In simple terms, dehydration means that the body does not contain as much water as it should. It can be quantified as a 1 per cent or greater loss of body weight as a result of fluid loss.(1,2) Dehydration can result from losing too much fluid, such as through sweating, or excessive urine production in diabetes; not drinking enough; vomiting; and diarrhoea.(3)
Dehydration is classified as mild, moderate, or severe based on how much fluid is lost or not replenished. Even mild dehydration, between 1-3 per cent loss of body weight, can impair physiological and performance responses. Moderate dehydration in excess of 4-9% of body weight decreases endurance and strength and is the primary cause of heat exhaustion.(4) A loss of more than 10% of body weight can become life-threatening. Clinical signs include dizziness, lethargy, agitation, irritability, restlessness, and confusion.(5,6)
Involuntary dehydration is a delay in full restoration of a body water deficit by drinking. It occurs when individuals are exposed to various stresses, including exercise, environmental heat and cold, altitude and water immersion. Involuntary dehydration appears to be controlled by various factors including social customs that influence what is consumed, the capacity and rate of fluid absorption from the gastrointestinal system, and the level of cellular hydration.(7)
Voluntary dehydration is a state of water deficit where fluids have been ingested to the point where the sensations of thirst are no longer strong enough to provide a drive to drink which is sufficient to fully replace the water loss.(6,8)
Early signs of dehydration include light-headedness, dizziness, tiredness, irritability, headache, sunken features (particularly the eyes), loss of appetite, flushed skin, heat intolerance, dry mouth and throat, chapped and dry lips, dry eyes, skin that becomes loose and lacks elasticity. There maybe a burning sensation in the stomach, the feeling of an ‘empty stomach’ or even abdominal pain. Urine output will be reduced, and the urine will be dark in colour and have a strong odour.(4,9,10,11,12) In infants, the fontanelles (the soft spot on the top of the head) may be markedly sunken in.(3)
Physiological thirst results from dehydration and is stimulated by increases in cellular tonicity (cellular dehydration) and decreases in extracellular fluid volume (reduction in the amount of total body water, or extracellular dehydration). The osmolality and volume of the extracellular fluid (ie blood) are directly controlled by the body and influence the osmolality and volumes of the other compartments (both intracellular and extracellular).(13)
Osmoreceptors in the brain (anterior hypothalamus, near the supraoptic nuclei) monitor changes in plasma osmolality. There are no sensors that can detect the amount of total body water, or the circulating volume. However, since volume and pressure are directly related, the body measures pressure changes in the form of vascular stretch (the fullness of the blood vessels) at various sites and uses this to monitor the effective circulating volume. Arterial blood pressure is monitored by baroreceptors in the carotid sinus and aortic arch. Stretch receptors in the right atrium of the heart monitor venous pressure and fluctuations in kidney perfusion are monitored by stretch receptors in the renal afferent arteriole.(10,13,14) Despite our understanding of the brain structures and receptors involved in thirst, little is known about how the messages to initiate thirst are transmitted to the brain.(15)
Once detected, the body’s response to dehydration involves the hypersecretion of antidiuretic hormone (ADH, or vasopressin) from the brain. This increase in plasma ADH acts first on the kidneys, which respond by excreting less water. For an adult, urine has a typical osmolality of 1200 mosmol/litre (although this can range from 40 to 1400 mosmol/litre). Infants urine is more dilute, with a typical value of about 700 mosmol/l, because they have a reduced capacity to concentrate urine.(9)
The main constituents of urine are the nitrogen-containing breakdown products of protein metabolism (principally urea) and sodium chloride, plus other substances, eg sulfates, phosphates and other electrolytes. In order to excrete these waste products (the solute load) a minimum amount of water is needed by the kidneys. This means that kidneys have an obligatory water requirement below which they cannot function properly, and this limits the extent to which they can concentrate the urine, in order to save water.(9)
Thirst is triggered when the kidneys cannot make the urine any more concentrated in order to prevent an increase in plasma sodium and osmolality. Functional magnetic resonance imaging in humans indicates that activity in the rostral anterior cingulate cortex area of the brain appears to reflect thirst level.(16)
