Absolute power, not sex, promotes perspiration (2011)

Ichinose-Kuwahara et al. (2010) claim to have demonstrated ‘a sex difference in the effects of physical training on the sweating response’, a finding that has been widely reported by the media. Our analysis of the data presented in the paper, however, suggests a simpler explanation involving neither sex nor training status – the harder you work, the more you sweat. The conclusions of Ichinose-Kuwahara et al. (2010) were drawn from plots of sweating responses in men and women at the same relative exercise intensity (percentage of maximal oxygen uptake; ). Since sweating is known to depend upon absolute exercise intensity (Saltin & Hermansen, 1966; Davies, 1979), we decided to replot the data from Ichinose-Kuwahara et al. (2010) with absolute rather than relative work rate on the x-axis (Fig. 1). Replotting of Ichinose-Kuwahara et al. (2010) Fig. 2 sweating rate data against absolute work rate (taken from their Table 2) The means (±SEM) have been fitted with a power function of work rate: Sweating rate (in milligrams per centimetre squared per minute) = 0.0004 W1.68. Figure 1 reveals that the sweating rate can, apparently, be explained in its entirety by a single, simple function of absolute work rate without regard to either sex or training status. Since estimated body surface area is ∼10% less in women, we also plotted the estimated total sweat loss (in litres per hour) against absolute power, but this did not expose any latent differences (data not shown). We have also replotted the number of activated sweat glands per unit area of skin against absolute work rate (Fig. 2), and again there does not appear to be any obvious sex or training effect. Our graph suggests that at >100 W all of the available sweat glands are active (male, female, trained or untrained). This implies that any further increases in sweating rate in any group must come from a greater production from each gland. Replotting of Ichinose-Kuwahara et al. (2010) Fig. 2 data regarding the mean number of activated sweat glands against absolute work rate, fitted with an exponential function of work rate (rate constant ∼0.03 W−1 and maximal activated sweat glands ∼120 glands cm−2) The observation that sweat production is related to absolute work rate is not new. Davies (1979) measured sweating in men and women, athletes and non-athletes during exercise. He concluded that there were no apparent sex differences in thermoregulatory function, and that maximal sweating rates were linearly related to . It can be useful to consider relative rather than absolute work intensities, especially to normalize individuals within an experimental group. However, when applied between groups it can lead to erroneous conclusions. Finally, Ichinose-Kuwahara et al. (2010) also plotted body temperature against sweating in their Fig. 3, which they claim reveals sex-dependent effects of training. However, the data from all four groups (our Fig. 3) can be fitted by a single function (see Appendix, eqn (4)) using the maximal oxygen extraction reported for each group. Thus, the differences between the groups are entirely accounted for by (in millilitres per minute) alone, without requiring any reference to sex or training status. Replotting of Ichinose-Kuwahara et al. (2010) Fig. 3 data for sweat output per gland against temperature The four curves are plots of the same function [eqn (4)] but using the (in millilitres per minute) for each group (from data in Table 1 of Ichinose-Kuwahara et al. 2010). In summary, there may well be sex differences in sweating responses, but the data set of Ichinose-Kuwahara et al. (2010) does not show them.