This is a topic that I feel is largely ignored. A lot of training content out there comes with an implicit presumption of ideal conditions. We all like to assume we all have optimal training conditions, all the time, but the reality is us adventurers often find ourselves in the most extreme environments this world has to offer. Perhaps the most challenging of all is the hot and humid air of the tropic an subtropic. When you wake up drenched in sweat and step into a wall of heat so thick your breath sticks, you know you’re not in “normal” marathon training territory. Subtropical humidity flips the script—this is survival engineering, not just running.
VO2 Max Drops in the Heat, and Why It Matters
First, know this: In subtropical climates, functional VO2 max plunges. The science is clear—your heart and lungs may be up for the job, but your skin and blood are the limiting factor. As humidity climbs above 60–70%, sweating stops working because the air can’t absorb any more moisture. Instead, your body shunts blood to the skin, trying desperately to cool itself, and your maximum oxygen uptake (VO2 max) drops by 8% for every 5°C above optimal race temperatures. It’s not weakness—it’s purely a thermodynamic reality.
“Combined thermoregulatory and circulatory stress ultimately limits the capacity to perform an all-out exercise to exhaustion.“
When I first moved south, I lost at least 10–15% race pace in the heat despite months of “acclimation.” Over time, my splits normalized but never returned to my cool-weather best. You will have to accept that reality, or face disappointment.
Hydration—Strategy Beats Quantity; How to Calculate Your Sweat Rate
The old mantra “Just drink water” is not a wise move. You are fighting a battle of retention, not just consumption. Subtropic marathoners can dump as much as 2-3 liters per hour in peak effort, far more than your gut can absorb.
Calculating your sweat rate is a scientific and reliable way to optimize your hydration plan, especially in hot, humid climates like Miami or Honolulu. In my view, the gold standard is to measure body mass loss during a controlled run scenario, account for any fluids consumed, and adjust for the actual duration. Here’s how:
- Weigh yourself nude (or in dry running clothes) immediately before and after a run lasting at least 60 minutes. Record the total fluid consumed in milliliters (ml) during the session.
- Subtract your post-run weight from your pre-run weight (kg), multiply by 1,000 to convert kg to grams (which is equivalent to ml lost), and add the fluid consumed.
- Divide the total by the number of hours exercised to get sweat rate per hour.
Sweat Rate (ml/hr) = [ (Pre-run body weight (kg) – Post-run body weight (kg)) × 1,000 + Fluid intake during run (ml) ] / Duration (hours)
For example,. if you lose 0.7 kg in one hour and drink 500 ml during the run:
Sweat Rate = [ (0.7 × 1,000) + 500 ] / 1 = 1,200 ml/hour
This sweat rate helps set your real-world hydration targets for racing and training, ensuring you stay ahead of dehydration and perform better in heat and humidity.
Recovery—A New Approach, Not a Repeat
Hot, humid air isn’t just “more sweat”—it’s cellular stress. Recovery windows shrink, inflammation spikes, and muscle repair slows. Because of that, you will need to focus on the following:
- Cool-down: Get into shade quickly, use cold towels, cold drinks, or “ice vest” methods.
- Sleep: Humid climate = poor sleep for runners, meaning slower recovery; use A/C, fans, moisture-wicking sheets.
- Nutrition: Double up on anti-inflammatory foods (berries, leafy greens, omega-3s) to speed muscle repair.
- Rest days: Don’t double on high-heat days. 20°C and 90% humidity days merit two rest days after a long run.
If you are interested in deep diving into post heat recovery, I would recommend the Cardiovascular Challenge in Heat article in PubMeb and/or or the ExtraMilest excellent write up on training in hot and humid weather.
Chafing, Blisters, and Tactical Gear—Critical Adjustments
Heat and humidity aren’t just hard—they’re destructive.
- Chafing: Pre-load the skin with BodyGlide, Squirrel’s Nut Butter, or Vaseline in all high-friction zones. Reapply halfway through, especially around nipples, inner thighs, underarms, and toes.
- Blisters: Double up on moisture-wicking socks (Balega, Injinji). Consider paper-thin toe caps for problem toes.
- Gear: Wear light-colored, mesh ventilated kits. Use hats with UV coating, sweat-wicking wristbands, and sunglasses with anti-fog coatings.
Field Note: If you’re using a tactical vest or pack, add absorbent shoulder pads or liner to catch sweat and reduce salt buildup. For marathoners crossing over from ultra or military backgrounds, a high humidity kit means cutting every gram and trimming every edge off your usual gear.
Oxygen Availability—Running in a “Low-Altitude” Pressure Cooker
Why do heart rates spike more in humidity than in dry heat?
Less evaporation = harder cooling. The body pumps excess blood to the skin, leaving less for muscles, meaning higher HR and lower oxygen utilization.
Monitor heart rate and HRV—accept higher effort at lower speeds.
If training with a monitor, set new “heat zone” thresholds for Red/Yellow pacing. My Garmin Fenix 8 makes this easy with its heat acclimatization analytics.
For additional detail on this, I recommend the following study on ScienceDirect: Mechanism underlying the influence of humidity on thermal comfort and stress under mimicked working conditions
Acclimation and Training Tweaks
Give yourself 2-3 weeks to adapt—progressive overload works for heat as much as for hills.
Ramp up exposure: Start with short, easy runs outside and slowly increase effort and duration over 2–3 weeks.
- Pacing: Reduce starting pace by 10–25% and monitor symptoms.
- Split & block training: When temps peak, split your session (e.g., easy pre-dawn run, strength at lunch, short intervals at dusk).
- Cross-train: Mix in rowing, cycling, or swimming to reduce sweating burden some days.