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Water and Hydration

As we've noticeably entered the heat of summer with high temperatures and humidity, it becomes prudent to look at water intake and hydration status to ensure safe and healthy physical activity. Dehydration, particularly during the summer months, can come as a result of excess fluid loss, like in sweating, and/or inadequate fluid intake.

It should go without saying that water is THE most essential nutrient to the human body. Likely everyone is already aware of the chances of survival without food (weeks) compared to survival without water (days). The major reason for this requirement of water intake is the fact that a majority of the human body is composed of water where nearly all spaces inside and outside of cells are filled with a water-based body fluid and where its concentration is controlled by solvents and membranes. (Nix, 2013) This total body water can be considered intracellular (inside the cells) or extracellular (outside the cells) with membranes separating these compartments of water and electrolyte solvents (including sodium, chloride, potassium, magnesium, phosphate, bicarbonate and protein) determining the balances between these compartments of water. (Nix, 2013) The general guidelines for recommended water intake suggest approximately 3-4 liters/day (1 gallon is about 3.7L) for adults coming from both fluids and food. (Nix, 2013) Foods that contain water largely consist of fresh foods (fruits, vegetables, fresh meats/dairy, etc) whereas dried foods or processed foods are devoid of water content. There may be additional water needs due to losses like in conditions of sweating (physical activity, high temperature/high humidity), functional losses (like in illness), age, or other losses like from diuretics (medications, acute alcohol intake). (Nix, 2013) The actions of water in the body include acting as a solvent, a means of transport, lubrication and thermoregulation. (Nix, 2013) For the purpose of this post, we will focus on thermoregulation.

So what is thermoregulation?

Thermoregulation is the body's mechanism for protecting against overheating by

1.) Radiating body heat

2.) Heat loss through conduction with another surface (immersing in water)

3.) Heat loss via convection (airflow from a fan)

4.) Evaporation of sweat.

*Take note that it is the evaporation of sweat, not simply the act of sweating, that removes heat from the body. This is why air humidity plays a large role in the increased risk of dehydration during the summer months as the moisture in the air prevents sweat on the surface of the skin from evaporating, thus inhibiting a primary mechanism for heat removal. Unsurprisingly, caution is placed on moderate to high intensity physical activity lasting longer than 30 minutes in environments where the air temperature is above 95 degrees Fahrenheit with 60% or higher relative humidity due to this inability for the body to evaporate sweat and reduce internal core temperature. (McArdle, et al. 2015) As well, efforts to remove sweat from the surface of the skin (like continually drying the skin with a towel during exercise) inhibits this process of heat loss as again, it is not the ACT of sweating that cools the body, but the evaporation of that sweat. (McArdle, et al. 2015) In addition to higher air temperatures and indices of humidity, clothing or equipment (helmets, pads, etc) that cover large surfaces of the skin can interfere with this evaporation, and interestingly it is only in wet clothing that evaporative heat loss can occur (i.e. changing into dry clothes will not cool you down faster). (McArdle, et al. 2015) **If anyone has ever tried to quickly shower and change clothes immediately after a workout, they have noticed this phenomenon where they continue to sweat (body temperature remains high) despite their efforts as they have not allowed time for this evaporation of sweat to occur.

But what does water intake have to do with thermoregulation?

Think of thermoregulation as the process of maintaining core temperature when conditions arise that alter it (exercise, heat, etc) by releasing excess heat. When the body becomes overheated, it will use water as a medium for dissipating this excess heat (via sweat). As water is the main constituent in blood volume, when water loss occurs (from sweating, respiration, waste, etc) much of the water that is lost is from the blood. In cases of dehydration (or low body water content), that decrease in blood volume (and blood pressure) from extensive loss of water triggers hormonal effects (the renin-angiotensin-aldosterone system, and vasopressin/anti-diuretic hormone) that then work to reabsorb water and sodium, increase potassium excretion, and ultimately increase blood pressure and blood volume back to normal. (McArdle, et al. 2015) More on this in a bit...

A topic that is continually highlighted this time of year is the potential issue with dehydration, namely in young athletes training in the summer heat. Under normal circumstances, the general guidelines for water intake corresponding with physical activity include sipping water during a workout up to one hour, and with the possible addition of an electrolyte and carbohydrate drink after this primarily based on sweat/water loss. (Nix, 2013) However when exercise intensity and/or air temperature and humidity are increased (as in the summer months), then more careful adjustments must be made to ensure adequate hydration status. By adequate hydration status, we are focusing on not just water intake, but also electrolyte intake. Factors like increased body size, high intensity exercise, wearing protective clothing or equipment, and a hot and humid environment all increase sweating and subsequent water loss and where more than 2% of body weight lost within a training session can signal hypo-hydration, or dehydration. (Nuccio, et al. 2017) This would be the equivalent of a 180lb male losing about 3.5lbs within a training session due to excessive sweating, and thus water loss. Interestingly though, in this meta-analysis of hydration level in various team sports, it was observed that cognitive and physical performance impairments were associated with a comparatively higher level of body weight loss (3-4%) than the 2% loss to signal dehydration. (Nuccio, et al. 2017) This 3-4% of body weight loss during a training session was not typically observed in team sports but more often in individual athletes (like endurance events) and notably due to heat stress. (Nuccio, et al. 2017)

How can dehydration affect sport performance?

