Water, an essential nutrient, is often ignored in reports of dietary surveys and nutrition. Although it is ubiquitous in foods and beverages, the attention is often focused on the minerals or calorific values of the fluids imbibed rather than the water per se. Water is often taken for granted by many in Western countries due to its abundant availability through water systems. In developing countries, however, water and sanitation raise significant problems. This review overviews (i) the global perspective of the potable water supply, (ii) human rights and water, (iii) dietary guidelines and sources of water and (iv) the physiology of water balance. Gaps in knowledge and understanding around hydration and water requirements are also discussed. Nutritionists are urged to look at the bigger picture of the global water supply and to use good judgement and common sense when advising on water requirements.
Water is essential for all forms of life and is relatively ubiquitous. In human nutrition, water is often ignored in research reporting dietary intake and food consumption patterns. Nutrition journal reviews in 19991, 20092 and 20103 have highlighted water as an essential, and often ignored nutrient, while others have addressed the importance of water and hydration for the individual.4, 5, 6, 7 Discussion of individual requirements for water and recommended dietary intake guidelines has received some attention,8, 9, 10, 11 but there is not a clear consensus in public health or clinical medicine of what the message should be and how it should be delivered. This may be due to the limited amount of objective research that has been undertaken concerning actual water intake and turnover among populations who (i) live in different environments, (ii) have varied levels of activity and (iii) are at different life stages.
The ability to continue to support food production on this planet is dependent on water, yet rapid population growth and economic development combined with global climate change are impacting on the availability of potable water.12 Of particular concern is the adequacy of safe drinking water for populations. Disasters and emergencies, add to the challenge where often the priority for short-term survival is potable water for drinking and then cooking; cleaning becomes a secondary consideration.
The complexity of the challenge of understanding water as a nutrient and source of life on this planet may be portrayed as clusters of factors that impinge on an individual’s water balance and turnover. A very simple schematic (Figure 1) modelled on the Foresight Obesity System Maps13 depicts some major effectors of potable water availability and water balance.
The millennium development goals promulgated in 2000 included a goal to ensure environmental sustainability.14 In the year 2000, more than one billion people were without sustainable access to safe drinking water and basic sanitation, and the goal set for 2015 was to halve this number. Progress towards millennium development goals has been uneven and most goals including water and basic sanitation will not be met.15, 16
On 28 July 2010, through Resolution 64/292 the United Nations General Assembly explicitly recognised the human right to water and sanitation. This resolution was based on the understanding that clean drinking water and sanitation are essential to the realisation of all human rights and living a life in human dignity. The right to water now stands alongside the right to food: first recognised by the United Nations in 1945. Similar to food security, water security means that water is sufficient, safe, acceptable, physically accessible and affordable.
The right to water and access to water are considered crucial to well-being and quality of life. Yet, the link between the right and access for an individual is dependent on governance and operationalization,17 soft-path strategies that require the participation of local communities and alternative technologies.15 As the number of people living in water-stressed regions continues to increase and effects of climate change and population growth continue to impact18 innovative and adaptive solutions become more imperative.
The proportion of body mass that is water can range from 30 to 70%, association with a body fat percentage of 60 and 5%, respectively. Under normal conditions the hydration of the fat free mass varies by only a few percentage points19, 20, 21 with the average of 73.2% water.
Water as a molecule has unique chemical and physical properties; that are related to specific functions in the human body.22 Its distribution is tightly regulated1 and any changes in water balance, including shifts within the body, stimulate the response of exquisitely sensitive physiological mechanisms to maintain tight control of osmolarity and sodium balance. Water is continually lost from the body through respiration, urine formation, insensible loss, sweat and faeces. It is gained from fluids including water and food and also from catabolic processes where water is released.
Objective measures of water balance
Direct measurements of total amount of water gained and lost are not possible, and determinations that rely on interviews or recall of food intake are not reliable.23 Clinical fluid management assessment and practice are predicated on observations24 that are not supported by a strong evidence base. One objective way to measure water turnover is to administer an accurately measured dose of deuterium labelled water. Within 3–5 h, the isotopically labelled water equilibrates with body water, and total body water may be determined.25 The labelled water is lost from the body through all routes and is diluted by the water gained. The time course of this loss provides a measure of water turnover and it is assumed that water balance is maintained, that is, input=output. Dependent on the dose and biological half-life of the label, the usual time of measurement is over 7–14 days. The advantage is that the individuals are free-living, and the only requirement after dosing is that timed samples of urine or other body fluid are collected. The elimination rate may be determined by a two point calculation where only a baseline, a sample after the dose has equilibrated, and another sample from the finish of the period are required.26 The multipoint method of calculation requires more samples, preferably at regular intervals, and this may give better combined precision and accuracy,27 with the coefficient of variation 5.4% compared with 3.6% for the two point determination. Either way the accuracy of water turnover measured by deuterium dilution far exceeds other methods of assessment of water turnover, but is not suitable for the measurements of rapidly changing water balance.
