Why does our body sweat?

2 to 4 million

of sweat glands across the entire body.

≈ 1 litre

of sweat lost per day on average without physical activity.

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1. Sweat primarily serves to regulate body temperature.

The sweating process primarily has a thermoregulatory role.

In other words, it enables the body to maintain a relatively stable internal temperature, at around 37°C, despite variations in the environment or increases in heat production by the body itself. This is a crucial point, because the functioning of enzymes, cells and organs depends on a precise thermal balance. If body temperature rises too high, this functioning can be disrupted.

One of the main internal factors that can cause a rise in temperature is physical activity. When a muscle contracts, not all of the chemical energy consumed is converted into useful movement. A substantial proportion is dissipated in the form of heat. In practice, this means that during physical exertion, the muscles generate a great deal of heat in addition to the mechanical work required. The more intense the exercise, the greater this heat production becomes. This excess heat then needs to be removed.

This is where perspiration comes into play. The eccrine sweat glands, distributed over almost the entire surface of the body, secrete sweat onto the skin’s surface. This sweat consists predominantly of water, with a small amount of mineral salts and other dissolved compounds. In itself, the presence of water on the skin is not sufficient to cool the body. The actual cooling mechanism arises from its evaporation. Indeed, when sweat evaporates, it absorbs thermal energy at the skin surface. In other words, to move from the liquid state to the gaseous state, water requires energy, which it draws in part from the body’s heat. This is how sweating enables efficient heat dissipation.

It is estimated that approximately 1 gram of evaporated sweat allows the body to dissipate nearly 2.4 kJ of heat. The amount of sweat produced therefore depends closely on the amount of heat that the body needs to eliminate.

What are the differences between eccrine and apocrine sweat glands?

There are two main types of sweat glands:

Eccrine glands: Very numerous and distributed over almost the entire surface of the body, they are active from birth and secrete a watery sweat, low in lipids, principally in response to an increase in body temperature.
Apocrine glands: Located only in certain areas (armpits, genital region), they become active at puberty and produce a thicker secretion that is rich in organic compounds, particularly in response to emotions or stress.

It should be noted that if the air is dry and circulates well, sweat evaporates easily and cools the body efficiently. Conversely, when the air is humid, evaporation becomes more difficult. Sweat may then run off without really fulfilling its cooling role. This is why, at the same temperature, we sweat more when the air is humid. It is the nervous system that controls this process. The hypothalamus, a region of the brain involved in regulating body temperature, receives information from thermoreceptors located in the skin and within the body. When it detects a rise in temperature, it activates the eccrine glands via the autonomic nervous system, more specifically the cholinergic sympathetic fibres, which triggers sweating.

Note : It is interesting to note that the amount of sweat produced is not fixed. It can adapt over time, particularly in the case of heat acclimation. After several days of repeated exposure to hot conditions, the body becomes more efficient: it starts to sweat earlier and often more, while losing proportionally less sodium in the sweat. This adaptation improves heat tolerance and reduces the risk of hyperthermia.

Thus, we sweat primarily because the body needs to protect its internal temperature.

2. We also sweat as a result of emotions.

Perspiration is not limited to a thermal response. It can also be triggered by emotions such as stress, anxiety, fear or even excitement. Unlike heat-related sweating, which is primarily driven by the hypothalamus in response to an increase in body temperature, emotional sweating is triggered by brain structures involved in emotional processing, such as the limbic system, particularly the amygdala. When the brain perceives a situation as stressful or threatening, it activates the sympathetic nervous system, which leads, among other effects, to stimulation of the sweat glands.

Emotional sweating is very rapid and can appear just a few seconds after an emotional stimulus.

This type of sweating affects specific areas of the body, in particular the palms of the hands, the soles of the feet, the armpits and sometimes the forehead, where apocrine sweat glands are found. Naturally, emotional sweating varies greatly from one individual to another. Some people exhibit increased sensitivity of the sympathetic nervous system, which may result in more pronounced emotional sweating, sometimes referred to as hyperhidrosis when it becomes excessive and affects quality of life.

