Printer Friendly, PDF & Email


“Drink a lot of water even before you feel thirsty or in quantities indicated by your doctor.” (Gouvernement du Québec, Heatwave, 2007)

Staying Hydrated

For those who are extremely hot (as a result of environmental conditions or strenuous exercise), the single most important precaution they can take to prevent the development of a heat-related illness (apart from getting out of the heat, for those who are environmentally exposed) is to maintain the body’s water level.1 The best way to do this is to drink water at a rate equivalent to its loss.2

Currently, many public messages, recommending the appropriate quantity and frequency of water consumption during extreme heat, are often confusing or misleading.3


Under conditions of extreme body heat, the major thermoregulatory mechanism (method the body uses to keep its core temperature within acceptable limits) is sweating. In fact, approximately 90% of the body’s ability to cool itself relies on the evaporation of sweat from the body.4 This translates into a considerable cooling effect.

Sweating also results in water loss from the body. This varies, not only with the degree of heat experienced by the body but also with the degree of acclimatization (see earlier article: Acclimatization). With acclimatization, the physiological changes that occur include a more conservative (decreased fluid volume and salt content) sweating mechanism.5, 6 Such factors as activity level, type of clothing, opportunity to cool off periodically are also important considerations.


Dehydration impairs the body’s ability to maintain optimal core temperature. Many organs depend on water to sustain membrane gradients of ions in the cells to maintain appropriate electrical activity and to support normal cell function. Changes in ion and electrolyte concentrations can interrupt cardiac muscle contractions, cause muscle spasms7, gastro-intestinal disturbances, and lead to problems with the normal functioning of muscles and nerve cells.8

One key point to note is that the sense of thirst (particularly in the elderly) is not a useful index of the need to consume water and become rehydrated. At a water loss corresponding to a reduction in body weight by 1%, there are already consequences, such as, increased strain on the heart as a result of pumping blood that is more viscous. This could be a significant factor in direct heat-related illnesses, such as, heat exhaustion or indirect heat-related causes of death such as heart attack.

Water and thirst

During extreme heat events or vigorous exertion, the consumption of water before feeling thirsty should be encouraged. Once thirst is perceived, a person is already suffering from mild dehydration (Table 1).

Table 1: Progression of dehydration by percent of total water loss

1%already classified as mild dehydration and characterized by reduced blood
volume and compromised cardiovascular capacity
2%dehydration is well established9 and is classified by marked thirst6
4%dry mouth and throat6
5%shown to impair work abilities10
6-8%loss of saliva production, speech becomes difficult and large increases in
heart rate6
10%mental incapacitation6
12%no longer able to swallow and cannot recover without medical assistance6

It is not until the body has lost about 2% of its weight as water that the sense of thirst develops. It is important, when hot, to attempt to consume water at a rate that corresponds to the degree of sweating; obviously much greater in someone exerting themselves outdoors, in the sun, than in someone who is sedentary in a cool environment. The American Conference of Industrial Hygienists (ACGIH) recommends that physically fit workers consume a cup of cool water every 20 minutes as a control against heat stress11, but this recommendation would not apply to those who are sedentary.   

One of the few studies on water replacement needs in high temperature environments was conducted by Greenleaf and Sargent in 1964.12 They studied extremely fit men who were acclimatized to 49 °C and found that, although heat was the largest predictor of water consumption, voluntary consumption rates were insufficient to maintain hydration. Subjects, with free access to water, did not drink enough to compensate for water loss and frequently did not feel thirst, despite losing an average of 6% of their body mass to dehydration. When water loss was measured and subjects consumed water equivalent to their loss rate, they reported feeling less exhausted.

How much water should a person drink to make up for sweat losses?

This question could be answered by regular weighing on precise scales, for example, a 1% loss in body weight for a 70 kg person is 0.7 kg. This may be an onerous or impractical task, suggesting a need for a simple metric to estimate hydration needs. The rate of water loss varies tremendously from one individual to another. For any one person, this could be determined mathematically, from knowledge of their metabolic/work rate, ambient temperature and humidity, clothing, degree of acclimatization, etc. Although this may be possible to determine for an individual, it is not practical from a public health perspective. It would be useful to have a general guide or simple equation that people could use to estimate water needs during hot weather (e.g., the hydration equivalent to Canada’s Food Guide to Healthy Eating)


Drinking too much water can also be harmful for those who have medical conditions (e.g., renal and heart failure patients) that require limiting water intake and for the general population. Overconsumption of water, in those who are physically healthy (may happen in certain sports events or with certain psychiatric conditions), causes a decrease in plasma electrolyte concentrations13, resulting in hyponatremia.14, 15 Hyponatremia is the state of low plasma sodium levels[16] which, by disrupting the osmotic balance across the blood brain barrier, results in a rapid influx of water to the brain17 and may lead to health complications and even death.18 Deaths have been reported in the literature, following rapid consumption of large quantities (approximately 15 l) of water.19

