Heat training has become increasingly popular in endurance sport.

Athletes are riding indoors without fans, training in hot rooms and, increasingly, wearing heat suits designed to trap body heat during exercise. One of the more provocative descriptions being used is that heat training is a “poor man’s altitude.”

There is some logic behind the comparison.

Both heat and altitude can expand blood volume. Both can produce cardiovascular adaptations. And research now suggests that several weeks of repeated heat training can even increase total haemoglobin mass.

But that does not mean heat training and altitude training are doing the same thing.

They begin with completely different physiological stresses, activate different primary pathways and — importantly — an increase in haemoglobin mass produced during heat training has not consistently translated into the same proportional improvement in aerobic performance.

In other words:

Increasing haemoglobin mass is not necessarily the same thing as creating an altitude adaptation.

Altitude starts with oxygen

The primary stimulus at altitude is simple: reduced oxygen availability.

As altitude increases, atmospheric pressure falls. Although oxygen still makes up approximately 21% of the air, the partial pressure of oxygen decreases.

That reduces the amount of oxygen available to move from the lungs into the blood.

The body detects this reduction in oxygen availability and activates a sophisticated hypoxia-response system involving hypoxia-inducible factors, or HIFs.

One of the key downstream responses occurs in the kidneys, which increase production of erythropoietin — EPO.

EPO stimulates the bone marrow to produce more red blood cells.

The pathway is essentially:

Reduced oxygen availability
→ hypoxic signalling
→ increased EPO
→ increased red blood cell production
→ increased haemoglobin mass
→ increased oxygen-carrying capacity

This is one of the fundamental reasons endurance athletes have used altitude for decades.

A major meta-analysis of altitude studies estimated that haemoglobin mass increased by approximately 1.1% for every 100 hours of adequate altitude exposure, although individual responses can vary considerably.

Altitude therefore directly challenges the system endurance athletes ultimately rely on:

the transport of oxygen from the environment to working muscle.

Heat starts somewhere completely different

A heat suit doesn't primarily challenge oxygen delivery.

It challenges the body's ability to control its temperature.

By trapping heat and reducing evaporative cooling, a heat suit causes skin temperature and ultimately core temperature to rise.

The body responds by increasing skin blood flow and sweating.

With repeated exposure, the athlete becomes better adapted to hot conditions. Among the most important adaptations is an expansion of plasma volume.

The basic pathway is therefore more like:

Heat accumulation
→ increased core temperature
→ increased skin blood flow and sweating
→ plasma-volume expansion
→ improved thermoregulation and cardiovascular stability in the heat

That is an extremely useful adaptation if an athlete needs to perform in hot conditions.

But it isn't the same physiological signal as hypoxia.

So why can heat training increase haemoglobin mass?

This is where the comparison becomes interesting.

A growing body of research suggests prolonged heat training can eventually increase haemoglobin mass.

In one study, elite cross-country skiers added five 50-minute heat-suit sessions per week for five weeks.

The heat-training group finished the intervention with approximately 30 grams greater haemoglobin mass than the control group, along with greater red-blood-cell volume.

Similarly, researchers studying elite cyclists found that five weeks of heat training increased average haemoglobin mass from approximately 893 g to 935 g — an increase of around 42 grams.

So the evidence is increasingly clear:

Heat training can increase haemoglobin mass.

But that raises a much more important question.

Does that additional haemoglobin translate into better aerobic performance?

This is where heat and altitude begin to diverge.

The haemoglobin–performance “uncoupling”

The cross-country ski study produced a particularly interesting result.

Despite the significant increase in haemoglobin mass and red-blood-cell volume, researchers detected no corresponding advantage in VO₂max, running economy, lactate-threshold measures or a 15-minute performance test compared with the control group.

The elite-cyclist study showed something similar.

Haemoglobin mass increased substantially more in the heat group, but this greater increase did not produce a significantly greater increase in VO₂max.

There were some potentially beneficial changes in other exercise variables, so this doesn't mean heat training was ineffective. But the extra haemoglobin mass did not translate neatly into a proportional improvement in maximal oxygen uptake.

That distinction matters.

The haemoglobin produced following heat training isn't somehow inferior haemoglobin. A red blood cell does not know whether the stimulus that contributed to its production came from heat or altitude.

Rather, the evidence suggests that haemoglobin mass is only one component of the entire oxygen-delivery and utilisation system.

Simply increasing one component does not guarantee a proportional improvement in the performance of the whole system.

