The Hydration Equation: Update on Water Balance and Cognitive Performance : ACSM's Health & Fitness Journal

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The Hydration Equation

Update on Water Balance and Cognitive Performance

Riebl, Shaun K. M.S., R.D.; Davy, Brenda M. Ph.D., R.D., FACSM

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ACSM’s Health & Fitness Journal 17(6):p 21-28, November/December 2013. | DOI: 10.1249/FIT.0b013e3182a9570f
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Although it often is overlooked as an essential nutrient, water is vital for life because it serves several critical functions. Total body water comprises approximately 45% to 75% of a person’s body weight (27). Muscle mass is 70% to 75% water, whereas water in fat tissue can vary between 10% and 40% (25). Water acts as a transporter of nutrients, regulates body temperature, lubricates joints and internal organs, provides structure to cells and tissues, and can help preserve cardiovascular function (26). Water consumption also may facilitate weight management (15,17). Water deficits can impact physical performance (25,38), and recent research suggests that cognitive performance also may be impacted (4,13,20–22,35,36). This article will address water balance, hydration assessment, and the effect of water balance on cognitive performance.


Water balance (i.e., input vs. output) is influenced by dietary intake, physical activity level, age, and environmental conditions. Although total body water balance is regulated tightly during a 24-hour period (25), deficits and excesses can occur. Dehydration develops from inadequate fluid intake or excessive fluid losses, and overhydration can result from excessive water (or fluid) intake with or without proper electrolyte replacement (25,33).

Water Output and Its Regulation

The skin, kidneys, lungs, and digestive system are all sources of water output (Figure). Environmental factors (e.g., humidity, temperature) and intensity and duration of physical activity also impact urine output (e.g., increased urine output in colder climates, decreased urine output in hot climates and greater water loss via sweat with longer-duration activities) (25). Respiratory water loss averages 250 to 350 mL per day in sedentary adults; however, physical activity can increase losses to about 600 mL per day (19,25). Insensible water loss, which includes sweat loss, can vary with environmental conditions (i.e., wind speed, humidity, and sun exposure), activity level, body composition, degree of physical fitness, and other variables (e.g., clothing worn, sweat rate) (19,25,38). On average, insensible water losses are about 450 mL per day; however, during vigorous physical activity in a hot environment, losses in excess of 3 L per hour are possible (37). Urine output generally ranges 1,000 to 2,000 mL per day but can be altered by exercise and heat strain (25). Gastrointestinal and fecal water output accounts for 100 to 300 mL per day (19,25,27). Total water output is estimated to be approximately 1,500 to 3,100 mL per day for adults in temperate climates (19,25).

Average daily fluid balance in adults. 1 cup = 237 mL. (Based upon estimates reported by Jequier E, Constant F. Water as an essential nutrient: the physiological basis of hydration. European Journal of Clinical Nutrition 2010;64(2):115–23.)

When water loss exceeds intake, blood volume decreases and plasma osmolality increases. The reduction in blood volume decreases blood pressure, leading to increases in renin and angiotensin II concentrations. The latter, along with aldosterone, promote sodium and chloride reabsorption in the kidneys and, thus, water via osmosis and decreased urine output. Increased blood osmolality and angiotensin II stimulate the hypothalamus and arginine vasopressin (AVP) is released, promoting renal water retention and reduced urinary output. Increased plasma osmolality also stimulates thirst through peripheral osmoreceptors in the mouth and gastrointestinal tract to replace the remaining water lost. Baroreceptors promote AVP release and thirst when reductions in plasma volume are significant; however, this mechanism is not as sensitive as the osmotic regulation of thirst (31).

Water Input and Its Regulation

Water input comes from food and beverage ingestion and normal metabolic processes (Figure 1). There are regulated or physiological (e.g., osmoreceptors in the brain and mouth, baroreceptors in blood vessels and atrium) and nonregulated (e.g., social, cultural, behavioral) factors that influence water intake (25,35,43) and fluid balance. The thirst sensation is triggered with a body water loss of 1% to 2% — a range where physical and cognitive performance may decline (4,9,21,22,25,34,38). Typically, plasma osmolality is maintained tightly between 280 and 290 mOsm/kg; however, an increase of approximately 1% to 3% creates a drive to drink (12,43).

Fluid water intake generally accounts for approximately 70% to 80% of total water consumed (25), and approximately 20% to 30% of total water intake comes from solid foods (5,19,25). In a typical sedentary adult, this represents approximately 7 cups (1,575 mL) from beverages, approximately 3 cups (675 mL) from foods, and approximately 1 cup (300 mL) from normal metabolic processes (27). Despite popular myths, coffee can be considered a source of fluid (7,25), and although alcohol may increase fluid losses short-term, it is not believed to result in significant water loss over a day’s time (25).