The threshold for the induction of thirst occurs at a point where a person is slightly dehydrated to a level of 0.8 to 2% loss of body weight (as a result of water loss) at which stage the average plasma osmolality is 294 mosmol/kg. The average normal plasma osmolality is 287 mosmol/kg (range 280-296 mosmol/kg).(9,17) The threshold for release of ADH (284.7 mosmol/kg) is lower than that governing the desire to drink (294 mosmol/kg). Thus fine adjustments in water balance are met by changes in plasma ADH concentration and consequent changes in urine output, whilst thirst and resultant water intake are invoked at a later point.(18)
In addition to the homeostatic mechanisms mentioned above, there are also numerous psychological and environmental influences that add to the stimulation of thirst. These include availability of the beverage; taste, temperature, and palatability of the beverage; knowledge about the importance of proper hydration; drinking in association with meals; social and societal norms and habits; and even the presence of others who are consuming a beverage.(10)
Given free access to fluid, humans become thirsty and drink sufficient fluid to restore water balance. This may be simply because of habit or social norm or in response to subtle oropharyngeal cues, such as the act of swallowing, which helps regulate the volume of fluid ingested by providing information to the brain on the amount that has been drunk.(10) However, at times of stress the body becomes insensitive to thirst. This lack of thirst leads to voluntary dehydration, were there is a delay in complete rehydration following body water loss.(19)
Few studies have been performed to investigate the thirst in children. Studies that are available have tended to focus on the influence of flavourings, sodium content, palatability and drink composition, all of which can affect the volume of fluids voluntarily consumed.(20)
Other factors that may add to the stimulation of thirst in children include learned behaviour and social aspects, such as drinking in association with meals and as part of daily routines. Depending on age and ability, children and infants need varying degrees of support in order to maintain an adequate hydration level. They may feel thirsty, but have limited capacity to express their need, or to access drinks.
In addition, children may not recognise the signals of thirst as readily as adults and should be encouraged to drink at regular intervals. Since drinking patterns are established in childhood, this encouragement helps children to develop healthy drinking habits which will be of benefit throughout their lives.(21) A survey of 2-7 year olds drinking habits, found that fruit squash was the most frequently consumed drink and had replaced plain drinking water especially among the pre-school age group. Children as young as 2 years had been conditioned to the sweet taste of squash, to the extent that they refuse to drink water as an alternative.(22)
Voluntary dehydration is a condition in which people, especially children, do not drink appropriately in the presence of adequate fluid availability. Children living in a hot climate are especially susceptible to developing voluntary dehydration. In hot climates the threshold for thirst corresponds to Uosm of about 800 mosm/kg H2O. This means that thirst is only felt when the urine is already highly concentrated and the body is dehydrated. In one study, seventy per cent of school-age children in southern Israel were found to be in a state of chronic dehydration.(23) In another study comparing Jewish children with a Bedouin group, over 80 per cent of the Jewish group were found to be chronically dehydrated (defined as Usom exceeding 800 mosm/kg H2O), compared to only 50 per cent of the Bedouin group. It is possible that the Bedouins, who have lived in a very hot climate of the desert for many generations, may have developed a mechanism that lowers the thirst threshold, or that they are more aware of the importance of drinking water even without feeling thirsty. Children who are exercising in a hot climate are also susceptible to voluntary dehydration. A group of 10-12 year olds, those who were allowed to drink when thirsty, showed a progressively increasing fluid loss of 0.3 per cent of body weight per hour, due to insufficient drinking when exercising in the heat. On average they drank only 72 per cent of the amount required to replace fluid loss.(24)
There is no universally accepted laboratory method to characterise individual hydration status. Water requirement vary between and within individuals depending on several factors, including climate, physical activity, diet and renal solute load, and body composition, especially the fat free mass (which contains most of the body’s water). (25,26) Small changes in body hydration levels are particularly difficult to detect because the body makes constant adjustments to maintain water balance.(19) In addition, opinions vary regarding definitions of differences in hydration status, which may be referred to as dehydration, hypohydration, euhydration or hyperhydration.
