Training in the heat: for young athletes, the risk isn't (only) about sex

It isn't about sex, it's about exposure: in U.S. high schools exertional heat illness is 87.7% male, but that drops to 50.9% once American football is excluded (Kerr et al., 2013). On physiology, direct evidence on sex differences during puberty is still scarce (Topham et al., 2022). What actually matters is acclimatization over the first two weeks (Casa et al., 2009).

In U.S. high schools, exertional heat illness is 87.7% male — but strip out American football and it drops to 50.9%, close to parity (Kerr et al., 2013). Boys aren't better at handling heat: boys play more football, in the hottest season of the year. Here's what the physiology actually says about adolescent girls training in summer, what it doesn't say yet, and what changes in the first week of preseason — the week when almost everything happens.

Exertional heat illness has a reputation as an American-football problem — pads, helmets, a 16-year-old collapsing during an August two-a-day. In U.S. high school sports surveillance data, that's partly true: 87.7% of events involve boys. But that number tells a different story than it seems to. Strip out football, and the gap between boys and girls nearly disappears. Sex doesn't explain who's at greater risk — which sport is played, with what equipment, and at what point in the season does. That distinction matters for any Italian sports club opening its gates in late August, with the thermometer still above thirty degrees.

In brief

  • Exertional heat illness is 87.7% male in U.S. high schools, but drops to 50.9% once American football is excluded: the gap is about exposure, not sex (Kerr et al., 2013).
  • At matched relative exercise intensity, a review finds no physiological differences between children/adolescents and adults in heat dissipation (Rowland, 2008).
  • Whether girls actually sweat less than boys during puberty isn't firmly established: direct comparisons remain scarce (Topham et al., 2022).
  • Heat acclimatization builds over 1-2 weeks at the start of the season, gradually increasing duration and intensity (Casa and Csillan, 2009).
  • One in three heat illness events (33.6%) happens with no medical professional present: an organizational gap before it's a medical one (Kerr et al., 2013).

Is heat illness an American-football problem, or does it affect girls too?

The raw numbers say "football." The adjusted numbers say "exposure." In U.S. high school sports surveillance (2005-2011), exertional heat illness occurred at a rate of 1.20 per 100,000 athlete-exposures, with football alone posting a rate 11.4 times that of all other sports combined — which is why 87.7% of overall events involve boys, in a country where high school football is nearly all-male (Kerr et al., 2013, Am J Prev Med). Strip football out of the count, and boys drop to 50.9% of events: essentially parity.

The lesson isn't about football itself — irrelevant to Italian soccer, volleyball, or track — but about the mechanism it reveals: risk follows exposure, not sex. A sport with intense sessions, heavy equipment, or little shade, concentrated in the hottest weeks of the year, produces more events regardless of who plays it. A summer preseason in track or girls' soccer in late August shares the same risk structure — heat, exertion, little acclimatization — even without helmets and pads.

Do adolescent girls tolerate heat worse than adults?

The old certainties don't hold up under direct comparison. For decades, exercise physiology attributed a set of heat disadvantages to children: lower exercise economy, a higher body-surface-area-to-mass ratio, reduced sweating capacity, lower cardiac output at a given workload. These are real traits, but a review that directly compared children and adults — at matched relative intensity of effort, not absolute workload — found no group differences in heat dissipation capacity (Rowland, 2008, J Appl Physiol).

The "matched relative intensity" detail isn't a technicality: a child running at the same absolute pace as an adult is working at a higher relative intensity, and that's exactly where older comparisons distorted the picture. Corrected for this, adults' "natural" heat advantage shrinks considerably. That doesn't mean an adolescent is as safe as a trained, acclimatized adult — it means chronological age alone doesn't explain the risk, and treating it as if it did is a mistake.

Is it true that girls sweat less than boys during puberty?

Here the honest answer is: we don't know with the certainty it's often presented with. It's a common belief that adult women, and children in general, have lower maximal sweating capacity than adult men, with potentially reduced evaporative heat loss. But isolating the specific effect of sex from that of pubertal maturation — and separating both from overlapping factors like body mass and metabolic heat production during exercise — is methodologically difficult, and studies directly comparing sudomotor activity between adolescent girls and boys remain scarce (Topham et al., 2022, Am J Physiol Regul Integr Comp Physiol).

The review that explicitly asked the question — are girls disadvantaged? — concludes the direct evidence still isn't sufficient to answer with confidence. It's the same kind of gap that runs through much of women's sports medicine: what's missing is the direct comparisons, not the hypotheses. Until that data arrives, the sound approach is to treat acclimatization and environmental conditions as the levers that actually matter — for everyone, regardless of sex — rather than building separate protocols on a difference that isn't yet demonstrated.

How heat acclimatization is built, and why it outweighs everything else

It's an adaptation you build, not a trait you either have or don't. Heat acclimatization is the physiological and behavioral process by which the body learns to exercise in hot conditions: plasma volume increases, the sweat response becomes more efficient and starts earlier, heart rate drops at a given workload. Guidelines for the start of the season in U.S. secondary schools recommend building it over one to two weeks, gradually introducing session duration, intensity, and equipment instead of starting at full load on day one (Casa and Csillan, 2009, J Athl Train).

It's a principle, not a rigid protocol built for a different context: the practical translation for an Italian club is that the first week of summer preseason — the week when enthusiasm pushes everyone to do everything at once — is also the week with the smallest margin for error. Building up load gradually in those days isn't excessive caution: it's the variable that, according to the available data, outweighs an athlete's sex or age.