3. Sweat contributes to maintaining the skin’s balance.

Although perspiration is primarily a thermoregulatory mechanism, it also plays a more discreet yet genuine role in maintaining the physiological balance of the skin. Sweat produced by the eccrine glands is not composed solely of water: it also contains a variety of molecules, such as sodium, potassium, lactate, urea, as well as certain amino acids, which contribute to maintaining skin hydration.

Some of these compounds are part of what are known as natural moisturising factors, whose role is to retain water in the stratum corneum, the outermost layer of the epidermis.

Beyond hydration, sweat also appears to contribute to the skin’s defence against external aggressions. The sweat glands are able to produce and secrete antimicrobial peptides, such as dermcidin, cathelicidin and lactoferrin. These molecules help to limit the proliferation of certain micro-organisms on the surface of the skin, thereby contributing to the maintenance of the skin microbiome. Perspiration could therefore, to some extent, play a role in protection against certain skin infections, although this function is still being explored.

Sweat also plays a role in maintaining the acidity of the skin surface. Indeed, certain of its components, such as lactate, help to maintain a pH that is slightly acidic, around 4.5–5.5, which is favourable to the balance of the microbiota and to the integrity of the skin barrier. This acidity in particular limits the growth of pathogenic bacteria and supports the activity of enzymes involved in the renewal of the stratum corneum.

Note : While moderate sweating can help to hydrate and protect the skin, excessive or prolonged sweating, or sweating associated with occlusion, such as in the case of tight clothing, can on the contrary weaken the skin barrier and promote irritation. The impact of sweat on the skin therefore depends on the context and on its intensity.

4. Can sweating “detoxify” the body?

The idea that sweating helps eliminate toxins from the body is widespread. Saunas and intense exercise are indeed often presented as ways of accelerating this elimination. However, from a physiological point of view, this perspective is highly simplified. It is true that sweat contains traces of exogenous compounds, such as metals, pollutants or even alcohol. However, their presence in sweat does not mean that sweating constitutes a major elimination mechanism. Sweat is produced from the interstitial fluid, itself derived from blood plasma, and its composition partly reflects that of this compartment. It is therefore more a phenomenon of passive diffusion than an active detoxification process.

The genuine organs involved in the elimination of unwanted substances are the liver and the kidneys. The liver converts toxins into metabolites that are more easily eliminated, while the kidneys filter the blood to excrete these substances in the urine. For example, around 90% of ingested alcohol is metabolised by the liver, with the remainder being eliminated in small amounts via exhaled air, urine and sweat. The contribution of sweating to this elimination is therefore marginal.

Just as sweating does not make it possible to eliminate fat mass, excessive sweating, for example after a sauna session, is not necessarily accompanied by an increased removal of undesirable substances.

5. Certain diseases or medications can alter sweating.

The normal functioning of sweating is sometimes altered. These changes may result in excessive sweating (hyperhidrosis) or, conversely, insufficient sweating (hypohidrosis or anhidrosis). Certain diseases directly affect the functioning of the sweat glands or the nerve pathways that control them. For example, diabetes or multiple sclerosis can disrupt the regulation of sweating. In other cases, such as congenital anhidrosis, the sweat glands are absent or poorly functional, which compromises the body’s ability to dissipate heat and can lead to heat intolerance.

Conversely, hyperactivity of the sympathetic nervous system following hormonal changes, such as hyperthyroidism or even simply the menopause, can cause excessive sweating. Certain medications can also stimulate sweat production, such as antidepressants and analgesics.

These alterations are not insignificant.

Insufficient sweating can limit the body’s ability to cool itself, increasing the risk of heatstroke. Conversely, excessive sweating can affect quality of life, particularly from a social and emotional perspective. This is why, if you notice a change in your sweating, you should not hesitate to discuss it with a healthcare professional so that the cause can be identified and appropriate solutions can be offered.