Physical Activity and Perspiration

Original Editor - Kapil Narale

Top Contributors - Kapil Narale, Rishika Babburu, Kim Jackson and Uchechukwu Chukwuemeka  

Introduction

Sweat is mainly a byproduct of thermoregulation, and is essential to occur in humans. Some people may like it, while others may find it disgusting. Sweat evaporation from the skin surface is also important for human temperature regulation. Despite this, there are many body-balancing functions of sweat. Similar to the kidneys, sweat glands have an important excretory function, as they eliminate excess micronutrients, metabolic waste, and toxic material from the body. The excretion of certain materials in sweat may lead to health imbalances, specifically micronutrient imbalances.        

In this page, we'll be exploring the different constituents, benefits, consequences, and mechanics of sweat.

Here is an introductory video before all the sweaty content:

Purpose of Sweating

Thermoregulation

It is said that the main purpose of sweating is to release heat from the body for temperature regulation , through evaporative functions . During exercise, an excess amount of heat is produced by the working muscles as a byproduct of metabolism. Also, during exercise outdoors, heat is transferred from the air to the body, especially when the air temperature is greater than the skin temperature.

With sweating, body heat is transferred to water, which is on the surface of the skin, and mainly secreted as eccrine sweat which is produced by sympathetic cholinergic receptors.

The energy of sweat vaporization is 580kcal of heat per kg of evaporated sweat.

As mentioned by the Heat Balance Theory, the relationship between the evaporative needs for heat balance and the maximum evaporative capacity of the environment can determine the amount of sweat produced.

The main forms that the body gains heat are through exercise intensity, which can be described as metabolism, and from heat in the environment. These are the main facilitators of autonomic nervous system (ANS) activity. It should be noted that not all sweat is evaporated, and some or much of it may drip off the individual. In humid environments, with conditions of low sweat efficiency, a greater sweat rate may be required for a given amount of evaporation.

Here is a brief video on thermoregulation, mentioning when the body may be too hot or too cold:

Skin Health

Eccrine sweat is important in maintaining an epidermal barrier, from the release of water, natural moisturizing factors, and antimicrobial peptides, onto the skin surface. These natural moisturizing factors include amino acids, lactate, urea, Na⁺, and K⁺, which act as moisturizers for the outer layer of the stratum corneum (outer layer of skin) to stay hydrated. Most of these moisturizers are derived from eccrine sweat, and the amino acids on the skin are derived from the stratum corneum themselves.

Sweating is shown to increase stratum corneum hydration, which may occur as a result of the moisture transfer from the eccrine sweat gland coil directly into the skin before surface sweating begins.

An important therapeutic remedy for reversing dermatitis or dry skin conditions may be to maintain sweating on the skin's surface.

On a more sensory note, moisturizing/wetting the hands with eccrine sweat on the palmar surfaces may improve tactile sense, and strengthen grip, as a fight or flight mechanism.

As mentioned above, the eccrine sweat glands produce and excrete antimicrobial peptides such as dermcidin, cathelicidin, and lactoferrin, which all act as a protective mechanism against skin infection.

Physical Activity, environment and its effect on sweating

Noting that the purpose of sweating is for thermoregulation, exercise/workout, training, competition, or any activity in the heat, this will increase sweat rates.

Although this is the case, sweat rates may vary between individuals. It is seen that the threshold temperature for sweating can generally decrease during exercise.

Athletes and workers who sweat profusely benefit from rehydrating themselves. There are two noted benefits because of this: it improves cardiovascular function, and reduces any impairments of sweating.

It is seen that individuals who are heat acclimated will start to sweat earlier, and will have a higher sweat rate while exercising. It is also noted that individuals with a larger body mass and BMI will sweat more easily compared to individuals with a smaller body mass. However, two individuals with the same body mass and the same amount of heat acclimation may have differing sweat rates due to genetic variations. Physical training and heat acclimation both increase sweating at a given mean skin temperature, as physical training has a positive relationship with sweating.

An interesting fact is that American football players, in their full uniform and practicing or playing in a hot and humid environment, experience one of the highest sweat rates among athletes. This can be attributed to the fact that football players have large body masses, and their uniforms retain a lot of heat. In a hot and humid environment, the average sweat loss in a football player, from 180lbs - 240lbs, can range from 4-5L/hour during a practice session. When football players are practicing twice a day, their daily sweat amount can range from 9-12L. They must replace this lost water, otherwise may face dehydration or be at risk for a heat injury.

