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.

Intervention to diminish dehydration and kidney damage among sugarcane workers

“…Although other causal factors have been proposed to have a role in the MeN epidemic [ie, pesticides (22,23), nephrotoxic medication, alcohol nonsteroidal anti-inflammatory drugs (NSAID) etc] to date none have been shown to be a major cause of MeN (24). The known detrimental effect of heat strain and dehydration on work performance (25,26) plus the current causal evidence relating MeN, chronic heat stress, and dehydration demands attention.…”

Thermal stress, human performance, and physical employment standards

“…Exercise-associated hyponatremia is defined as having a plasma sodium concentration below 135 mmol·L −1 within 24 h of physical activity (Hew-Butler et al 2015). It is noteworthy 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 (Cotter et al 2014). Moreover, the thirst mechanism in-built in humans cannot completely prevent dehydration (US Army 2003).…”

Fluid Intake Habits in Type 1 Diabetes Individuals during Typical Training Bouts

“…However, these guidelines have been designed for healthy athletes, and should be applied with some caution for specific cases in clinical populations, like those with diabetes. The latest National Athletic Trainers’ Association position statement suggests considering fluid replacement volume based on individual fluid losses to avoid a 2% or higher BM loss [18]; there is currently ongoing debate in the literature whether 2% BM loss inevitably leads to reduced performance, or whether people should follow the dictates of thirst [19]. However, recent results recommend preventing BM losses greater than the commonly accepted cut off of 2%, to optimize fluid replacement, as thirst sensation may be insufficient [20-22].…”

“…Certainly, the optimal fluid replacement protocol to preserve euhydration during exercise is heavily debated. The most commonly referred strategies are as follows: (i) replacing fluid losses in order to avoid a 2% BM loss or (ii) following dictates of thirst [19, 31-33]; nevertheless, drinking to thirst has often been found insufficient to prevent dehydration [21, 34, 35], while fluid loss estimated through BM change was shown to impair endurance performance in a blinded study [22]. To date, the influence that T1DM may exert on thirst perception (i.e., the capacity of stimulating thirst depending on changes in blood osmolality), due to chronic fluctuations in blood glucose and osmolality, has not been fully defined, although high glucose levels increasing blood osmolality are likely to signal for increased thirst sensation [36, 37].…”