Using the deuterium dilution approach others have found high interindividual variability in the water turnover of adults,28 even though only small age differences in these populations were seen. One water balance study29 found no difference in water turnover between subjects aged 23–46 years and 63–81 years. Other studies have assessed water turnover in the middle-aged and elderly,28, 30, 31 but very few have examined younger age groups. Only one32 has focused on young adults outside of specialized sports activities, two have looked at child water turnover32, 33 and one has examined water turnover of overweight adolescents.34 These and other studies have reported wide interindividual variation, which add to the difficulty of accurate public health messages around how much fluid an individual requires in a day.
Public health messages should be clear, achievable and based on evidence. For example, for survival the World Health Organisation recommends that each individual needs between 2.5 to 3 l each day in drink and food, but comments that this depends on climate and individual physiology.35 This simple advice is relevant to a wide range of emergency situations. In order to provide context to advice for individual water requirements, the data from the study of water turnover in children and young adults in temperate New Zealand32 has been examined further. When water turnover was regressed against body mass (Figure 2), the regression equation for predicting water turnover (l per day) was: 0.949+0.024 body mass in kg (SEE=0.71 and r2=0.52). In rounded terms 1 l plus 250 ml for every 10 kg of body weight. Furthermore, the Rush study32 showed that the variation in water turnover when indexed to total energy expenditure, was higher in women and men than in girls and boys, respectively. The variation is right skewed (Figure 2); demonstrating that there is a tendency for mean consumption to be greater than the median. Although not measured, social imbibing and habitual sipping rather than drinking for thirst may explain the higher water turnover in these adults.
Guidelines not tramlines
Three underlying principles govern guidelines around fluid intake. The first is that fluid intake does not have to be water because requirements may be met by other sources of fluid including tea, coffee, milk and food. Second, beverages with no or few calories should take precedence over the consumption of beverages with more calories.36 Third, because of the variations in physical activity levels, climate, air conditioning and body size and surface area there is a wide variation in actual fluid requirements. Government guidelines for adequate fluid intake are based on observations.The often quoted ‘drink at least eight glasses of water a day’ has no underpinning scientific evidence10, 37 and does not account for other sources of fluid. Although it is recognised that water intake ‘ahead of thirst’ may cause exercise associated hyponatraemia,38 this is considered a rare occurrence in the general population, and there is not enough evidence to set a recommended dietary intake or upper limit for fluid intake. Furthermore, there is no clear evidence for lack of benefit of increased water ingestion,39, 40 and limited reports of positive effects of water consumption on cognition, physical performance, child health, primary and public health.41 This limited evidence may be used by Big Food multinationals to help market health42 and in particular water and the ‘importance of hydration’.41
The evidence base to support claims of widespread dehydration and the need to drink ahead of thirst does not exist. Yet, the need to be ‘optimally hydrated’ still receives a lot of attention. Reports of dehydration have been associated with cognitive decline,43 and the advocation, particularly, by the sports drink industry to drink ahead of thirst are based on limited evidence that this will help to avoid heat stroke, dehydration and improve health and performance.38, 44 Furthermore, there is a danger that the advice to drink ahead of thirst may be associated with hyponatraemia,45 independent of whether it is water or a sports drink.
In temperate climates and for moderate energy expenditure, requirements for fluid intake (including the water in food) recommendations are 1 ml/kcal.46 Popkin et al.6 applied this ‘rule’ to The National Health and Nutrition Examination Survey 2005-2006 data, and identified that children aged <14 years were consuming around 75% of the water recommended. Evidence that this translates into dehydration and puts children at risk is lacking, as are biomarkers for hydration status.
Market forces versus public health messages
In more affluent societies there is a public perception that although bottled water costs more, it tastes better and is safer than tap water.47 Evidence for a growing preference is provided by increasing market share for bottled water.39 The annual sales of bottled fluid water has burgeoned in 76 countries from 175000 million litres in 2005 to 237000 million litres in 2010.48 In 2010 the two countries accounting for sales of 30000 million litres each (25% of total) were China and the United States. For the United States this approximates to 96 l for each person each year. While this may be seen as a move away from caloric beverages, it may also be seen as successful marketing of the importance of hydration.
Water is absolutely necessary and fundamental to life, and like food access to adequate water is a human right. Essential to the health of ecosystems, food supplies and individuals, water is both an under-researched nutrient and overexploited resource in the world. From a global perspective and current practice, water insecurity will increase and have an impact on ecological and human health.
The information around individual water balance is lacking and guidelines and public health messages around water are not clear. More evidence is required, including how to translate accumulating knowledge into safe practice and recommendations.
More than 2000 years ago Hippocrates c (460–377 BC) said ‘If we could give every individual the right amount of nourishment and exercise, not too little and not too much, we would have found the safest way to health’. Future research should not only examine basic physiology and water requirements, but also take into account the wider impacts of global climate, change, local environments, water availability and potability, water distribution systems and commercial concerns.
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European Journal of Clinical Nutrition (2018)