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How much water do we really need?

2008; Wiley; Volume: 33; Issue: 4 Linguagem: Inglês

10.1111/j.1467-3010.2008.00724.x

1471-9827

Jean Lunn, Rebecca Foxen,

Sodium Intake and Health

As is becoming increasingly typical for a British summer, water was everywhere! But this year it was doing much more than just falling from the sky. Water has been appearing in the headlines of the major newspapers, as well as being featured in health and lifestyle magazines, and the question that everyone has been trying to answer has been ‘does anyone know how much water we really need?’ Water is an essential requirement for life – without water, humans can only survive for 2 to 3 days at the most. On average, water makes up 60% of body mass (range 45–75%), which is equivalent to approximately 45 l in a 70kg man and 30 l in a 55kg woman (Olsson & Saltin 1970). Every day we gain and lose water although there is a huge degree of inter-individual variation. This makes setting any blanket recommendations for the entire population very difficult. However, when asked, many health professionals or members of the general public would most likely be familiar with the old adage to drink around 6 to 8 glasses of ‘water’ a day. Indeed, many believe that we need eight glasses of pure water a day just to stay healthy. The use of such recommendations has been called into question in a recent editorial in the Journal of the American Society of Nephrology (Negoianu & Goldfarb 2008). Its authors have concluded that, in the absence of any clear evidence, there is no proof of any health benefit for drinking 6 to 8 glasses of water a day, except perhaps for people in hot climates. This editorial triggered headlines in the Daily Mail (2008a), Daily Mirror (2008), Daily Telegraph (2008), Guardian (2008) and Independent (2008) along with a story on BBC News (2008), proclaiming that there is no evidence that drinking eight glasses of water a day improves skin tone, aids dieting or prevents headaches. Further confusion has been added following news reports of tragic cases where individuals have either died or have had their health seriously compromised by drinking too much water (e.g.Daily Mail 2008b; The Sun 2008). In such instances, the consequences of following ill-advised detoxing regimens or mistakenly over-consuming water during daily life or exercise can be incredibly serious, as the body attempts to cope with the dilution of many important electrolytes. Water is essential for our bodies to work properly (see Manz & Wentz 2005). Water (and fluid in general) has many functions in the body. It acts as a lubricant for joints and eyes. It helps us swallow. It provides the medium in which most reactions in the body occur. It acts as a cushion for the nervous system and helps get rid of waste. It helps to regulate body temperature. The body cannot produce enough water to meet all of its needs, so some must be provided by the diet. If we do not consume sufficient water, dehydration occurs. Dehydration can be very serious, and a loss of 15% to 20% of body weight as water is fatal. Dehydration by as little as a 2% loss of body weight can result in impaired physiological and performance responses. Symptoms of such mild dehydration include headache and fatigue, and dehydration has been associated with an increased risk of urinary stones and cancers of the urinary tract and bowel (see Kleiner 1999; Thomas et al. 2008). Drinking too much water can be equally problematic. If an individual consumes too much water, this can lead to water intoxication with potentially life-threatening hyponatraemia. This occurs when the concentration of sodium in plasma falls to below 130 mmol/l (normally around 140 mmol/l). Often, in the aggressive communication of the importance of consuming adequate fluids to avoid dehydration, the risk of consuming too much plain water is overshadowed. In severe cases, hyponatraemia can result in lung congestion, brain swelling, headache, fatigue, lethargy, confusion, vomiting, seizures and coma (see O'Brien et al. 2001). Such overconsumption is particularly associated with athletes and marathon runners, although recent headlines have highlighted the dangers of drinking excessive amounts of pure water on ill-advised detoxing regimens. Body water balance is determined by the difference between the sum of water intake and endogenous water production, and the sum of water losses (Table 1). Water is lost from the body mainly via the kidney and in sweat. Such losses vary widely with intake, diet, activity level, temperature and clothing. Indeed, no two people have the same sweat rate – it varies greatly in the general population. Other losses of water from the body are said to occur ‘insensibly’ via the lungs, skin and in faeces. The main dietary sources of water are outlined in the next section, but water is also produced in the body from the aerobic metabolism of hydrogen-containing compounds. The exact amount of water produced is dependent on energy expenditure and the specific substrate oxidised. Extrapolation of data available suggests that an energy expenditure of roughly 2500 kcal/day produces approximately 250 ml of metabolic water per day (Grandjean & Campbell 2004). The amount of water in the body, called total body water, is distributed between two compartments, namely the fluid inside cells (intracellular fluid) and the fluid outside cells (extracellular fluid). Total body water, the water content of the intracellular and extracellular compartment, and the balance between water input and output are all under strict homeostatic control whereby the body is able to modify excretory pathways and stimulate intake (thirst) to correct any imbalances. Elaborate feedback mechanisms, which act mainly on the kidney, are capable of sensing and reacting to changes in the concentration of body fluids by 1% to 2%. Indeed, it has