Nevertheless, there are many ways of measuring the hydration status, or the level of dehydration. These include dilution techniques; neutron activation analysis; bioelectrical impedance; haematocrit and haemoglobin analysis; use of tracers to measure fluid absorption and equilibration; plasma and serum osmolality measures; body mass difference; urinary indices (volume, colour, protein content, specific gravity and osmolality); and perceptual rating of thirst.(27)
For laboratory assessment, the chosen test needs to be precise, accurate and reliable. Other factors that must be taken into account when choosing which test to perform are the availability of technical expertise, time and financial resources. Where these criteria are met, isotope dilution techniques, neutron activation analysis, osmometry, and measurements of haematocrit and haemoglobin are preferred. To monitor hydration during daily activities, the preferred method should be accurate, safe and inexpensive, such as urine specific gravity, urine colour, and body mass measurement techniques. Some of these methods are described below.
Urine specific gravity
Urine specific gravity refers to the density of the sample in comparison to pure water (which has a specific gravity of 1.000). Normal urine specimens usually range from 1.013 to 1.029 in healthy adults, but exceeds 1.030 during dehydration. Specific gravity can be measured quickly and accurately with a handheld refractometer. The validity and reliability of dip-stick test methods requires further investigation.(27)
Urine osmolality
In an environment where water is freely accessible, urine osmolality (Uosm) in the range of plasma osmolality can be assumed to indicate adequate hydration. Uosm in excess of plasma osmolality suggests water is being conserved by the body, and Uosm below plasma osmolality implies a water surplus. Uosm in 24 hour urine samples can be used, in addition to free water reserve* as a parameter to quantify individual 24 hour euhydration and allow the calculation of Adequate Intake values for total water.(25) Although urine osmolality has been successfully used to monitor hydration status, it may not accurately reflect hydration status when used immediately after exercise, and in addition, large intercultural differences in mean Uosm exist.(27)
*Free water reserve corresponds to the difference between the measured urine volume and the ideal urine volume necessary to excrete the actual 24 hour urine solutes at a mean –2s.d. value of maximum Uosm.
Urine colour
Reduced urine output is as a useful indicator of dehydration and signals the need for increased water intake.(9) Although urine specific gravity and osmolality are effective methods for indicating moderate levels of dehydration, the particular test chosen depends on the sensitivity and accuracy with which hydration status needs to be known, as well as the technical requirements and expense involved.(28,29)
Urine colour has been shown to be at least as good an indicator of hydration as plasma or urine osmolality or urine specific gravity.(30,31) It also indicates body water loss as effectively as plasma osmolality, plasma total protein concentration, or plasma sodium concentration.(31) Armstrong et al(32) devised a standardized colour reference chart for urine colour using a numbered scale including colours ranging from very pale yellow (number 1) to brownish green (number 8). This was used by athletes to determine whether they were well hydrated, euhydrated or hypohydrated. Individuals who maintained a pale yellow urine colour always were within 1 per cent of their baseline euhydrated body mass. Individuals should seek to produce urine that is "very pale yellow", "pale yellow" or "straw coloured" to indicate that they are well hydrated. It has been noted, however, that without a colour chart, colour interpretation can be confusing, for example moderately yellow might be interpreted as "dark" when contrasted against "pale yellow" or "clear".(18) Urine colour is not, however, a foolproof method of assessment, but it is likely to be effective in athletic and industrial settings that do not require high precision.(27)
