Sweating is our body’s natural cooling system. It keeps our temperature in check working on an automatic internal thermostat. Think of our pores as the permanent filters in the body system providing the opening for sweat to be released. Is there more to sweating than what’s obvious? And how much should we be hydrating when we do? Here’s what personal trainers need to know about drinking water and sweating it out.

The human body has several mechanisms in place to regulate temperature, but when it comes to cooling down, none is more important than perspiring. Perspiration is the combined effects of evaporative cooling and heat loss through the sweat itself.

Sweating Triggers

Exercise raises the body’s core temperature, and “working up a sweat” rightly refers to an external sign of intense physical effort, as through exercise. Sweating isn’t always tied to intense physical activity, however. Other expressions, including “don’t sweat the small stuff” and “no sweat”, refer to the link between perspiring and intense effort and/or emotional state.

While sweat is odorless, topical bacteria in hairy areas of the body can cause odor and wetness when a person is anxious or stressed  – right?

  • Deodorants and antiperspirants make up a $5 Billion market segment in the USA – which Statista equates to an average annual cost of ~$15 per consumer. Saddlebag, under-arm sweat is not good in a big business meeting, yet that category known as apocrine sweat is not the topic of this post. Rather, eccrine sweat from about 3 million sweat glands is as it relates to clients’ training, exercise, and healthspan.

Temperature and humidity are the external factors most often associated with the body’s sweat response, even at rest; but certain emotions, such as anger, anxiety, embarrassment, and fear can also trigger a perspiration response. These triggers point to the fact that the sweating mechanism is controlled by the autonomic nervous system, and is therefore not something that is not under conscious control.

Cooling System

Sweat is a combination of water and minerals. “When sweat evaporates from your skin, it removes excess heat and cools you…When the water in the sweat evaporates, it leaves the salts (sodium, chloride, and potassium) behind on your skin, which is why your skin tastes salty,” Craig Freudenrich, Ph.D. wrote in How Stuff Works. “The loss of excessive amounts of salt and water from your body can quickly dehydrate you.”

We have a serious number of sweat glands and the amount varies by gender. In an August 2013 Huffington Post article Six Things You Didn’t Know About Sweat reported that each of us has two to four million sweat glands. Although women have more sweat glands than men, men produce more sweat. The article also states that an athlete who exercises intensely in the heat can sweat out 2-6% of their body weight.

We all sweat at different degrees. How quickly someone sweats is not necessarily, a sign of their fitness level. Although, many exercise enthusiasts measure the value of their workout by how much they sweat.

Not sweating at all is not good, especially when temperature regulation is required. Some people are born with anhidrosis or hypohydrosis, where perspiration is absent. If you have such a personal training client it is of paramount importance to not overwork or overheat them, since the results could indeed be deadly.

For most of our clients however, sweating is part of the workout–a by-product. But hydration needs to accompany a measurable amount of perspiration.

Electrolyte Loss

Even though several electrolytes are lost in sweat including- sodium, potassium, iron, magnesium, chloride, and calcium, the main losses come from sodium and potassium. Studies looking at sweat losses during a 7-hour exercise stress protocol (Five 1hr exercise sessions – 20 min rest) found the sweat content did not change much after sessions 1, 3, and 5 and was related to initial sweat content (Montain et al 2007).

This suggests that a replacement drink could be specifically designed if the initial content is known and the length of the event and environmental conditions are known, too. Lack of water can lead to dehydration, a condition that occurs when the body has insufficient water to carry out normal functions. In the second part of this article, we will look at assessing hydration in athletes.

What Affects Sweating During Exercise?

Factors that affect the sweat rate, measured as the volume of sweat/unit time are:

  • Level of physical conditioning;
  • Acclimation to the environment- someone used to the heat will sweat earlier and more;
  • Environmental conditions- air temperature and humidity being higher- sweating greater;
  • Exercise intensity- the harder you exercise, the more you sweat due to increased body;
  • Clothing- the thinner and more breathable the material, the less sweating.

The contents of sweat can vary greatly between individuals, as well as from when the sample is taken, and from what part of the body. Both the level of conditioning and amount of acclimatization can have some bearing on this, as well.

Hot and/or humid weather, even without exercising or engaging in activity, is enough to cause perspiration and call for additional intake of fluid. The sweating mechanism works because there is a gradient for drying the sweat by the dry air passing over it. However, when the humidity is high (above 85%), there is not a sufficient gradient for drying and, as a result, there is no cooling.

The body temperature then increases and continues to sweat even more. Thus, there is a very large amount of sweating in very humid environments and participants must be careful not to overheat since the sweating mechanism is largely ineffective. Heated indoor air, as is common in the cooler months, can also cause the skin to lose moisture during the cooler months. In addition, altitudes over 8,200 feet (2,500 meters) can lead to an increase in urination and more rapid breathing, which in turn depletes fluid reserves.