When fluid is consumed, osmoreceptors in the mouth are stimulated, which reduces AVP secretion. This allows the kidneys to release excess water and preserve water balance. If plasma osmolality decreases and blood volume increases, the thirst sensation fades. The desire to drink may cease before achieving water balance (13), however, plasma osmolality will remain elevated and thirst sensations may return until water homeostasis is achieved (12,43).

Hyperhydration and Hyponatremia

Typically, healthy individuals can maintain water balance through urination when excess fluid is consumed; hyperhydration is not commonly encountered (19,25). However, during extreme and extended-duration exercise, excessive consumption of hypotonic fluids and sodium losses that exceed the rate of replacement, and sometimes even in the absence of overconsumption of fluids, can cause hyponatremia (25,33,38). Hyponatremia, which is defined as a blood sodium concentration lower than 135 mmol/L (25), can have serious health implications (19,25). Hyperhydration (i.e., “water intoxication”) can present with symptoms such as fatigue, lethargy, disorientation, confusion, headache, nausea, vomiting, and, if not treated properly, coma and death (23,25). The signs and symptoms of dehydration and overhydration can be similar (i.e., light-headedness, dizziness, headaches, nausea, fatigue) (4,21,22,30). When working with clients, health and fitness professionals can use a variety of methods to assess the presence and nature of water imbalance to ensure that clients receive proper treatment.


Hydration refers to having adequate fluid within body tissues, and it can be determined through a variety of methods. Dilution techniques, plasma osmolality, neutron activation analysis, and bioelectrical impedance spectroscopy can be used to assess hydration status in a laboratory setting, whereas thirst, 24-hour urine volume, change in weight (i.e., body mass), urine color, and urine specific gravity (USG) can be used in the field (3). Others have reviewed these techniques, their ease of use, and potential limitations extensively (2,3,11,38); however, a brief discussion of practical measures to assess hydration status is provided.

USG is an accurate and rapid indicator of hydration status (2). A urine specimen is placed on the glass plate at one end of a handheld refractometer and, on holding it up to natural light and looking through the eyepiece, a fitness professional can read the USG. Normal ranges are from 1.013 to 1.029; a USG of 1.030 or higher suggests dehydration, and 1.001 to 1.012 may indicate overhydration (2). USG is more indicative of recent fluid consumption versus overall chronic hydration status (8); however, it can be used in conjunction with other practical measures of hydration status such as changes in body weight (19,38). To obtain accurate information, weight should be measured on waking on three successive days, after voiding, and before consumption of any fluids (3,38). If fluctuations exceed more than 1% from baseline, water imbalance may be present (3). Although subjective, urine color can be a marker of hydration status when used in combination with a more quantifiable method such as USG (6,8,38). A person’s urine sample is compared with a color chart that identifies euhydration or the need to consume additional fluids (8,32). A lighter color indicates adequate hydration, whereas darker colors indicate the need for fluid consumption. However, diet, supplements, and medications can affect body weight and urine color (19,32); thus, these factors must be considered when using this method.


Cheuvront and Sawka (11) suggest that athletes use the WUT framework (available at: http://, which takes into account not only body mass but also the degree of thirst and urine parameters. In addition, a client’s usual fluid intake can be measured using the beverage intake questionnaire (BEVQ-15) (24), which can be administered rapidly by the practitioner (∼3 to 4 minutes) to provide a valid and reliable estimate of total beverage intake (including water, juice, and sports drinks) in terms of volume and calories (24). Although there are several measures to estimate hydration status, all have limitations (3); using multiple methods may allow the health and fitness professional to obtain the most accurate assessment of a client’s hydration status (5,6,8,38).


Water needs can vary from person to person — and no one person will need the same amount of fluid from one day to the next, thus,developing a recommended dietary allowance for water is challenging. The Institute of Medicine (IOM) established an Adequate Intake (AI) for water, which is a guideline to help most healthy individuals avoid dehydration (25,26). Table 1 outlines the AI for total water and total fluid intake for various age groups. On average, Americans typically consume about 1 L (approximately four cups) of drinking water per day (40).