In addition to direct consumption of water, foods can contribute to hydration status. The water content of foods ranges from almost nothing for some foods (e.g., approximately 1–2% for roasted nuts and cereals) to nearly 100% for others (e.g., cucumbers, lettuce, and watermelons contain more than 90% water).20 Information regarding the nutrient content of most foods can be accessed through the USDA Nutrient Database for Standard Reference.20


It is important that heat-exposed individuals maintain their body stores of water. Apart from getting out of the heat, drinking water, particularly before feeling thirsty, is the single most important thing people can do to mitigate the risk of experiencing a heat illness. However, precise recommendations are difficult to make and additional tools are needed to provide practical, evidence-informed public health advice.

Gaps and questions

  • Apart from recommending they weigh themselves regularly, how should members of the general population be advised on how to maintain body water stores below a loss of 1–2%?
  • We have the science, but not the tools; should it not be possible to develop a series of tables, showing work rate (with general descriptors of the work), clothing, ambient temperature, ambient humidity, etc., and include the incremental water consumption requirements per unit time for each variable?
  • What is the impact of high humidity on water needs during hot weather? Humid air decreases evaporative cooling (when water drips from the body rather than evaporating), but does it affect the degree of water loss?
  • What are the advantages (and disadvantages) of beverages, other than water? (see upcoming article on this topic)


  1. Buono MJ, Wall AJ. Effect of hypohydration on core temperature during exercise in temperate and hot environments. Pflugers Arch. 2000;440(3):476-80.
  2. Miller VS, Bates GP. Hydration, hydration, hydration. Ann Occup Hyg. 2010;54(2):134-6.
  3. Horswill CA, Stofan JR, Lovett SC, Hannasch C. Core temperature and metabolic responses after carbohydrate intake during exercise at 30 degrees C. J Athl Train. 2008;43(6):585-91.
  4. Frisancho AR. Human adaptation: A functional interpretation. Ann Arbor: University of Michigan Press; 1981.
  5. Buono MJ, Ball KD, Kolkhorst FW. Sodium ion concentration vs. sweat rate relationship in humans. J Appl Physiol. 2007;103(3):990-4.
  6. Wheeler D. Temperature regulation. Surgery. 2006;24(12):446-51.
  7. Stofan JR, Zachwieja JJ, Horswill CA, Murray R, Anderson SA, Eichner ER. Sweat and sodium losses in NCAA football players: a precursor to heat cramps? Int J Sport Nutr Exerc Metab. 2005;15(6):641-52.
  8. Parsons K. Heat stress standard ISO 7243 and its global application. Ind Health. 2006;44(3):368-79.
  9. Armstrong LE, Casa DJ, Millard-Stafford M, Moran DS, Pyne SW, Roberts WO. American College of Sports Medicine position stand. Exertional heat illness during training and competition. Med Sci Sports Exerc. 2007;39(3):556-72.
  10. Byock I. Patient refusal of nutrition and hydration: Walking the ever-finer line. Am J Hosp Palliat Care. 1995;12(2):8, 9-13.
  11. American Conference of Governmental Industrial Hygienists. Heat stress and strain: TLV physical agents - Documentation. 7th ed. Cincinnati: ACGIH; 2009.
  12. Greenleaf JE, Sargent F, 2nd. Voluntary dehydration in man. J Appl Physiol. 1965;20(4):719-24.
  13. Noakes TD, Speedy DB. Case proven: Exercise associated hyponatraemia is due to overdrinking. So why did it take 20 years before the original evidence was accepted? Br J Sports Med. 2006;40(7):567-72.
  14. Rodriguez NR, DiMarco NM, Langley S. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. J Am Diet Assoc. 2009;109(3):509-27.
  15. Rodriguez NR, Di Marco NM, Langley S. American College of Sports Medicine position stand. Nutrition and athletic performance. Med Sci Sports Exerc. 2009;41(3):709-31.
  16. Almond CSD, Shin AY, Fortescue EB, Mannix RC, Wypij D, Binstadt BA, et al. Hyponatremia among runners in the Boston marathon. N Engl J Med. 2005;352(15):1550-6.
  17. Maughan RJ, Griffin J. Caffeine ingestion and fluid balance: A review. J Hum Nutr Diet. 2003;16(6):411-20.
  18. Yeates KE, Singer M, Morton AR. Salt and water: A simple approach to hyponatremia. CMAJ. 2004;170(3):365-9.
  19. Anastassiades E, Wilson R, Stewart JSW, Perkin GD. Fatal brain oedema due to accidental water intoxication. Br Med J. 1983;287(6400):1181-2.
  20. U.S. Department of Agriculture, Agricultural Research Service. USDA National Nutrient Database for Standard Reference, Release 22. Nutrient Data Laboratory Home Page. 2009 [cited 2010 May 4].

May 2010