Hydration, dehydration, underhydration, optimal hydration: are we barking up the wrong tree?

Abstract

According to the Medical Subject Headings of the US National library of medicine “dehydration is the condition that results from excessive loss of water from a living organism” [1]. Even though dehydration describes the state of body water deficit, some scientists have suggested that dehydration refers to the process of losing water, while hypohydration is the state of water deficit, and rehydration is the process of gaining water from a hypohydrated state towards euhydration [2]. To define dehydration or hypohydration in a laboratory setting, scientists have been using acute changes of body weight as the gold standard [3]. For instance, if someone weighs 70 kg in a euhydrated state, the acute loss of − 1.4 kg is equivalent to dehydration of − 2% of body weight (− 1.4 kg/70 kg × 100%). Unfortunately, outside of laboratories where experimentally-induced dehydration is controlled, we rarely have a recent baseline euhydrated body weight to be able to accurately examine the presence and the degree of water deficit. For this reason, different blood, urine, and clinical biomarkers have been used to assess hydration status [4]. The majority of research on water homeostasis and its effects on the human body has focused on how water deficit impacts exercise performance, mainly in hot environments [5]. Edward Adolf in his classic work “Physiology of Man in the Desert” was one of the first to study the effect of water intake on thermoregulation and performance [6]. He also introduced the term voluntary dehydration when he observed that during “rapid sweating”, humans do not drink enough to maintain body water. He concluded that: “...when he is active and needs much water his thirst sensations are inadequate”. During the last 30 years we have learned that even a mild degree of dehydration (< 2% of body weight) can impair exercise performance and increase heat strain, especially in the heat [5, 7]. The degree of exercise-induced dehydration often ranges between 2 and 5% of body weight and it is accompanied by elevated plasma osmolality, decreased plasma volume, and increased urinary biomarkers (i.e. urine osmolality) [5]. Influenced by this observation and based on the mathematical symmetric property stating that if A = B, then B = A, we have mistakenly assumed that the backward association is also true. Thus, if exerciseinduced dehydration leads to increased urine biomarkers, then elevated urinary biomarkers should correspond with water deficit and dehydration. So, when we read data indicating that a majority of children, adults, and athletes have elevated levels of urinary osmolality or specific gravity we mistakenly conclude that a large portion of the population is dehydrated [8–11]. Furthermore, when we read data indicating that a majority of people across the world do not meet the dietary guidelines for water intake we also conclude that most people are dehydrated. Is it possible that people with free access to water when they do not meet the water intake guidelines or when they have elevated urinary biomarkers are dehydrated? Probably not. Let’s examine the data from the National Health and Nutrition Examination Survey (NHANES) in the US. If we compare the 10th (1694 mL/day) and the 90th (7934 mL/ day) percentile of water intake distribution in the US we will notice that they have nearly identical plasma osmolality (279 and 280 mmol/kg, respectively) [12]. Similarly, people who chronically consume either low (low-drinkers) or high (high-drinkers) amounts of water have similar plasma osmolality, but low-drinkers have greater vasopressin [13]. In 2015, Johnson and his colleagues published a study that identified lowand high-drinkers through an initial screening * Stavros A. Kavouras [email protected]