It can be noted that in a hot and humid environment, the performance of prolonged aerobic exercise, such as a half-marathon, full marathon, or long triathlon, is adversely affected. The hot environment can also decrease athletic performance in high-intensity anaerobic events, such as rugby, soccer, track and field, or sprinting.

There are three main contributors to impaired exercise performance in the heat:

1. Faster muscle fatigue,
2. Cardiovascular impairments, and/or
3. Central Nervous System impairment.

These factors can act independently, or interact with each other, to produce decreased exercise performance. In many cases, these may depend on exercise intensity and duration.

It is possible to conclude that sweating occurs when the core temperature reaches a fixed amount, which would surpass a threshold temperature, which would be around 0.2^oC above core temperature, for fit individuals compared to unfit individuals.

In addition, concerning passive heating, it was concluded that the threshold temperature for sweating during exercise is 0.7^oC lower than the threshold temperature for sweating during passive heating. As mentioned above, exposure to thermal heat stress may be more of a factor for hidromeiosis, compared to the stress from physical activity.

American Football Players

American Football players are known to have one of the highest sweat rates in athletes, especially in their full uniform, and practicing in a hot and humid environment. The Football players have a large body mass (between 180- which would quickly increase sweat production, and their large bulky uniforms facilitate the retardation of heat loss.

The average sweat rate in football players can range from 4-5L/hour during a practice session in a hot and humid environment. During the preseason practice sessions, football players have a sweat loss of about 9-12L/day with practicing twice in a given day.

Sweating in Male and Female Runners

When upper body sweat rate and sweat volumes were compared between males and females, there were some interesting findings. Males and females generally showed similar sweat responses, but there were a few differences. Males had a higher sweat rate in the mid-front, sides, and mid-lateral back, compared to females. Females showed a greater sweat rate in the upper arm, lateral lower back, and upper central back. In both males and females, sweating was seen to be the greatest along the spine, and greater on the back rather than the chest.

Females are known to have a lower overall sweat rate, linked to a higher core and skin temperature threshold.

In a study conducted, where males and females were asked to run, and their sweat output mass was measured, it is interesting to note the differing masses. Here is a detailed analysis of sweat amounts in different regions of the body in males vs. females:

Males

Anterior/Front of the body

• Slightly higher sweat amount on the chest/breast area
• Much higher sweat amount on the shoulders (over the deltoids)
• Higher sweat amount on the lateral aspect of the body/sides
• Much higher sweat amount on the mid-torso region
• Higher sweat amount at the abdominal region

Posterior/Back of the body

• Much higher sweat amounts on the shoulders (over the deltoids)
• Much higher sweat amounts on the upper back, over the shoulder blades (scapulae)
• Much higher sweat amounts on the lateral aspects of the midback (away from the spinal column)
• Much higher sweat amount on the mid-to-lower spinal column (lower thoracic and lumbar regions)
• Same sweat amounts as females on the lower lateral aspect, above the hips
• Much higher sweat amounts at the waistline

Females

Anterior/Front of the body

• Slightly higher sweat amount on the upper arms

Posterior/Back of the body

• Higher sweat amounts at the spinal column of the upper back (thoracic region)
• Slightly higher sweat amounts on the upper arms (triceps area)
• Same sweat amount as males on the lower lateral aspect, above the hips

As mentioned, although males and females have the same number of sweat glands throughout the body, males have a higher sweat amount per region.

It is seen that females sweat more on the upper arms (anterior and posterior) and in the middle of the upper back. Although they have lower amounts elsewhere, the presence of sweat still does exist in each area.

It is noted that the different zones/regions had more of a difference between males and females, than the difference of the sexes themselves. It was also seen that the mid-central back (spine) had the highest sweat rate in both males and females, and thus the entire back had more sweat than the entire chest, as a whole. The periphery, the upper arms, had the least sweat. While males and females did not show a great difference, there was a distinction between sexes and sweat distribution between the different regions.

In this present study, it was seen that males were sweating 13% more than females. After the exercise test, it was seen that the males' sweat rate declined faster than the females' sweat rate.

Gender/Sex and Sweating

It is important to note that, although there are known differences, it is unclear whether the differences in sweating between Men and Women are due to Physical characteristics or physiological characteristics.

