sweatsciences
Heat training for endurance

Heat might be the accessible altitude.

A young line of research asks whether repeated heat exposure does what an altitude camp does — build more oxygen-carrying blood. The early numbers are genuinely interesting, and the part that's actually in your control is the boring one: the fluid and sodium you lose during a heat block are what decide whether it becomes adaptation or just strain. Here's the honest state of it, and how a block is run.

Get your sweat number for the block →

What the evidence actually shows.

Across the published trials, roughly five weeks of six sessions a week, about 50 minutes at a conversational effort near 90°F, raises hemoglobin mass by 2–5% — the same range altitude camps produce. One head-to-head in trained cyclists found no significant difference between the heat and altitude arms. Once built, about three sessions a week holds it; stop the heat entirely and it fades within roughly two weeks. That's a real, repeatable physiological signal.

The honest version — this is not settled. Nearly all of this work comes from a single collaborative research group working with trained and elite cyclists, and independent replication is still thin. Hemoglobin mass is a surrogate, and across the trials most failed to show a clear gain in VO₂ max, lactate threshold, or actual performance — only the largest one did. Read this as cautious optimism about a promising mechanism, not a proven way to get faster. It is not a reason to cook yourself.

Why hydration is the training here.

A heat block only works if your body can recover from each session — and recovery runs on fluid. Let yourself get dehydrated and the same heat flips from a signal that builds you to a stress that wears you down. That's not a footnote to heat training; on a five-week block of near-daily hot sessions, it's the whole difference between adapting and digging a hole. You'll also sweat a lot more than usual for weeks, so replacing it accurately matters more than in any other phase.

And your number moves during the block. As you acclimate over about two weeks, two things shift in opposite directions: your sweat rate goes up (you start sweating sooner and more) while your sweat gets less salty (you conserve sodium better). So the formula that fit in week one is wrong by week three. That's exactly why it's a number you re-run, not a label you memorize. Re-run yours →

How a block is run.

Dose~50 min, conversational effort, hot. A hot room, a trainer in a warm space, or post-session sauna — the trials used easy intensity, not hard efforts in the heat.
Frequency~5–6 sessions/week for ~5 weeks to build, then ~3/week to maintain into a goal race.
Fuel itReplace fluid and sodium to your measured losses after every session — this is the switch between adaptation and strain, and your number rises through the block.
TimingIt fades in ~2 weeks without heat, so don't finish the block long before your event.
Safety first. Heat stress is real stress. Ease in, never train hard in the heat while acclimating, stop if you feel dizzy or unwell, and if you have cardiovascular conditions or are pregnant, talk to a clinician before a heat block. This is education, not a training prescription.

Sources: Lundby & Robach, J Physiol 2025 (review) · Cubel et al., Exp Physiol 2026 (heat vs. altitude head-to-head; only 7 cyclists did both) · Lundby et al., J Appl Physiol 2023 (the one adequately powered trial where performance moved).

Keep reading

The class version

Hot yoga & the heat

The same hormetic angle for a hot-studio practice, with the mitochondria layer.
Measure it

Your sweat rate

Weigh in weekly through the block — it rises as you acclimate.
The physiology

Why salt, not just water

What that sodium is doing while heat does its work.