title: Are we being drowned in hydration advice? Thirsty for more?
abstract: Hydration pertains simplistically to body water volume. Functionally, however, hydration is one aspect of fluid regulation that is far more complex, as it involves the homeostatic regulation of total body fluid volume, composition and distribution. Deliberate or pathological alteration of these regulated factors can be disabling or fatal, whereas they are impacted by exercise and by all environmental stressors (e.g. heat, immersion, gravity) both acutely and chronically. For example, dehydration during exercising and environmental heat stress reduces water volume more than electrolyte content, causing hyperosmotic hypohydration. If exercise continues for many hours with access to food and water, composition returns to normal but extracellular volume increases well above baseline (if exercising upright and at low altitude). Repeating bouts of exercise or heat stress does likewise. Dehydration due to physical activity or environmental heat is a routine fluid-regulatory stress. How to gauge such dehydration and - more importantly-what to do about it, are contested heavily within sports medicine and nutrition. Drinking to limit changes in body mass is commonly advocated (to maintain ≤2% reduction), rather than relying on behavioural cues (mainly thirst) because the latter has been deemed too insensitive. This review, as part of the series on moving in extreme environments, critiques the validity, problems and merits of externally versus autonomously controlled fluid-regulatory behaviours, both acutely and chronically. Our contention is that externally advocated hydration policies (especially based on change in body mass with exercise in healthy individuals) have limited merit and are extrapolated and imposed too widely upon society, at the expense of autonomy. More research is warranted to examine whether ad libitum versus avid drinking is beneficial, detrimental or neither in: acute settings; adapting for obligatory dehydration (e.g. elite endurance competition in the heat), and; development of chronic diseases that are associated with an extreme lack of environmental stress.
authors.affiliation: Exercise and Environmental Physiology, School of Physical Education, Sport and Exercise Sciences, Division of Sciences, University of Otago, PO Box 56, Dunedin 9054, New Zealand.
authors.forename: James David
authors.initials: JD
authors.lastname: Cotter
authors.affiliation: Faculté de Médecine, Université de Lorraine, U 1116 -INSERM-UL, 9, Avenue de la forêt de Haye, CS50-184 - 54505 VANDŒUVRE, Les Nancy cedex, France.
authors.forename: Simon N
authors.initials: SN
authors.lastname: Thornton
authors.affiliation: Defence Medical and Environmental Research Institute, DSO National Laboratories, Singapore ; Yong Loo Lin School of Medicine, National University of Singapore, Singapore ; Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore.
authors.forename: Jason Kw
authors.initials: JK
authors.lastname: Lee
authors.affiliation: High Performance Sport New Zealand, Auckland, New Zealand ; Sports Performance Research Institute New Zealand (SPRINZ), Auckland University of Technology, Auckland, New Zealand.
authors.forename: Paul B
authors.initials: PB
authors.lastname: Laursen
keywords: Adaptation; Dehydration; Exercise; Renal; Thirst; Water