The Institute of Medicine’s Water Intake Recommendation*

Whereas the AI addresses water needs of the general public, the health and fitness professional must consider an individual’s physical activity regimen and environment when assessing hydration needs (25,38). ACSM’s Exercise and Fluid Replacement guidelines can be used when counseling clients on appropriate hydration strategies to avoid dehydration and overhydration. Dehydration can impact physical performance negatively (25,34,38), and the magnitude of decrements in physical performance may be influenced by fitness level, environmental acclimatization, and mode of activity (25,38). As the level of dehydration increases, physical performance decreases — that is, performance suffers with greater degrees of dehydration (25) — and recent literature suggests the same for cognitive performance (9,36,41).


Cognition refers to the process or act of knowing — a person’s awareness and judgment. Cognitive functions can include a person’s concentration or attentiveness, concept learning, critical thinking, and memory (39). Likewise, motivation, mood, arousal, and physical health affect cognitive processing (39). Cognitive performance is a measure of cognitive function (39), or how someone uses their judgment, memory, reasoning, and concentration to complete one or more tasks. Many tests exist to measure cognitive performance; however, debate on which assessment method is superior persists among practitioners (29). There are few standardized assessment methods, and causal mechanisms as to how dehydration may impact cognitive performance are unknown (30,35).

The degree of precision and/or rapidity of a response commonly is evaluated in cognitive performance assessments (39). For example, the time it takes for someone to respond to a visual stimulus would measure speed/reaction time and a word recall would measure an individual’s cognitive accuracy. Table 2 provides definitions of common terminology and examples of cognitive performance assessment methods.

Common Measures and Methods of Cognitive Performance Assessment

Two cognitive assessments that may be of practical use for the fitness professional are the ruler drop test and Trail Making Test (TMT) A and B (1,14,42,44). To conduct a ruler drop test, the practitioner holds a ruler vertically hovering above the outstretched dominant hand’s index finger and thumb of a client. The 0-cm line of the ruler is parallel to the client’s thumb. The client catches the ruler following the practitioner “dropping” it without notification. The distance is recorded and converted into a reaction time or interpreted as follows: poor, greater than 28 cm; below average, 20.4 to 28 cm; average, 15.9 to 20.4 cm; above average, 7.5 to 15.9 cm; and excellent, less than 7.5 cm (14). The TMT can measure vigilance and consists of form “A” on which a client is asked to connect 25 randomly placed numbers in sequence with a pen/pencil (42). The second form (i.e., “B”) is similar to the first except, in addition to numbers, alphabetical letters are incorporated (42). For example, a client would have to connect the number 1 with letter A and connect letter A to number 2, which would then be connected to letter B, and letter B would be connected to number 3, and so on (42). The outcome measure is time to completion, and mistakes do not stop timing. Average score for form A is 29 seconds, with scores greater than 78 seconds considered below average (1). For form B, average score is 75 seconds, and below average is 273 seconds (1).

Because of the complexity involved with cognitive processes, a battery of assessments should be administered to obtain the most accurate analysis (45). One such battery is called the Montreal Cognitive Assessment 7.1 (MoCA) (10). This screening tool was developed to assess mild cognitive impairment and early Alzheimer dementia through attention and concentration, executive functions, memory, language, visuoconstructional skills, conceptual thinking, calculations, and orientation analyses (10). The MoCA can be administered in approximately 10 minutes, and a normal score is considered 26 of 30; however, scores of 24 may be acceptable (10). Although the tool was tested extensively in adults aged 49 years or older, it also can detect mild cognitive impairments in younger active individuals (16). The health and fitness professional may find the MoCA useful because of its rapid administration and scoring; however, if clients participate in contact sports or have experienced a concussion in years past, scores may be lower than suggested norms (16). The test, administration, and scoring instructions can be found at



Cognitive performance had been reported previously to decline at or above a 2% body water loss (22,25). The level of reduction in cognitive performance can depend on environmental and individual factors (e.g., level of fitness, acclimatization, and dehydration tolerance) (41), and it appears that, as the level of dehydration increases, efficiency of cognitive processing decreases (36). In long-distance walkers and runners, increased water intake has been associated with increased visual attentiveness and short-term memory (9). In women, aspects of mood (i.e., vigor, alertness, fatigue, calmness, confusion, happiness) were affected negatively during fluid deprivation (36). Children also may have decrements in cognitive function as a result of inadequate water intake (20).

Recently, we have learned that even mild dehydration — a bodywater loss of 1% to 2% — can impair cognitive abilities (4,21). This amount of dehydration equates to about 1.5 to 3 lbs of body weight loss for a 150-lb person, which could occur through routine daily activities (4). Because many individuals experience fatigue later in the day when their workout time approaches, this could be important for fitness professionals to discuss with their clients. Problems with cognitive performance that can occur with mild dehydration include poor concentration, increased reaction time, and short-term memory problems, as well as moodiness and anxiety (4,21). Water consumption affects cognitive performance in adults (18), and an adequate daily water intake is important for maintaining optimal cognitive function.

