Train Hot, Race Fast: How Heat Acclimation Boosts Performance

The short version

Spend 3–5 weeks doing some of your training in the heat — or soaking in a hot bath after easy runs — and your body upgrades itself in ways that make you faster, even on a cool race morning.

What the research found

Two recent studies, a 2025 paper in the Journal of Physiology and findings presented at the 2026 American Physiological Society Summit, looked at what happens when runners consistently expose themselves to heat over several weeks. The protocol didn't require anything exotic: either running easy in hot conditions for 60–90 minutes, or sitting in a sauna or hot-water bath for 20–30 minutes right after a normal workout, repeated three to five times a week.

After two to three weeks of this, the adaptations were meaningful:

• Core temperature at a given effort dropped — your body gets better at offloading heat before it builds up.
• Heart rate at the same pace fell — less cardiovascular strain for the same workload.
• Sweat rate increased — earlier, more aggressive cooling.
• Hemoglobin mass and VO2max went up — the heat stress triggers a plasma and red-cell volume expansion that's essentially the same mechanism behind altitude training.

The last two points are the ones that matter most for cool-weather racing. Your cardiovascular system doesn't know you're back in 60°F weather — it just knows it has more oxygen-carrying capacity than it did six weeks ago.

How to actually do this

You don't need to move to Phoenix. The post-run sauna or hot bath approach is practical anywhere:

• Finish your easy run, then spend 20–30 minutes in water or air at 40°C / 104°F or above.
• Do it 3–5 days a week for 2–3 weeks during your build or peak phase.
• Drink freely the whole time. The studies confirmed that drinking to thirst doesn't blunt the adaptation.

Start this block at least three weeks before your target race so the blood volume gains have time to consolidate.

The runner-facing note

Heat acclimation is one of the few legal, low-cost interventions with a measurable VO2max effect. If you have a race on the calendar and three open weeks, this is worth adding to the plan.