Dehydration can negatively impact cognitive and physical performance due to a high rate of fluid loss in sweat being associated with an increase in plasma osmolarity and decrease in plasma volume, which leads to reduced stroke volume and a subsequent increase in heart rate to compensate. (Nuccio, et al. 2017) The increase in heart rate negatively impacts aerobic performance and where this reduced plasma volume and increased plasma osmolarity actually decreases the sensitivity of sweating and skin blood flow responses to being overheated, which leads to further heat storage. (Nuccio, et al. 2017) If thermoregulatory processes are not able to effectively remove heat from the body, an individual can undergo heat cramps, heat exhaustion, and even heat stroke which can permanently disable multi-system organ function in those who are most susceptible. (McArdle, et al. 2015) As well, it is proposed that the decreased plasma volume and increased osmolarity could decrease cerebral blood flow, reduce brain volume, or possibly increase permeability of the blood-brain barrier. (Nuccio, et al. 2017)

What to do about dehydration...

Water loss can occur in a variety of ways, the most commonly noted is through excessive sweating, but significant losses of body water can also occur from being immersed in water (like in swimming where there is increased urine production) and diuresis (medications, dangerous weight loss efforts, acute alcohol intake). (McArdle, et al. 2015) The general guidelines for maintaining hydration status (aside from maintaining water intake to meet your needs, like the recommended 3-4L/day) include drinking ~20oz 2-3 hours prior to exercise, another 10oz after a warm up, and 28-40oz during every hour of exercise. (McArdle, et al. 2015) As a point of reference, most plastic water bottles are ~16oz. Any losses in body weight during activity (due to water loss) should be replaced within 2 hours after the activity with 20-24oz for every pound of weight lost from sweating. (McArdle, et al. 2015) This would imply that a 180lb male that lost 3lbs in a training session (from sweat/water loss) should take in 60-72oz (or 2L) of water within the hours after exercise. However this does NOT necessarily mean chugging down 2L of water but rather a combination of water and foods, and yes there is merit behind the addition of electrolytes.

Now about these electrolytes...

The role of the electrolyte sodium, or sodium chloride as it is found in food, is commonly linked to hypertension (high blood pressure) where anti-hypertensive diets are based around reducing total sodium intake to approximately 1,500mg per day compared to the general guidelines suggesting 2,300mg per day for non-hypertensive individuals. (Gropper, et al. 2013) Some of the theories regarding this link are based on the water retention that occurs with sodium intake (with salt, comes water) which can increase extracellular volume and release substances (like calcium) that can increase contractility (like the contractile muscles of the vessels). (Gropper, et al. 2013) Sodium retention also decreases nitric oxide production and promotes urinary calcium excretion, both of which can also contribute to increasing blood pressure. (Gropper, et al. 2013) Potassium however is noted to have an inverse affect where it is associated with decreasing blood pressure by promoting excretion of sodium and reducing excretion of calcium and magnesium. (Gropper, et al. 2013) Potassium can also relax vascular smooth muscle and reduce peripheral resistance, thus reducing blood pressure. (Grooper, et al. 2013)

*[There are theories to suggest that while reducing sodium is an aim for reducing hypertension, that increasing potassium should also be viable as a part of a therapeutic anti-hypertensive diet. An interesting notion to point out though is the fact that relative concentrations of these electrolytes could be determined based on water content to where low water intake could be associated with increased concentrations of sodium, which proposes the notion of simply increasing water intake to decrease risk of hypertension. There are study findings to suggest that habitually drinking water (at least 6 cups everyday) is associated with a significantly reduced risk for hypertension and where regular coffee consumption (3 cups everyday) showed no association with increasing risk for hypertension, despite claims against caffeine intake. (Suheti and Sunandar, 2016)]

Why am I talking about blood pressure when the topic is dehydration?

