still underestimate why.
The correct answer?
C. Providing the amino acids needed to support mitochondrial protein synthesis and aerobic enzyme production 🧬⚙️ ✅
Endurance training doesn’t just stress muscle fibers, it drives massive turnover of mitochondrial proteins and aerobic enzymes that power fat and carb oxidation.
Without enough amino acids coming in, those adaptations might slow down. Over time, that means fewer watts, slower marathon pace, and a lower aerobic ceiling.
Here’s how the votes shook out:
⬜️ A. Improving satiety so it’s easier to hit high daily calorie needs 🍽️🙂 – 7
🟨 B. Supporting immune function when training stress is high 🛡️🤧 – 33
🟩 C. Providing the amino acids needed to support mitochondrial protein synthesis and aerobic enzyme production 🧬⚙️ – 155 ✅
⬜️ D. Stabilizing blood sugar between long or double-session days 🔄🍚 – 23
Bottom line?
If you’re stacking mileage and workouts, protein isn’t just for repair, it’s raw material for building the aerobic engine itself. Underfuel it, and no amount of Zone 2 or threshold work will fully stick.
🏃♂️ Stryd Training Tip — Match Your Race to Your Power-Duration Curve
Ever feel like a race is harder than expected, even when training has gone well?
The issue often isn’t fitness or pacing. It’s that your physiology doesn’t match the race demands.
Your Power-Duration Curve (PDC) shows your highest sustained power across durations, based on recent best efforts. It reveals where your performance capacity is strongest—and where it drops off—as duration increases.
Two runners can test similarly (Threshold) and still race very differently. The difference is where their power is expressed along the curve.
The most actionable insight comes from Actual vs Modeled.
Toggle Modeled Ability on in Stryd. If your Actual curve consistently sits below the Modeled curve in a specific duration band—and that range has been tested recently—that gap is a clear training signal.
💡 Bottom line: Your curve doesn’t define your limits. It shows where targeted training will matter most.