Body mass
Changes in body mass (or weight) are often used to detect changes in hydration status. For individuals consuming an adequate diet, any acute change in body weight will result form changes in total body water (ie when corrected for the mass of fluid and food intake, urine and faecal losses, and sweat loss), because no other body constituent is lost at a similar rate. Serial measurement of body weight is therefore a sensitive, accurate, straightforward, and cost-effective indicator of hydration status, especially during dehydration that occurs over a period of 1 to 4 hours, with or without exercise. Using this method, levels of dehydration can be detected from as little as a 1% loss of body weight.(19,27)
Saliva parameters
Saliva is 97-99.5% water, and studies have shown that saliva flow rate, osmolality and total protein concentration track body mass changes during progressive acute dehydration.(33) In particular, changes in saliva osmolality and total protein concentration during dehydration were sensitive to a body mass loss of 2.1% and were as sensitive as urine osmolality at tracking changes in acute hydration status during hypertonic-hypovolaemia. Changes in saliva flow rate during dehydration were less sensitive to whole body water losses. Further work is needed to show whether saliva parameters are sensitive to both hypertonic and isotonic hypovolaemia, during acute and chronic dehydration.(34)
Assessment of dehydration in children
It is important to be able to assess the degree of dehydration quickly and accurately in infants and young children. Although most parents understand what dehydration is, one study found that only about two-thirds could identify more than one sign of dehydration in their children.(35) Nevertheless, parents' report of history and observations for children with suspected dehydration can be valuable and help healthcare staff to predict the severity and outcomes of the dehydration episode.(36) Because there is no reliable way of clinically measuring dehydration, laboratory tests are often used. There can, however, be a discrepancy between clinical assessment of dehydration and laboratory parameters which confirm the condition. Anormal anion gap and urea concentration have been shown to be reliable laboratory method for assessing dehydration in children, whereas creatinine concentrations and mean pH can show similar values regardless of whether dehydration is present or not.(37) A review of the methods available for assessing dehydration in infants and children concluded that the most useful individuals signs for predicting 5% dehydration were an abnormal capillary refill time, abnormal skin turgor and abnormal breathing pattern. Using a combination of examination signs was even more accurate. Laboratory tests and other indicators were less useful in estimating the degree of dehydration.(38)
(see also, Older people: detecting dehydration in older people)
Last updated: December 2006
(1) Kristal-Boneh E, Blusman JG, Chaemovitz C, Cassuto U. Improved thermoregulations caused by forced water intake in human desert dwellers. Eur J Appl Physiol. 1988;57:220-224
(2) Brooks GA, Fahey TD. Exercise Physiology: Human Bioenergetics and its applications. New York, NY: John Wiley & Sons 1984
(3) MedlinePlus. Medical encyclopaedia. Dehydration. www.nlm.nih.gov/medlineplus/ency/article/000982.htm
(4) Kleiner SM. Water: An essential but overlooked nutrient. J Am Diet Assoc 1999:99:201-7
(5) Food and Nutrition Board. Recommended Dietary Allowances. 10th ed. Washington DC: National Academy Press 1989
(6) D’Anci KE, Contant F, Rosenberg IH. Hydration and cognitive function in children. Nutr Rev 2006;64:457-64
(7) Greenleaf JE. Problem: thirst, drinking behaviour, and involuntary dehydration. 1992;24:645-56
(8) Biology-online. Voluntary Dehydration. www.biology-online.org/dictionary/Voluntary_dehydration