Why we get thirsty

What is the experience of thirst? When fluids are lost, there is a steady increase of sodium in our blood.

“Sodium being the electrolyte responsible for fluid retention, holds the key as to why we thirst,“ as DicoveryHealth.com states. An electrolyte is a “medical/scientific“ term for salts, specifically ions. The term electrolyte means that this ion is electrically charged and moves to either a negative (cathode) or positive (anode) electrode.

Throughout the day, whether active or not, there is a gradual reduction in our bodily fluids from chemical reactions, including exercise of all types. When this concentration reaches certain levels, the thirst center in our brain is triggered, creating a sensation of thirst.

Guyton’s Textbook of Medical Physiology states that “the total amount of water in a man of average weight (70 kilograms) is approximately 40 liters, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 75 percent of the body weight, but it progressively decreases from birth to old age, most of the decrease occurring during the first 10 years of life. Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent.

It is important to note that these figures are statistical averages, and will vary based on the type of population, age, and number of individuals sampled, as well as the methodology. Consequently, there cannot be a figure that is exactly the same for all people.

Optimal Hydration

Water is lost from the body through a combination of breath, urine and bowel movements, as well as perspiration. The body’s natural way of replenishing its water supply is to consume drinks and foods that contain water.

The more you sweat, the more you need to hydrate. In addition to regulating body temps, it also assists with internal chemical reactions like digestion, circulation, and waste excretion. Such bodily functions as expanding and contracting muscles, breathing (keeping the lungs moist), controlling internal and external temperatures, lubricating our joints, and allowing all other organs to do their jobs owe their functionality to H2o.

The Institute of Medicine, an independent, non-profit organization whose stated mission is to serve “as adviser to the nation to improve health” recommends that men drink a total of about 3 liters (roughly 13 cups) of beverages a day and that women drink 2.2 liters (roughly 9 cups) of beverages daily.4 Beverages are an obvious source, but water is present also in many foods, perhaps most obviously fruits and vegetables. Food, in fact, accounts for about 20 percent of the average person’s fluid intake each day.

Water is everywhere in the body, from the jelly inside the eyeballs to the fluid inside our skulls in which the brain floats. Our blood is what carries nutrients to all tissue and rids those tissues of lactic acid, a by-product of muscles during exercise, and other metabolic waste products. Because our blood is 90% water, without it our tissues would starve and we could poison ourselves in our own biochemical wastes.

Decreased volume in the blood vessels is called hypovolemia. Because water moves freely between the compartments, extracellular fluid deficit causes intracellular fluid deficit (cellular dehydration), which leaves the cells without adequate water to carry on normal function.

Keeping cool and staying hydrated can help conserve energy that can be used for exercise periods or other daily activities. Some folks often hesitate to drink a lot of H2O because they believe it will increase water retention, yet the opposite occurs. When there is enough water in the system, the body senses the shortage is over and the kidneys can resume getting rid of toxic wastes produced by the body. When this happens, the liver can return to its primary function of converting stored fat into energy.

Therefore, with a proper balance of caloric intake and physical exercise, the body is better able to reduce the amount of stored fat.

The amount we need to hydrate optimally varies based on factors such as air temperature, humidity, clothing, diet, exercise intensity, and individual needs. There are various opinions on how much fluid we need, but it’s usually between 8 and 10 cups of water daily. Another method of determining your ideal intake is taking your body weight in pounds, dividing it in half, and calling it ounces:

A 150lb individual will need about 75 oz a day. (With a maximum of 90 oz a day for anyone!). Always heed your medical professional’s advice!

Assessing Hydration For Athletic Performance

The body’s ability to exert at high intensities – the stimulus for muscle growth and speed development – is dependent on proper hydration status.

When looking at the balance of water loss to intake and absorption, the body is either hyper-hydrated (too much), dehydrated (too little), or euhydrated (normal hydration). Even a loss of 2% of body weight can significantly reduce performance in a warm or hot environment. Assessing dehydration has been, and remains, an inexact science. This is due in part to the variations within each individual.

With that said, there are several methods in common use to assess hydration during physical exertion, such as during an athletic competition.

The simplest method, but perhaps, least accurate, is to look at the urine color. In a study by Armstrong et al (1998), they found urine color was a better indicator of hydration status than urine volume. They noted that a person should drink until the urine was either very pale yellow or pale yellow. Under normal eating conditions, this can be a good indicator; however, many athletes may ingest supplements (particularly B complex) during a competition and that can markedly change the color to a more golden yellow color.

In addition, muscle breakdown will result in a darker appearance; a possible outcome during endurance events.

Another method for assessing hydration is to measure acute changes in body weight using a scale. A simpler approach, look at the percentage of body weight lost during a competition. The exact decrement in performance is based on several factors including the athlete’s tolerance or acclimatization, the type of event, and the environmental heat.

