The finding of our study (Szinnai et al., 2005) cited by Jéquier and Constant (2010) does not support their conclusion that: ‘Severe dehydration therefore affects the function of many systems and is a life-threatening condition’. Instead we found that cognitive-motor function was preserved up to a dehydration level of 2.6% of body weight (BW).
A number of other erroneous concepts are included in their article. We limit ourselves to the most important.
The authors emphasize ‘the necessity of drinking water’. They propose that humans should drink ‘1.5 l of water/day, as water is the only liquid nutrient’ so that ‘voluntary drinking of water is a key behavior for maintaining water balance’. Why do the authors limit their recommendations to water and not to other fluids, including milk, tea, coffee, fruit juices and so on? Limiting the intake of water will disturb fluid balance in ‘populations at particular risk of dehydration, very young and the elderly’ as shown by studies in babies (Brown et al., 1986). There are remarkable ontogenetic and cultural differences in urine osmolality, an indirect marker of hydration, in children and adolescents from Germany (860 mosm/kg) and Poland (392 mosm/kg). But large differences in fluid intake are not associated with adverse health outcomes (Manz and Wentz 2003). In older adults, the relative contributions of beverages to total water intake were 36–41% and coffee was the predominant water source (Zizza et al., 2009).
The authors propose that ‘elderly subjects are at risk of dehydration’. This statement applies to the frail elderly adults, but healthy older adults maintain water balance comparable to those of younger adults (Bossingham et al., 2005). But elderly subjects are prone to the development of hyponatremia, as they have a reduced capacity to excrete a water load. It is surprising that this review focuses exclusively on dehydration and hypernatremia, whereas the established risks of over hydration leading to water intoxication and hyponatremia are ignored.
The authors state that ‘the need to drink hypotonic drinks during endurance exercise is well established’ but they do not reference this claim. Yet exercise-associated hyponatremia that was first described more than 25 years ago (Noakes et al., 1985) occurs as a result of the excessive intake of water or hypotonic fluids associated with inappropriate arginine vasopressin secretion, which limits renal-free water clearance. As severe hyponatremia associated with cerebral edema is life threatening, any review of hydration must explain how to prevent overdrinking during exercise.
Finally, the authors state that: ‘Drinking water before being thirsty is a good habit for maintaining a good body hydration’. This popular mantra is not evidence based. Advice to ‘Drink water before being thirsty’ or to drink ‘as much as tolerable’ during exercise overemphasized the risk of dehydration but ignore the risk of hyperhydration. The International Marathon Medical Directors Association advises athletes to drink only in response to thirst, and a recent meta-analysis study has found that drinking either more or less than according to the dictates of thirst causes an impaired exercise performance (Goulet, 2011).
Bossingham MJ, Carnell NS, Campbell WW (2005). Water balance, hydration status, and fat-free mass hydration in younger and older adults. Am J Clin Nutr 81, 1342–1350.
Brown KH, Creed de Kanashiro H, del Aguila R, Lopez de Romana G, Black RE (1986). Milk consumption and hydration status of exclusively breast-fed infants in a warm climate. J Pediatr 108 (5 Part 1), 677–680.
Goulet EDB (2011). Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. Brit J Sport Med (in press).
Jéquier E, Constant F (2010). Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr 64, 115–123.
Manz F, Wentz A (2003). 24-h hydration status: parameters, epidemiology and recommendations. Eur J Clin Nutr 57 (Suppl 2), S10–S18.
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Szinnai G, Schachinger H, Arnaud MJ, Linder L, Keller U (2005). Effect of water deprivation on cognitive-motor performance in healthy men and women. Am J Physiol Regul Integr Comp Physiol 289, R275–R280.
Zizza CA, Ellison KJ, Wernette CM (2009). Total water intakes of community-living middle-old and oldest-old adults. J Gerontol A Biol Sci Med Sci 64, 481–486.
The authors declare no conflict of interest.
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Arnaud, M., Noakes, T. Should humans be encouraged to drink water to excess?. Eur J Clin Nutr 65, 875–876 (2011). https://doi.org/10.1038/ejcn.2011.40
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