How to recognize exertional heat stroke

The sign that separates an emergency from passing discomfort is neurological, not how much someone is sweating. Exertional heat stroke is clinically defined as a severe rise in core body temperature — typically but not always above 40°C — accompanied by signs of central nervous system dysfunction: ataxia, confused or bizarre behavior, delirium, seizures (Morris and Patel, Heat Stroke, StatPearls, 2023). It typically affects young, healthy people exercising intensely in hot, humid conditions — exactly the profile of a training adolescent, not a frail older adult.

If an athlete who has been training in heat is walking in an uncoordinated way, responding incoherently, or appears confused, that isn't "heat tiredness" to wait out on the sideline: it's a medical emergency requiring immediate cooling and qualified help. Sector guidance emphasizes this precisely because the line between discomfort that resolves with shade and water and an emergency requiring an ambulance is thin, and erring on the side of optimism is the most common mistake (Casa et al., 2015, J Athl Train).

What a club can do, starting Monday

None of these is a medical prescription. They're organizational choices, nearly all of them free:

BAB's role

BAB doesn't replace a heat emergency plan and doesn't prescribe acclimatization protocols. It does what, session after session, no one has time to track by hand: it gives the athlete a way to report privately how she's feeling in the heat — energy, dizziness, nausea, trouble concentrating — and gives the club aggregated signals, never the individual's health data, to see whether the first week of preseason is going the way it should.

The gap between 87.7% and 50.9% in Kerr's data didn't come from a hunch: it came from someone logging every event, sport by sport, across six seasons. Same principle, much smaller scale: the signals that matter only show up if someone collects them.

Sources

This article is for informational purposes only and does not constitute medical advice or a clinical assessment. If an athlete who has been training in heat shows confusion, uncoordinated movement, or isn't responding coherently, that's an emergency: she needs immediate cooling and qualified help — not sideline observation.

Are girls at higher risk of heat illness than boys when they train?

Not in the data, and it's a common misreading. In U.S. high school sports surveillance, exertional heat illness affected boys in 87.7% of cases — but almost all of that gap comes from American football, by far the highest-risk sport (a rate 11.4 times that of all other sports combined). Excluding football, boys account for 50.9% of events: essentially parity (Kerr et al., 2013). Sex doesn't explain the gap; which sport is played, and in which season, does.

Do adolescents tolerate heat worse than adults?

It's an older assumption than it sounds, and direct comparisons don't support it. For decades, exercise physiology attributed several heat disadvantages to children: lower exercise economy, a higher body-surface-to-mass ratio, and lower cardiac output at a given workload. A review that directly compared children and adults at matched relative exercise intensity found no group differences in heat dissipation capacity (Rowland, 2008). That doesn't make heat harmless in adolescence — it means age alone doesn't explain the risk.

Is it true that girls sweat less than boys during puberty?

Here honesty means saying we don't know for certain. Both adult women and children in general have been described as having lower maximal sweating capacity, but isolating the effect of sex from that of pubertal maturation — and from overlapping factors like body mass and metabolic heat production — is methodologically difficult. Studies that directly compare sudomotor activity between adolescent girls and boys during exercise remain scarce, and the available review concludes the evidence for a specific disadvantage in girls is still limited (Topham et al., 2022). It's not a tidy answer, but it's the one the data currently allow.

What is heat acclimatization, and why does it matter more than sex or age?

It's the physiological and behavioral process by which the body adapts to exercising in heat: plasma volume increases, sweating becomes more efficient and starts earlier, heart rate drops at a given workload. Guidelines for U.S. secondary schools recommend building it over one to two weeks at the start of the season, gradually increasing session duration, intensity, and equipment (Casa and Csillan, 2009). It's the first week of preseason — not an athlete's sex or age — that accounts for much of the risk.

How do you recognize exertional heat stroke?

The sign that separates an emergency from a passing discomfort is neurological. Exertional heat stroke is defined as a core body temperature typically above 40°C accompanied by signs of central nervous system dysfunction — ataxia, confused or bizarre behavior, delirium, seizures (Morris and Patel, StatPearls). If an athlete who has been training in heat appears confused, moves in an uncoordinated way, or isn't responding coherently, that's a medical emergency: she needs immediate cooling and qualified help, not sideline observation.

What can a sports club actually control about heat risk?

More than it might seem, and it's all zero-cost organizational choices. In U.S. high school surveillance, one in three heat illness events (33.6%) occurred with no medical professional on site, and the most commonly cited risk factors — obesity, starting training in hot and humid conditions without an adaptation period — are all manageable before an athlete ever steps onto the field (Kerr et al., 2013). Sector guidance calls for a written plan to recognize and respond to heat emergencies, not improvisation (Casa et al., 2015).

Is drinking more water enough to stay safe in the heat?

Hydration is part of the answer, not all of it. It's one of the things heat acclimatization helps the body manage better over the first weeks, alongside increased blood volume and a more efficient sweat response (Casa and Csillan, 2009). But the heaviest risk factors in surveillance data — no adult able to recognize the warning signs, full-intensity sessions on day one of the season, no written plan — aren't solved with a water bottle. They're solved by organizing the first week differently.

Why is so much of this data about American football specifically?

Because it's the sport where the risk is most visible and most studied: U.S. high school football has an exertional heat illness rate 11.4 times that of all other sports combined, is played in summer in heavy equipment, and is why it has the most surveillance and the most guidelines (Kerr et al., 2013). The mechanism that matters — little acclimatization, intense heat, high exertion in the first week — isn't specific to football: it applies to a summer preseason in track, soccer, or volleyball just as much, even where the sport and the athlete population differ from the ones studied.