It is interesting to note that Men have higher sweating rates than Women. They have a higher cholinergic response, and therefore maximal sweating rates, compared to Women. However, when Men and Women with similar body mass, surface area, and metabolic heat production were compared, differences were only apparent in excess of usual environmental conditions, and metabolic heat production rate, which result in extreme evaporative needs for maintaining thermoregulation. This would help explain why Women have a lower upper limit for the secretion of sweat than Men. Although Men and Women have a similar response to sweating to thermal stress, even with increased exposure to more severe thermal stress, Women are seen to sweat less than Men.

Sweat gland density is higher in Women than Men, which can be attributed to the lower body surface area in Women, However there are a higher number of sweat glands in Men. Thus, the lower sweating rates from Women can be a result of lower output per sweat gland. There may be more, but smaller, sweat droplets in Women, which also may affect evaporative efficiency. On the contrary, higher whole-body sweat rates in Men may be a result of higher body mass, and higher metabolic heat production from higher exercise intensities.

When considering differences between Men and Women, factors such as body size, surface area to mass ratio, heat acclimation status, aerobic capacity, exercise intensity, or environmental conditions all have a greater contribution than the differences in gender and sex in deciding the exercise-heat stress related autonomic responses. These factors can affect the evaporative rate needed for thermoregulation.

Menstrual cycle

It is important to note that central temperature rises during the menstrual cycle. One notable difference between Men and Women is that the threshold central temperature for sweating changes as the resting central temperature changes during the menstrual cycle in females. Other possible factors that can affect the stimulation of the autonomic response are maturation/development, altitude/hypoxia, circadian rhythm, and menstrual cycle. These alterations, though, in the stimulation of regional sweating from an increased body core temperature are not directly related to whole-body sweating when it comes to exercise. In regards to the menstrual cycle, regional sweating during the luteal phase is decreased at a certain body temperature, since there is a greater threshold. There aren’t any major differences in whole-body sweating during the different menstrual cycle phases. As such, for trained female athletes, exercising in the heat, the different phases of the menstrual cycle don’t have any effect on their physiology or performance.      

Heat and Sweating

It should be noted that sweat rates differ between individuals. One aspect of exercising in the heat is becoming acclimated to the hot and humid conditions. As mentioned above, heat acclimation allows sweating to begin earlier with a higher sweat rate, generally at the commencement of the exercise. This occurs with an increase in sweat rate as well. With sweating commencing at the beginning of exercise, there is less heat storage at the start of exercise, and there is also a lower core body temperature. Also, heat acclimation can increase the amount of sweating 3x compared to those who are not heat acclimatized.

As mentioned, sweat rates increase with exercise intensity. In hot environments, sweat rates can reach 2.8L/hour. It is seen that some runners lose 8% of their body weight during a marathon, even if they are hydrating along the way. The loss of electrolytes could pose an issue as well.

While exercising, excessive body heat is generally diminished by the evaporation of sweat on the skin. With high humidity, the water vapour pressure of air is high, thus sweating is not as effective in decreasing the body heat since evaporation cannot efficiently happen.  

In hot conditions, it is seen that there is minimal conductive heat exchange between air and skin due to the small temperature difference between the environment and the skin.    

Wind speed is also an important factor. Wind can accelerate heat transfer from the skin, as the flowing air replaces the skin surface air which contains evaporating water, with more dry air, which enhances sweat evaporation.

Quick moving activities such as running or cycling will not have the same effect of high heat and humidity as slower moving activities such as beach volleyball or field events. It is noted that body heat release increases with wind speed, and an increase in ambient temperature reduces air density and air resistance.

The heat and light from the sun also need to be considered. Sunburns can aggravate thermal perception from exercise, which would limit thermoregulation from sweat gland responsiveness.

In addition, individuals with a high body mass will have a greater sweat rate, while individuals with a smaller body will have lower seat rates. There may also be genetic variations, in which two individuals with the same body mass and heat adaptation may have differing sweat patterns.

Micronutrient Balance - NaCl

Heat acclimation results in better salt conservation, with decreased sweat. After 10 days of heat acclimation, the concentration of sweat Na⁺ and Cl^- can range from about 30% - 60%. In fact, heat acclimation may occur after only two consecutive days of heat exposure, and sweat Na⁺ concentration will gradually decrease. It is seen that with heat acclimation, the rate of sweating tends to increase in the peripheral areas of the body, such as the forearms, and not so much on the chest or back.

