Everyone's Health

Heat, Bloating and Sweat for Survival

Why do some people sweat more than others?

By S. Seethaler (UCSD science writer)

Age is one factor. The ability to sweat increases with maturation. Compared to sweat glands in adults, those in children are less sensitive to increases in body temperature and produce sweat more slowly. Sweating capacity is also lower in older adults relative to younger and middle-aged adults.

Gender plays a role. Women have a greater sweat gland density – number of glands per unit area. Men produce more sweat per gland. Overall, women have a slightly lower sweat rate than men.

Heat acclimatization has a large effect on production of sweat and its composition. A person who is not acclimatized to the heat usually cannot produce more than 1 liter of sweat per hour. After a person is exposed to hot weather for a few weeks, the sweat rate can double or triple. At the same time, the concentration of sodium chloride in the sweat declines to conserve body salt.

The changes in sweating that result from heat exposure are controlled by hormones. Sweat comes from the fluid between cells, which is supplied by the blood vessels. Therefore, sweat is filtered blood plasma – the liquid portion of the blood. The sweating-related decrease in the water content of the blood leads to production of antidiuretic hormone by the pituitary gland, and to the production of aldosterone by the adrenal glands.

Antidiuretic hormone stimulates the kidneys to reabsorb water. Aldosterone stimulates the kidneys to reabsorb sodium. Repeated days of exercise in the heat can increase the volume of the blood plasma and the fluid between cells by 20 percent. Retention of water and salt prepare the body for subsequent sweat losses.

Aldosterone also stimulates reabsorption of sodium and chloride by the cells that make up the long, coiled tube of the sweat gland. However, potassium, calcium, magnesium and other electrolytes found in sweat are not conserved because the sweat gland does not have a mechanism to reabsorb them.

Sweating begins more quickly in physically fit people, and they produce more sweat than less fit people exercising at the same relative intensity.

Body size and composition also play a role in sweating by affecting the body's ability to radiate heat to the environment, so that more heat must be lost via evaporation. Other influences include hormonal imbalances and medications that stimulate the part of the nervous system that controls sweating.

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