Relationships between plasma volume, fluid regulatory hormones, and maximal oxygen uptake following fixed-workload heat acclimation in men and women

What carbon monoxide rebreathing is

A precise way to measure blood plasma volume, by having someone breathe a tiny, safe dose of carbon monoxide and tracking how much it dilutes in the blood. It matters here because a real change in plasma volume could be detected rather than estimated.

Drawn from background physiology, not from this paper.

Why they ran it

The authors note that plasma volume expansion is thought to support several adaptations to heat acclimation, possibly including a rise in maximal oxygen uptake measured in cool conditions.

They also note that the plasma volume response to heat acclimation is highly variable between people, and that the mechanisms behind that variability, including any role of sex, are poorly understood.

Drawn from the paper's stated purpose.

Eleven men and fifteen women completed a ten day heat acclimation programme, two hours a day of fixed workload treadmill exercise at 40 degrees Celsius and 40 percent relative humidity. Plasma volume was measured by carbon monoxide rebreathing and maximal oxygen uptake was tested in temperate conditions, both before and after. Serum aldosterone, estradiol and progesterone, plus plasma copeptin, were measured on the first and last day.

The protocol worked as heat acclimation: core temperature, heart rate and sweating rate during exercise in the heat all fell. Plasma volume did not change at the group level, moving from 3280 to 3380 millilitres, and maximal oxygen uptake did not change either, sitting at about 3.1 litres per minute before and after. Sex did not modify any of these responses.

The plasma volume response was highly variable between individuals, and most people did not increase it at all. Change in aldosterone tracked change in plasma volume across the whole group, and change in progesterone tracked it in the women, which is where the authors point for an explanation of who responds.

The numbers

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Why this might happen

Proposed by the authors This is the explanation the authors offer in their discussion. This study did not test it.

The authors point to the large person to person variability in the plasma volume response, with most individuals not increasing it at all, as the thing that needs explaining rather than the group average.

They report that fluid regulatory hormones tracked the change, aldosterone across the whole group and progesterone in the women, and suggest hormonal variability contributes to who responds and who does not.

Drawn from the authors' stated explanation in the paper.

What this does not show

This does not show that heat acclimation never expands plasma volume. It shows that this fixed workload version did not, on average, in this group. A protocol that holds thermal strain constant as fitness improves is a different exposure and was not tested here.
No performance outcome was measured. Maximal oxygen uptake was the aerobic measure. There was no time trial, no time to exhaustion and no field performance, so nothing here speaks to whether racing changed.
The hormone findings are correlations. Aldosterone and progesterone tracked plasma volume change. Nothing was manipulated, so these associations do not establish that either hormone drives the response.
The sex comparison was underpowered. Eleven men and fifteen women is a small basis for concluding that sex does not modify the response, and the authors report no interaction rather than demonstrating equivalence.

Where this leaves us

It contradicts the expectation that heat acclimation dependably expands plasma volume and nudges up aerobic capacity, at least when the workload is held fixed, and shifts the question from whether it works on average to who responds at all.

Young healthy men and women completing a ten day fixed workload heat acclimation programme, tested for aerobic capacity in temperate conditions.

A trial comparing fixed workload against fixed heat strain protocols with the same carbon monoxide rebreathing measurement, which would separate the protocol from the principle.

Caveats worth holding

The workload was held fixed, which means thermal strain can fall as participants adapt, and the authors flag this as a candidate explanation.
No control group, and maximal oxygen uptake was a secondary endpoint.
Menstrual and hormonal phase in the female participants was analysed after the fact rather than controlled.
Sourced from the abstract and record; the full discussion was not reachable.