The mechanism for NaCl conservation can be due an increased responsiveness of the sweat glands to circulating aldosterone. This has an effect on Na⁺ reabsorption in the eccrine sweat duct by increasing Na-K-ATPase activity. For NaCl conservation to occur with heat acclimation, there needs to be a salt deficit. This helps to explain the consumption of electrolytes to maintain balance in the body.

Refueling

It is uncertain whether the consumption of Na⁺ has an effect on sweat Na⁺ concentration during exercise. Sweat glands generally respond to salt deficiencies within 1-4 days. It has been shown that it may take many days or weeks for the influence of ingested Na⁺ to have an effect on sweat Na⁺ concentration. With much less Na⁺ ingestion, it was seen that there were no differences or there were minimal differences of sweat Na⁺ concentration or the rate of Na removal. It was also found that there were no differences in Na⁺ concentration when Na⁺ was ingested prior to 1.5 hours of exercise.

In terms of minerals, it is seen that minimal supplementary intake of various minerals, which would already be within the body's allowable limits, have a low impact on sweat mineral loss.

Since water loss of more than 2% during exercise in a hot environment can have a negative effect on endurance performance, maintaining water balance is quite important.

Dehydration and Sweating

Hydration is very important before, during, and after exercise in the heat, for optimal activity performance. However, there cannot be a set amount of fluid intake or loss, as there is a variable fluid need between individuals. Fluid loss by sweating and hydration status can be assessed by analyzing pre- and post-exercise variations in body weight, and urine color and volume.

The consumption of salt before, during, and after activity is important to maintain sodium balance in the body. Specifically in hot environments when sweating and loss of electrolytes is high, this would help to retain and distribute the ingested water throughout the body.          

Sweat-induced loss of body water, or in other words, dehydration, would impair athletic performance. Dehydration resulting in a 1-2% loss of body weight is enough to inhibit exercise performance. Dehydration greater than 3% of body weight hinders physiological performance and even has the risk of a heat injury.

 Heat acclimation would occur by continuously practicing the activity in the chosen/desired heat conditions. The acclimation would be met with a decrease in sweat sodium concentration, an increase in sweat rate, a decrease in core body temperature, and decreased heart rate with regular exercise in the heat. Work capacity will also be increased, with the reduction of the risk of exertional heat illnesses.

With heat acclimation, there is a 10-12% increase in plasma volume. With the increased plasma volume, central blood volume, stroke volume, and sweating capacity are all maintained. This helps the body to store more heat with a smaller temperature gain. With heat acclimation, there is an earlier onset of sweating and a greater sweat rate. Sweating starts to occur before heat adaptation. A greater extent of evaporative cooling can occur, which makes the sweating and the evaporative process more efficient.

Contrarily, as opposed to trained and acclimatized individuals, untrained and unacclimatized individuals lose more Na⁺ in sweat.

It is seen that dehydration may cause hidromeiosis, as mentioned above.

Ageing and Sweating

Older adults are seen to have a lower sweating response than younger adults. It seen that ageing does have an effect on regulating body temperature in sedentary individuals. This would be a lower response per activated sweat gland from a particular pharmacological stimulus. It is seen that there is a decline in sweat response throughout adulthood, and there are regional differences within the body as the function of the sweat gland declines. However, this may be more likely due to a decrease in aerobic fitness and acclimation, as one age. This may be due to the decrease in sensitivity of the sweat gland to cholinergic stimulation.  

It's worth noting that UV and environmental exposures may play a role in sweat gland responsiveness as an individual ages. It is interesting to note that there isn’t a big difference between sweating rates in older and younger adults with exercise in the heat, except for peak sweating rates in hot and dry climates.  

A study showed that younger and older male adults showed very little difference in heat control ability. Thermal tolerance, between older and younger adults, is minimally affected when factors such as fitness level, body composition, and chronic disease, are not considered.

Even within healthy and physically active older individuals, heat tolerance does not seem to be drastically affected with age. However, as older adults exercise less and experience deconditioning, this may have an effect on performance (VO₂ max), decreased heat tolerance, and therefore thermoregulation. Therefore, it is interesting to note that there is a more apparent difference in the younger population than the older population, during exercise compared to passive exposure to heat.

The links given below would be helpful for further readings about the information on sweat:

• Physiology of Sweat, and
• Disorders of Sweating