At 3% body weight loss, there is a “dry mouth” feeling and a decrease in urinary output. At 4-5% body weight loss, there is a 20-30% decreased work capacity, headaches, and difficulty in concentration. As the dehydration climbs into the 6-7% range there is a strong impairment in temperature regulation, increased respiratory rate, tingling and numbness in extremities and finally collapse (fainting) if exercise is continued.

Measuring the urine specific gravity (USG) and osmolality are good indicators when taken in the morning, as this includes the morning body weight after voiding. Because osmolality units are mosm/kg, it is necessary to obtain first an accurate body weight measurement. However, being able to perform this test is contingent upon having access to laboratory equipment.

Also, urine osmolality lags behind plasma values, thus making this a better indicator. Plasma and serum osmolality are good indicators, but they require a blood sample from the individual and not many athletes want any amount of blood removed during competition.

Thirst may seem an obvious indication of a need for hydration but, unfortunately, iis not a very reliable sign. This is because the thirst mechanism in humans is weak and often lags about 20 minutes behind the body’s present condition. This is made even more unreliable by the fact that some people tolerate thirst quite well while others are very susceptible to even minor fluctuations in hydration status.

Signs of Dehydration

Athletes need to stay hydrated for optimal performance. Studies have found that a loss of two or more percent of one’s body weight due to sweating is linked to a drop in blood volume. When this occurs, the heart works harder to move blood through the bloodstream. This can also cause muscle cramps, dizziness, fatigue, and even heat illness including heat exhaustion and heat stroke.

As mentioned above, difficulty concentrating, increased respiratory rate, tingling, and numbness are also signs of dehydration.

Causes of Dehydration

When exercising, NFPT recommends drinking water throughout your workout, as well as following it. The small intestine absorbs 8 – 10 ounces of water every 20 minutes, so you want to pace your fluid intake. I remember as a sprinter waiting until the end of the meet to down a large bottle of a sports drink, only to throw it up in a trash can. Now I know why!

Drinking as you’re actively sweating gives your body a chance to use it on demand, rather than guzzling a bunch at once. I see many gym-goers walking around with a gallon jug of water to help them measure how much they drink.

Weighing before and after a workout can be a quick estimate of how much fluid was lost. To replace lost fluids, you need to ingest 16 ounces of water for every pound lost.

Safe Sweating – A Readiness Checklist

A four-point checkup for your client’s readiness for sweat and bliss is:

  • Am I adequately conditioned for my workout environment (altitude/dryness, temperature, shade from sun…)?

  • Have I acclimated to my workout environment with proper rest, nutrition, and hydration?

  • Am I prepared to adapt my workout volume, intensity, and tempo if conditions change?

  • Am I wearing anti-wicking clothing that is more comfortable and breathable for warm workout conditions or heavy exertion?

Water or Sports Drink?

When do recommend sports drinks to our clients? NFPT recommends a sports drink of 6-8% sugar solution with 100 mgs/8 OZs of sodium concentration if exercise is intense or lasts more than an hour.

Sweat is going to happen. “Sweat cleanses from the inside,” said Dr. George Sheehan. “It comes from places a shower will never reach.” As trainers, we can continue to emphasize sweat and hydration benefits. And whether to hydrate with plain H2O, water alternatives, or sports drink.



  1. Armstrong LE (2000): Performing in Extreme Environments. Champaign: Human Kinetics.
  2. Montain, Scott J., Samuel N. Cheuvront, and Henry C. Lukaski. “Sweat mineral-element responses during 7 h of exercise-heat stress.” International journal of sport nutrition and exercise metabolism 17.6 (2007): 574.
  3. Armstrong LE, Soto JA, Hacker Jr FT, Casa DJ, Kavouras SA & Maresh CM (1998): Urinary indices during dehydration, exercise, and rehydration. Int. J. Sport Nutr. 8, 345-355.
  4. Institute of Medicine (US). Panel on Dietary Reference Intakes for Electrolytes, and Water. Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. National Academies Press, 2005.
  5. Sawka, Michael N., et al. “American College of Sports Medicine position stand. Exercise and fluid replacement.” Medicine and science in sports and exercise 39.2 (2007): 377-390.
  6. Montain, Scott J. “Hydration recommendations for sport 2008.” Current sports medicine reports 7.4 (2008): 187-192.
  7. Personal Fitness Training Manual, National Federation of Professional Trainers, 7th edition, page 146 (2017).
  8. Guyton’s Textbook of Medical Physiology. Arthur C. Guyton., John E. Hall.
  9. health.howstuffworks.com/skin-care/information/anatomy/how-sweat-works3.htm
  10. www.huffpost.com/entry/facts-about-sweat_n_3709248

Kim Becknell Williams

Kim Becknell Williams is a freelance writer with more than ten years of personal training experience. Certified through NFPT, she is a Functional Training Specialist and holds a Master Trainer level certificate for resistance, endurance and sports nutrition. Kim has written two books including Gym Etiquette 101. She enjoys writing a variety of lifestyle articles and fitness blogs.