Altitude appears more tightly coupled to the oxygen-transport system

Altitude is different because reduced oxygen availability is the original stimulus.

The entire organism is forced to respond to an environment in which oxygen delivery is compromised.

That includes changes involving ventilation, blood volume, erythropoiesis, cardiovascular function and potentially peripheral oxygen utilisation.

This may help explain why increases in haemoglobin mass following altitude exposure have shown a clearer relationship with changes in VO₂max.

An analysis involving 145 elite endurance athletes examined the relationship between changes in haemoglobin mass and VO₂max following altitude exposure.

The researchers found a significant relationship between the two. In the altitude group, the data indicated that a 1% increase in haemoglobin mass was associated with approximately a 0.6–0.7% increase in VO₂max.

The researchers concluded that altitude training in endurance athletes could be expected, on average, to increase VO₂max by more than half the magnitude of the increase in haemoglobin mass.

That does not mean every athlete who increases haemoglobin mass at altitude will automatically become faster.

Performance is far more complicated than a single blood measurement.

But it suggests something important:

With altitude, the haematological adaptation is occurring as part of a broader challenge to the oxygen-transport system.

With heat, increased haemoglobin mass appears to be a secondary consequence of a stimulus whose primary purpose is thermoregulation.

Heat training is valuable — but it isn't altitude

None of this means athletes shouldn't use heat.

Heat acclimation can be extremely valuable, particularly for athletes preparing to race in hot environments.

Heat training may improve:

• plasma volume

• thermoregulation

• sweating response

• cardiovascular stability in hot conditions

• tolerance to thermal stress

And research increasingly suggests it can increase haemoglobin mass as well.

The mistake is assuming that because one outcome overlaps, the interventions are interchangeable.

They aren't.

A heat suit primarily asks:

“How well can your body manage heat?”

Altitude asks:

“How well can your body function when oxygen availability is reduced?”

Those are different physiological questions.

There is another major difference: the dose

There is also a practical consideration that is sometimes missed in the heat-versus-altitude debate.

Producing a meaningful adaptation requires sufficient exposure.

In the studies showing increased haemoglobin mass from heat, athletes typically accumulated repeated heat sessions for several weeks.

That means deliberately adding thermal stress to an athlete's training program.

Altitude exposure can be accumulated very differently.

With a live-high, train-low model — including simulated altitude — an athlete can perform their important training sessions close to sea level while accumulating hypoxic exposure during recovery and sleep.

An athlete sleeping for eight to ten hours per night in an appropriately controlled simulated-altitude environment can accumulate a substantial hypoxic dose without turning every training session into an environmental stress session.

That distinction is particularly important for elite athletes, where maintaining training quality and managing recovery are already major priorities.

Perhaps the answer isn't heat or altitude

There is also an emerging third possibility.

Heat and altitude may actually be complementary.

Recent research has begun examining whether heat exposure can help augment or maintain some haematological adaptations associated with altitude.

That makes physiological sense because the two interventions begin with different stresses.

Rather than viewing heat as a cheaper substitute for altitude, athletes may ultimately use them strategically for different purposes:

Altitude for oxygen-system adaptation.

Heat for thermal adaptation.

And, in selected circumstances, both together.

Research published in 2026, for example, examined athletes training at moderate natural altitude and found that adding heat-suit sessions augmented the haemoglobin-mass response compared with moderate altitude alone.

That is a far more interesting direction than treating one intervention as a replacement for the other.

Is heat really the “poor man's altitude”?

Probably not.

It is a catchy phrase, but physiologically it oversimplifies what is happening.

Heat training can be effective. It can expand plasma volume and, with enough repeated exposure, research shows that it can increase haemoglobin mass.

But the primary signal remains thermal stress.

Altitude is different.

The primary signal is oxygen deprivation itself.

And while heat-induced increases in haemoglobin mass have not consistently translated into proportional improvements in VO₂max or performance, altitude-induced changes in haemoglobin mass show a clearer relationship with improvements in maximal oxygen uptake.

So perhaps the better way to think about the two is:

Heat trains the body to deal with heat.

Altitude trains the body to deal with reduced oxygen availability.

There is overlap between the adaptations.

But overlap does not mean equivalence.

For an athlete specifically trying to improve the body's oxygen-transport system, altitude remains the more direct physiological stimulus.

And with simulated altitude allowing athletes to accumulate that stimulus while they sleep — while preserving their normal sea-level training environment — the distinction becomes even more important.

Heat training is not poor man's altitude.

It's a different tool for a different physiological job.