Trail Notes | Female Athlete Physiology

it is not just the weather

Running Hotter

Heat, Hydration and Your Hormones.

If you have ever felt inexplicably hotter on a run that should have been manageable, or noticed your heart rate was elevated despite the same effort, your hormonal environment may sometimes play a part alongside the temperature outside.

Heat tolerance is not a fixed capacity. Some women notice it changes across the month, and again through perimenopause and menopause. Research on this is mixed, and individual responses vary a lot. Track your own patterns: we train the person, not the phase.

Understanding this does not mean running in heat becomes safe or easy. It means you stop attributing every hot day to fitness, willpower or poor preparation, and start working with it instead.

This Trail Note covers the physiology behind heat regulation, what some women notice across the month and in perimenopause, and practical strategies that help you manage heat and hydration whatever your cycle.

Trail Note  ·  01

How the body regulates heat during exercise

During exercise, the working muscles generate heat as a byproduct of metabolism. The body dissipates this heat primarily through sweating and cutaneous vasodilation (directing blood flow to the skin for heat exchange). The hypothalamus regulates this process, triggering sweating when core temperature rises above a threshold and increasing skin blood flow to facilitate heat loss.

The threshold at which sweating begins (the sweating onset threshold) and the sensitivity of the sweating response are not fixed. They shift with heat acclimatisation, fitness, hydration status, age, and possibly hormonal environment.

This is not a problem with the system. For some runners, the felt experience of heat may vary across the month, particularly in warm or humid conditions.

Trail Note  ·  02

Across the month: what some women notice

Resting core body temperature is typically a little higher (around 0.3 to 0.5 degrees Celsius) in the second half of the cycle, when progesterone is higher. Effects on sweating and exercise performance are less consistent between studies and between individuals. Some women notice higher perceived effort at the same pace, and many notice nothing at all.

On a cool day with low humidity, any effect is likely to be imperceptible in practice. On a warm day, in direct sunlight, on a technical trail with less airflow, or during a longer effort where heat accumulation compounds over time, it may matter more. Whatever your cycle, hot conditions are a real thermal challenge.

Trail Note  ·  03

Perimenopause: when the thermostat gets less predictable

In perimenopause, oestrogen levels become erratic rather than simply declining. Changes in oestrogen are thought to narrow the range of core temperatures the hypothalamus tolerates, which is linked to hot flushes and night sweats.

During exercise, this may compound the heat challenge. The thermoneutral zone (the range of core temperatures over which the body makes no active heating or cooling response) appears to narrow in perimenopausal and postmenopausal women, so exercise-induced rises in core temperature may trigger cooling responses earlier, which could lead to greater fluid losses for the same workload.

MHT (menopausal hormone therapy) is a medical decision with individual risk-benefit considerations, not a blanket recommendation. If you are curious about it, talk with your GP or a menopause-informed doctor.

Trail Note  ·  04

Hydration: what changes and why it matters more

Oestrogen and progesterone both play a role in fluid regulation, partly through aldosterone (the hormone that controls sodium and water retention at the kidneys). Some women notice their hydration feels less predictable at different points of the month or in perimenopause.

This does not necessarily mean you need dramatically more fluid. If your baseline shifts, responding to it earlier matters more. Coming into a warm-weather session well hydrated is important for everyone, and arriving even mildly dehydrated makes the heat harder to manage.

Electrolytes, particularly sodium, matter more when sweat losses are higher and fluid intake needs to be matched. Sodium helps retain absorbed fluid and supports plasma volume maintenance. In warm-weather training, including electrolytes rather than relying on plain water alone is a reasonable adjustment, especially for longer efforts.

Trail Note  ·  05

Practical strategies for running in heat

These are not about avoiding hard sessions or hot weather. They are about adjusting your approach so your effort is meaningful and your risk is managed.

Trail Note  ·  06

Practical strategies for perimenopause

The strategies above largely apply in perimenopause, with some additions. Because heat tolerance may be less predictable, planning for heat management becomes a consistent practice rather than something reserved for hot days.

Trail Note  ·  07

Heat acclimatisation still works

One of the important reassurances in this area is that heat acclimatisation, the physiological adaptations that come from regular moderate exercise in the heat, works in female athletes. Plasma volume expands, sweat onset occurs earlier, sweat rate increases and core temperature rise for a given effort is blunted. These adaptations take approximately 8 to 14 days of consistent heat exposure to develop meaningfully.

Building heat fitness helps whatever your cycle or life stage. This makes a strong case for building your heat training progressively rather than arriving at a summer race without specific preparation.

For perimenopausal athletes, acclimatisation may be especially worthwhile because heat tolerance can be less predictable. Building heat capacity before summer races, or before your first hot-weather training block, is one of the most practical investments you can make in warm conditions performance.

Trail Note  ·  08

On race day in the heat

If race day is hot, the most important adjustment is pacing. The natural inclination, particularly in the first third of a race, is to run with the people around you at a pace that feels possible. In heat, that pace is often too fast. The thermal load compounds over distance and the early debt is very hard to recover from.

Use shade deliberately. Slow on exposed sections even when the terrain is runnable. Pour water on your wrists, neck and head at every aid station. Eat salt if it is provided and you tolerate it. If you feel unusually dizzy, confused, stop sweating despite the heat, or feel nauseous beyond what effort alone explains, stop and seek medical support. Heat illness is a medical emergency.

The athletes who handle hot races well are not the athletes who override heat signals. They are the athletes who build heat fitness in training, arrive to the start hydrated, go out conservative, cool actively and keep making good decisions as the conditions evolve.

cool smart, run well, know your system

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