Heat Training for Endurance Athletes

Using Sweat Loss to Improve Oxygen-Carrying Capacity

Many endurance athletes use altitude training to improve performance because it can increase hemoglobin mass — one of the key determinants of aerobic capacity. However, altitude camps are expensive, logistically challenging, and not always accessible. Over the last few years, research has shown that heat training may provide a practical alternative. By adding structured heat stress to training, athletes can trigger physiological adaptations that may improve oxygen delivery and endurance performance.

This article explains:

• Why haemoglobin mass matters for endurance performance

• How heat training can increase it

• How to structure a heat training session

• How much sweat loss to aim for

• How to rehydrate afterwards

To help athletes apply this in practice, we’ve also built a sweat-rate calculator that allows you to monitor whether your heat sessions are providing the right stimulus.

Why Haemoglobin Mass Matters

Haemoglobin is the protein inside red blood cells that carries oxygen from the lungs to the working muscles.

The total amount of haemoglobin in the body (haemoglobin mass, or Hbmass) is one of the strongest physiological predictors of endurance performance because it determines how much oxygen the body can transport during exercise.

Higher haemoglobin mass generally allows athletes to sustain higher aerobic power outputs.

This is one reason altitude training is so popular. Living or training at altitude stimulates the production of red blood cells, increasing haemoglobin mass and improving oxygen transport.

However, researchers have recently discovered that heat exposure combined with endurance training may trigger similar adaptations.

(Schmidt & Prommer, 2010)

How Heat Training Can Increase Haemoglobin Mass

When you exercise in the heat, the body experiences significant thermal stress and fluid loss.

One of the earliest responses to repeated heat exposure is an expansion of plasma volume — the fluid component of blood. Within about 7–10 days, plasma volume can increase by roughly 5–10%.

This temporarily dilutes the blood slightly. The kidneys appear to detect this change and respond by stimulating red blood cell production in order to restore the balance between plasma and red blood cells.

Over several weeks, this process may increase:

• haemoglobin mass

• red blood cell volume

• total blood volume

Several studies have now shown ~2–4% increases in haemoglobin mass after around five weeks of structured heat training.

(Rønnestad et al., 2021; Rønnestad et al., 2022)

How Much Sweat Loss Do Heat Sessions Produce?

Interestingly, most successful heat training studies produce very similar levels of sweat loss.

In one study of elite cross-country skiers performing heat-suit sessions, athletes completed 50-minute rides wearing insulating clothing that limited heat loss.

During these sessions:

• athletes drank 500 ml of water during the session

• average fluid loss was ~1.6–1.7 litres

For athletes weighing around 70–75 kg, this equates to roughly:

~2–2.5% body mass loss per session

This level of dehydration appears to create enough cardiovascular and thermal stress to stimulate adaptation without being excessive.

(Rønnestad et al., 2022)

How to Structure a Heat Training Session

The heat protocols used in the Rønnestad studies are simple and easy to replicate. You don’t need a heat chamber — most athletes can create sufficient heat stress training indoors while wearing extra clothing layers.

A typical heat session looks like this:

Heat Training Session Example

Duration: 50 minutes

Intensity: Low intensity (Zone 1–2)

Environment: Indoor training

Clothing: Layers designed to limit heat loss (for example thermal layers, jacket, or rain shell)

Fluid intake during session: ~500 ml water

Goal: Generate enough heat stress to produce roughly ~2–2.5% fluid loss. These sessions are typically performed ~5 times per week for around five weeks alongside normal training.

Why Tracking Sweat Loss Matters

Without measuring sweat loss, it’s very easy to either:

• under-do the heat stimulus

• or dehydrate excessively

Tracking body mass before and after a session is a simple way to control the dose of heat stress.

The sweat calculator below allows you to enter:

• your pre-ride body weight

• your post-ride weight

• how much you drank during the session

From this, it calculates:

• total sweat loss

• percentage body mass loss

• whether you reached the recommended ~2–2.5% range

This helps ensure each session provides the intended heat stimulus.

Hydration After Heat Training

While dehydration during the session is part of the stimulus, it’s important to restore hydration afterwards.

In the Rønnestad heat training studies, athletes were encouraged to drink fluids gradually after the session until their urine returned to a normal pale yellow or straw colour. Urine colour provides a simple and practical way to monitor hydration.

After a heat session:

• Drink fluids gradually over the next few hours

• Include sodium in meals or drinks

• Monitor urine colour

The goal is simply to return hydration levels to normal before the next training session.

When Should Athletes Use Heat Training?

Heat training can be used at any point during the season. It can be especailly usful when you will be competing in a hot environment, as it can help you acclimate to race conditions in addition to enhancing your oxygen carrying capacity.

Most successful protocols last 4–5 weeks, with ~5 heat sessions per week. Shorter heat acclimation blocks can improve thermoregulation but may not be long enough to meaningfully increase haemoglobin mass.

Key Takeaways

Heat training may provide a practical alternative to altitude exposure for stimulating haematological adaptations. Research suggests that effective heat training typically involves:

• ~50-minute low-intensity sessions

• extra clothing to limit heat loss

• ~500 ml fluid intake during the session

• ~2–2.5% fluid loss

If you want to try this approach, use the sweat calculator below to monitor your sessions.

Like any training intervention, heat training works best when it’s integrated into a well-structured training program that balances load, recovery, and race preparation. If you’d like help implementing strategies like this into your training, take a look at our coaching packages or get in touch — we’d be happy to help guide your progression.

References

Schmidt, W., & Prommer, N. (2010). Impact of alterations in total hemoglobin mass on VO₂max.

Rønnestad, B. R. et al. (2021). Five weeks of heat training increases haemoglobin mass in elite cyclists.

Rønnestad, B. R. et al. (2022). Heat suit training increases hemoglobin mass in elite cross-country skiers.