A review of two studies from 2012 looking at the influence of sex and fitness on sweating. One study found that at the the same percent of VO2MAX aerobically fit sweat up to twice as much, while at same power output unfit sweat more inefficiently form their forehead. The other study found evidence suggesting women have a lower maximal sweat output.
Two studies came out this past year looking at the physical characteristics that influence sweating. One of the studies looked at the influence of aerobic fitness. The other study looked at the influence of gender. The amount one sweats is based on several physical characteristics. People with a greater body mass will sweat more simply because they have more metabolic processes going on, thus producing more heat. Body surface area plays a role because a smaller surface area means that more heat must be lost per unit area to generate the same amount of total heat loss. Sudomotor activity is the nervous system's activation of the sweat glands.
The study that compared sweating of aerobically fit and unfit individuals yielded some interesting results [Cramer, M. N., Bain, A. R. and Jay, O. (2012), Local sweating on the forehead, but not forearm, is influenced by aerobic fitness independently of heat balance requirements during exercise. Experimental Physiology, 97: 572–582].
The study found that at the same power output or evaporation requirement (Ereq) both fit and unfit individuals had approximately the same whole body sweat output. Interestingly, the researchers found that when they looked at specific regions of the body there was significant discrepancy. The unfit individuals had significantly greater sweat levels on the forehead, but no significant difference between fit and unfit individuals on the forearm. The study authors concluded that the sweat efficiency of the aerobically fit individuals was greater. This is because excess sweating on the forehead leads to dripping. Evaporative cooling is what generates heat loss, dripping sweat is just wasted fluids. When the study participants exercised at 60% of their VO2MAX, the fit individuals sweated significantly more at all areas of the body. This was expected because more cooling was needed to compensate for the fitter individuals' greater power output at 60% of their VO2MAX. These results can be seen in the figure below.
The study participants exercised for an hour on recumbent cycling machines. These exercise machines were chosen so that mechanical efficiency would not be a factor. Because the power output was the same for fit and unfit individuals, the evaporation requirement (Ereq) should be the same in the BAL trial. In this first study, only men were chosen as participants because of gender differences in sweating. Gender differences were discussed in a second, more recent study.
This second study found that when controlling for all physical variables, women have a lower maximal sweat efficiency [Gagnon D & Kenny GP (2012). Sex differences in thermoeffector responses during exercise at fixed requirements for heat loss. J Appl Physiol 113, 746–757.]
This study compared the thermoeffector responses of males and females during exercise. Females are generally smaller and have a lower VO2MAX than males. The study authors controlled for this by using males and females with similar fitness and physical size. The onset of heat regulation was the same in both genders, an approximate response time of five minutes. Therefore, the onset threshold is not significantly different in males or females. As the figure below shows, before the maximal sweat rate any differences in total heat loss can be explained by a difference in metabolic heat production.
Eventually the sweat production reaches a maximum. At this point heat loss from sudomotor activity (scientific name for nervous system control of sweating) is at a plateau even if metabolic rate is increased. The body's other method of heat loss is vasomotor activity, bringing warm blood to the body surface in dilated blood vessels for cooling. It appears that vasomotor activity is equal in both genders across all ranges of required heat loss. However, females have a lower maximum sweat production. Female's lower sweat production is because the individual glands produce less sweat; the same number of glands per unit area are activated in both genders. The authors acknowledge they do not know why there is less sudomotor sensitivity in females at maximum heat loss requirement. In addition, the menstrual cycle affects a woman's resting body temperature by ~0.3-0.5°C. The effects that the menstrual cycle has on female heat loss during exercise is not known.
In summary, fitness and gender influence an individual's sweating capability and efficiency. Fit people have a lower sweat-rate on the forehead, but the same whole-body sweat-rate at the same power output. Females have a lower maximal sweat production than males, but appear to have the same vasomotor activity.