In cases of dehydration, or low body water content, there is a subsequent low blood volume and low blood pressure. I had mentioned the hormonal effects that take place (like the adrenal hormone aldosterone and vasopressin/anti-diuretic hormone from the posterior pituitary) that act to increase water and sodium reabsorption, increase potassium excretion, and thus increase blood volume and blood pressure. Due to the important role that sodium specifically plays with rehydration, it is notable to include sodium in with the rehydration process (through liquids and foods) as a means of absorbing the water that you are taking in to normalize blood volume and blood pressure.

Adding sodium to fluids can benefit rehydration efforts, as stated, where sodium increases water retention (i.e. with salt, comes water) which equates to greater absorption of water (not eliminating it), increasing the thirst mechanism, and restoring lost plasma volume with balanced osmolarity. (McArdle, et al. 2015) Without adequate sodium replacement, excess fluid intake can lead to increased urinary output (i.e. NOT absorbing the water you are drinking) which can further exacerbate dehydration. (McArdle, et al. 2015) However, NOT everyone will require added sodium, but rather in circumstances of high water loss or dehydration, high amounts of sodium lost in sweat (known as a 'salty sweater') or low sodium intake due to a low sodium diet.

**This implies that NOT everyone will require supplements or sports drinks like Gatorade to re-hydrate. In my opinion, the chances are an individual completing a moderate workout in an air conditioned facility is NOT going to require additional sodium and electrolytes but where others who are training outdoors, or for long duration/high intensity, or submerged in water, may likely need the addition of electrolytes. My personal favorite: salted watermelon (throw a dash of salt on some watermelon chunks) to provide water, sodium and carbohydrates.

What about diuretics?

It should go without saying that the use of diuretics can increase risk for dehydration as they increase water loss through increased urination. Individuals taking diuretics as anti-hypertensive medications should monitor fluid and electrolyte intake, particularly for physical activity and in warmer temperatures as there will be an increased risk for dehydration from water loss. However the most commonly debated diuretics include caffeine and acute alcohol intake. **Most of us are familiar with the moderate diuretic effects of caffeine and/or alcohol where the observed increase in urination is associated with increased efforts for the liver and kidneys to detoxify and remove those compounds from the blood. There have been several studies to find no significant diuretic effects from caffeine so worry not about your regular coffee or tea. (Zhang, et al. 2015) Additionally, it is theorized that acute alcohol intake, particularly in hot/humid weather, can be associated with dehydration due to the combined water loss via diuresis from the alcohol, and water loss via sweat from the air temperature and humidity index. While there is certainly validity behind this theory, there are some interesting study findings to suggest that the diuretic effects of alcohol may be more pronounced in individuals who are already adequately hydrated compared to those who are dehydrated due to exercise. (Hobson and Maughan, 2010) According to this study, individuals who were slightly dehydrated from exercise had less of a diuretic effect from alcohol than those who were not already dehydrated. (Hobson and Maughan, 2010) What does this mean? All this really implies is the need to have water in the system in order to experience water loss i.e. if there is already a low water content, then the body is not going to increase water excretion via urination as a means of reserving body water. However, in this case, the individuals who had adequate amounts of water in their system prior to drinking alcohol were the ones to experience diuresis from the acute alcohol intake (increased urination). Translation: a healthy response of the body when it needs to excrete a potentially toxic compound!

What does this mean for your summer plans?

Hydrate accordingly. It is probably not the best idea to have a high intensity, long duration workout in the high heat and humidity followed by day-drinking with shots and other water-free alcoholic drinks. In my opinion, make sure that you are getting in enough water (and sodium) to make up for losses from exercise and heat/humidity, and break up alcohol intake with drinks that have a higher water content versus just straight liquor (cocktails with ice, light beer, a water on the side, etc).


Gropper S, Smith J. Advanced Nutrition and Human Metabolism. 6th edition. Wadsworth-Cengage Learning. 2013.

Hobson R, Maughan R. Hydration Status and the Diuretic Action of a Small Dose of Alcohol. Alcohol and Alcoholism. 2010; 45(4):366-373.

McArdle W, Katch F, Katch V. Exercise Physiology. 8th edition. Lippincott, Williams and Wilkins. Philadelphia, PA. 2015.

Nix S. Williams' Basic Nutrition and Diet Therapy. 14th edition. Elsevier Mosby. St. Louis, MO. 2013.

Nuccio R, Barnes K, Carter J, et al. Fluid Balance in Team Sport Athletes and the Effect of Hypohydration on Cognitive, Technical, and Physical Performance. Sports Medicine. 2017;

Suheti T, Sunandar K. Habits Drinking Ordinary Water Can Prevent Hypertension. Open Journal of Nursing. 2016; 6: 404-411.

Zhang Y, Coca A, Casa D, et al. Caffeine and diuresis during rest and exercise: A meta-analysis. Journal of Science and Medicine in Sport. 2015; 18(5): 569-574.

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