Most studies on hydration and cognitive performance are short-term (i.e., hours, days), and it is not certain if there are longer-term cognitive decrements resulting from hypohydration; however, a recent study suggests that, even after replenishing a fluid deficit, effects on mood may persist (36). Meaning, even after achieving euhydration, cognitive function may be compromised. This is an area in need of additional research.


Marion is a 38-year-old mother of 3 who works a full-time job from 8 a.m. to 4 p.m. 5 days a week. The BEVQ-15 revealed that she typically consumes approximately 2,700 mL of fluid from beverages daily. You often train her at your gym in the afternoons, and, on Monday, she came to you after successfully completing a 10K on Saturday and spending Sunday gardening and doing yard work with her family. It is the middle of August and when she shows up for her training session, she was stating how tired and lethargic she feels, that she has been making careless mistakes at work, that her head has been “pounding all day,” and that she almost canceled the training session because she felt nauseous driving over from work. Marion’s baseline body weight is 150 lbs and, from Friday on, her weights are as follows:

Friday: 150 lbs (0% change from baseline)

Saturday (race day): 151 lbs (1% increase frombaseline)

Sunday: 149 lbs (<1% decrease from baseline)

Monday: 145 lbs (∼3% decrease from baseline)

You question Marion about her fluid intake, and you find that her focus has not really been on hydration since finishing the race on Saturday. After obtaining a urine sample that was dark yellow, you analyze Marion’s USG, which was 1.033. Suspecting that her fatigue and mistakes at work may be signs of compromised cognitive function, you administer the MoCA (10), and her score is 22. Marion’s physical and cognitive signs and symptoms suggest that she is dehydrated, and her recent decreased morning body weight, USG, MoCA results, and urine color all confirm this. You provide Marion with guidelines for rehydration according to ACSM (38) and make plans to follow up with her tomorrow afternoon.


Clients may experience mild dehydration — a 1% to 2% water loss — during routine daily activities (4,21,36). This may be a common problem, considering that adults drink only 1 L (∼4 cups) of water a day on average (40), which is less than half of what is recommended currently by the IOM (25). The signs and symptoms of dehydration and overhydration can mirror each other, sharing light-headedness/dizziness, headaches, nausea, and fatigue — all subjective parameters sometimes used in hydration and cognition research (4,21,22,30,36). When working with clients, health and fitness professionals can use a number of means to assess water imbalances (e.g., USG, body weight, and urine color) to ensure that clients receive proper treatment. In addition, fitness professionals can educate their clients on monitoring their own hydration status through morning body weight and the WUT framework (11), and when counseling patients on fluid needs before, during, and after exercise, the health and fitness professional can use the ACSM Exercise and Fluid Replacement guidelines (38). However, if a client has a chronic medical condition such as hypertension, cardiovascular disease, or diabetes, referring them to a registered dietitian for a personalized hydration plan may be necessary.

Cognitive functions, such as concentration, vigilance, memory, and critical thinking can be measured through a variety of cognitive performance assessments. Although there is no consensus as to which method of assessment is superior (29), tests such as the ruler drop test, Trail Making Test A and B (42), and MoCA (10) may be practical means for the health and fitness professional to assess rapidly a client’s cognitive processing. Similar to physical performance, cognitive performance has been observed to decline at levels more than 2%body water loss (22,25), but recent research shows that mild dehydration (i.e., 1% to 2% body water loss) may impair cognitive performance (4,21). Current literature provides insight into how cognitive function may be influenced by hydration status. However, the long-term consequences of dehydration on cognitive parameters and the mechanism by which fluid imbalances impact cognitive performance are unknown (35) — areas where future research efforts are needed.


Water is a crucial nutrient, and euhydration is necessary for optimal daily function. Water balance is regulated precisely within the body, and many methods exist for assessing hydration status. Cognitive performance measures an individual’s attentiveness, critical thinking skills, and memory. Traditionally, a 2% or more body water deficit was thought to produce cognitive performance decrements; however, recent literature suggests that even mild dehydration — a body water loss of 1% to 2% — can impair cognitive performance. Counseling clients about their health and well-being should include conveying the importance of water for normal body function, as well as its effects on physical and cognitive performance.


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      Fluid; Cognitive Function; Hydration Assessment; Water Intake; Mood

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