Heat training raises haemoglobin mass reliably and performance only slightly
So it is a plausible substitute for an altitude camp, chosen per athlete by what they tolerate, and it belongs late in the priority list rather than early
Key Findings
Altitude: the dose does not determine the response. For a given magnitude of hypoxic exposure, the physiological response between individuals ranges "from near nothing to previously unseen augmentations" in haemoglobin mass. The average effect of an altitude camp is a poor description of what any one athlete will get.
Whether an individual's altitude response REPEATS is unknown. The authors state it remains inconclusive whether the response seen after one camp can be reproduced at the individual level at a subsequent camp. This matters more than it first sounds: it means a good camp is not yet evidence that the next one will work, and it removes the usual justification for repeating what worked.
Heat: the haematology is the reliable part. After a 3-to-5-week period of heat training — training in a warm environment, or in heat-capturing garments in a cool one — a robust increase in haemoglobin mass has been reported repeatedly.
The performance gain is the unreliable part. In the authors' words the associated performance gains are "only small". This is the sentence that separates the review from how it is usually summarised.
Verdict: heat training "might serve as a substitute for altitude training". The recommendation is to let athletes experiment with either, and to decide at the individual level, ensuring the applied stress — hypoxia or heat — is adequately tolerated.
Practical Application
Preparing a sea-level race in temperate conditions and choosing one environmental block: either is defensible, and heat is the cheaper experiment. No travel, no altitude accommodation, no hypoxic equipment; it can be done on an indoor trainer with extra clothing. The review does not declare a winner, and neither should the coach.
Put it late in the queue, not early. The authors are explicit that heat training should probably not be implemented before the rest of an athlete's preparation is in place, and not at the expense of other parts of their life. For an athlete whose sleep, fuelling or consistency is the actual limiter, a heat block is the wrong intervention regardless of what it does to haemoglobin mass.
Promise the blood, not the stopwatch. Say that haemoglobin mass reliably responds and that the performance effect is small. An athlete told to expect a step change will be disappointed by a result the literature would call a success.
Decide by tolerance. An athlete who sleeps badly at altitude and one who cannot complete sessions in heat are two different problems with the same answer: use the other stressor. The review makes tolerance the deciding variable, not a physiological prediction.
Do not repeat an altitude camp purely because the last one worked. Individual reproducibility across camps is unestablished (REF-498 reports the same instability for ordinary training blocks). Measure again rather than assume.
Limitations
Narrative review, not a meta-analysis. No pooled effect sizes, no formal risk-of-bias assessment. REF-199 — the systematic review with meta-analysis on the neighbouring question of COMBINING heat and altitude — is methodologically stronger evidence and should outrank this entry where the two overlap.
A widely circulated secondary summary of this review reports vote counts — 31 altitude studies against 9 heat studies, with 67% vs 83% showing a significant haemoglobin-mass rise and 47% vs 67% a significant performance improvement — and concludes from them that heat is more likely to work. Those figures are not in the abstract and the full text is paywalled, so they are recorded here as unverified. Two cautions regardless of whether they are accurate: counting how many studies crossed p < 0.05 ignores effect size, sample size and study quality; and with only 9 heat studies, one study moves the percentage by 11 points.The authors' own stated conclusion — small performance gains, individual choice — is the claim this entry carries.
Elite cohorts. The review is about elite endurance sport, and both stressors are studied in athletes with high training loads and professional support. Nothing here establishes the response in a Tier 1-2 athlete.
No head-to-head trial. The comparison is across separate literatures, not a randomised allocation of the same athletes to heat or altitude. That trial does not exist, which is precisely why the recommendation is to experiment individually.
"Robust" describes haemoglobin mass, not the mechanism to performance. A rise in haemoglobin mass is assumed to transfer to endurance performance; this review's own finding is that the transfer is weaker than the haematology.