(9) Principles of Human Nutrition. Ed M Eastwood. Chapter 8: Water, electrolytes, minerals and trace elements. London: Chapman & Hall 1997
(10) Kenney WL, Chiu P. Influence of age on thirst and fluid intake. Med Sci Sport Exer 2001;33:1524-32
(11) Sherriffs SM, Merson SJ, Fraser SM and Archer DT. The effects of fluid restriction on hydration status and subjective feelings in man. Brit J Nutr 2004;91:951-58
(12) Shah SI, Aurangzeb, Khan I, Bhatti AM and Khan AA. Dehydration related to abdominal pain. J Coll Physicians Surg Pak 2004;14:14-7
(13) The kidney at a glance. Eds C O’Callaghan and BM Brenner. London: Blackwell Science 2000
(14) Electrolytes, body fluids and acid base balance. Ed R Eccles, London: Edward Arnold 1993
(15) McKinley MJ, Denton DA, Oldfield BJ, De Oliveira LB, Mathai ML. Water intake and the neural correlates of the consciousness of thirst. Seminars in Nephrology 2006;26:249-57
(16) de Araujo IE, Kringelback ML, Rolls ET, McGlone F. Human cortical responses to water in the mouth, and the effects of thirst. J Neurophysiol 2003;90:1865-76
(17) Sansevero AC. Dehydration in the elderly: strategies for prevention and management. Nurse Pract. 1997;22:41-42,51-57,63-72
(18) Valtin H. "Drink at least eight glasses of water a day" Really? Is there scientific evidence for "8x8"? Am J Physiol Regul Integr Comp Physiol. 2002;283:R993-R1004
(19) Grandjean AC, Reimers KJ and Buyckx M. Hydration: issues for the 21st Century. Nutr Rev 2003;61:261-71
(20) Kenney WL, Chiu P. Influence of age on thirst and fluid intake. Med Sci Sport Exer 2001;33:1524-32
(21) Brander N. Information for parents. “Water is cool in schools”. http://www.eric.org.uk/infosheets.doc (accessed 13/12/06)
(22) Petter LPM, Hourihane J O’B and Rolles CJ. Is water out of vogue? A survey of the drinking habits of 2-7 year olds. Arch Dis Child 1995;72:137-140
(23) Bar-David Y, Landau D, Bar-David Z, Pilpel D and Phillip M. Urine osmolality among elementary school children living in a hot climate: implications for dehydration. Amb Child Health 1998;4:393-397
(24) Bar-Or O, Dotan R, Inbar O, Rotshtein A, Zonder H. Volluntary hypohydration in 10- to 12-year old boys. J Applied Physiol 1980;48:104-8
(25) Manz F and Wentz A. 24-h hydration status: parameters, epidemiology and recommendations. Eur J Clin Nutr 2003;57(Suppl 2):S10-S18
(26) Ritz P and Berrut G. The importance of good hydration for day-to-day health. Nutr Rev 2005;63:S6-S13
(27) Armstrong LE. Hydration assessment techniques. Nutr Rev 2005;63(II):S40-54
(28) Shirreffs SM. Markers of hydration status. J Sport Med Phys Fit 2000;40:80-4
(29) Shirreffs SM. Markers of hydration status. Eur J Clin Nutr 2004;57(Suppl 2):S6-9
(30) Kavouras SA. Assessing hydration status. Curr Op Med Nutr Met Care 2002;5:519-24
(31) Wakefield B, Mentes J, Diggelmann L, Culp K. Monitoring hydration status in elderly veterans. Western J Nurs Res 2002;24:132-42
(32) Armstrong LE, Maresh CM, Castellani JW, Bergeron MF, Kenefick RW, LaGrasse KE, Riebe D. Urinary indices of hydration status. Int J Sport Nutr 1994;4:265-279
(33) Walsh NO, Montague JC, Callow N, Rowlands AV. Saliva flow rate, total protein concentration and osmolality as potential markers of whole body hydration status during progressive acute dehydration in humans. Arch Oral Biol 2004;49:149-154
(34) Walsh NP, Laing SJ, Oliver SJ, Montague JC, Walters R and Bilzon JLJ. Saliva parameters as potential indices of hydration status during acute dehydration. Med Sci Sport Exer 2004;36:1535-1542
(35) Gittelman MA, Mahabee-Gittens M, Gonzalez-del-Rey J. Common medical terms defined by parents: Are we speaking the same language? Ped Emerg Care 2004;20:754-758
(36) Porter SC, Fleisher GR, Kohane IS, Mandl KD. The value of parental report for diagnosis and management of dehydration in the emergency department. Ann Emerg Med 2003;4:196-205
(37) Shaoul R, Okev N, Tamir A, Lanir A, Jaffe M. Value of laboratory studies in assessment of dehydration in children. Ann Clin Biochem 2004;41:192-6
(38) Steiner MJ, DeWalt DA, Byerley JS. Is the child dehydrated? JAMA 2004;291:2746-54