been estimated that water balance is regulated to within 0.2% of body weight over a 24-hour period (Grandjean et al. 2003). Such water balance studies have formed the basis of estimations for the amount of water required each day by a healthy adult. Table 2 outlines an estimation of the minimum daily water losses and production, which can be used to predict water requirements. As stated previously, there is much inter-individual variation, and such calculations serve as a population estimate only. We obtain water from all foods and drinks consumed. Food-water content is usually less than 40% in cereals and cereal products, 40–70% in hot meals, more than 80% in fruit and vegetables, and approximately 90% in human and cows' milk (Przyrembel 2006). Table 3 gives the water content for a selection of commonly consumed foods. It has been estimated that roughly a third of water consumed is from food, with the remaining two-thirds from beverages. When we talk about water from drinks, it is important to remember that we can get our fluid requirements from a number of sources; it is not necessary to only drink ‘pure’ water, although this is a good choice. Like everything else we consume, selection depends on personal preference and availability. Other drinks like squash, fruit juices, tea and coffee contribute to our daily requirements too. Caffeine is a mild diuretic (i.e. it increases urine output to a small extent), but drinks that contain caffeine (e.g. tea, coffee, cola) also contribute to fluid intake. Contrary to what is often reported in the health and lifestyle magazines, it is not necessary to drink anymore water to compensate for consuming these drinks. With alcoholic drinks, the dehydrating effect can be greater, depending on the type of drink consumed (e.g. weak shandy compared with wine). Drinking water alongside alcoholic drinks will minimise any dehydrating effects. For most nutrients in the diet, there are dietary reference values (DRVs), which provide an estimated daily guideline amount, suitable for the majority of the population. In the UK, estimated requirements for particular groups of the population are based on advice that was given by the Committee on Medical Aspects of Food and Nutrition Policy (COMA) back in the early 1990s (DH 1991). COMA examined the available scientific evidence and estimated nutritional requirements of various groups within the UK population. These were published in the 1991 report Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. However, at this time, no DRVs were estimated for water. Speaking with experts involved in the compilation of the 1991 report, it is clear that there was never any suggestion of a set of DRVs for water and, indeed, as requirements vary hugely with circumstances such as environmental temperature, humidity and physical activity, determining a valid reference figure would be very complex (M. Wiseman, personal communication). In the absence of any official DRVs, bodies that advise the public on healthy diets have looked elsewhere to obtain an intake figure that could be of use as a point of reference for the general population. Such recommendations have been incorporated into the food-based dietary guidelines that advise on dietary pattern associated with good health. The Food Standards Agency (FSA) outlines recommendations for water consumption as part of its ‘Eight Tips for Eating Well’, advising us to ‘drink more water’. It is recommended that in the UK climate, we should drink approximately 1.2 l of fluid every day to prevent dehydration, and that we may obtain extra fluid from the foods we eat (FSA 2008). Recommendations have also been made by the National Health Service. They estimate fluid requirements to be around 6 to 8 glasses of fluid, which is how ‘water’ recommendations are most commonly remembered and an amount that has been the focus of much debate in the headlines. However, in the United States, the dietary reference intakes (DRIs) – the US equivalent of DRVs – provide a guideline for daily water consumption. For water, this has been determined as an adequate intake (AI), which is believed to cover the needs of all healthy individuals in the group to which it relates and prevents the effects of dehydration. It is suggested that an AI should not be interpreted as a specific requirement as there is insufficient data available to be sure that the AI would meet the needs of all individuals. However, the AI is the best estimate. For adult males, aged 19–70 years, the AI is 3.7 l/day of total water, 3.0 l of which is to be consumed as total beverages including drinking water. For adult females, aged 19–70 years, the AI is 2.7 l/day of total water, 2.2 l of which is to be consumed as total beverages including drinking water (IOM 2006). Separate recommendations are made for children, pregnancy and lactation. The United States currently has the Healthy Beverage Guidelines to provide guidance on the relative health and nutritional benefits and risks of various beverage categories. These guidelines were the result of a systematic review of the literature by an expert panel (Popkin et al. 2006). As well as the review, the panel also developed pictorial images to represent suggested beverage consumption patterns (10% of energy from beverages) for a person with a 2200 kcal energy requirement per day. There is currently no such advice in the UK, and it is left to the individual to balance fluid from various sources throughout the day. In Australia, a similar approach has been taken to that in the United States by establishing an AI as opposed to a DRI, as it is suggested that no single level of water intake would ensure adequate hydration and optimal health for half of all apparently healthy people, in all environmental conditions. For adults males aged between 19 and 70 years the AI is 3.4 l/day of total water, 2.6 l of which should be consumed as fluids. For adult females, aged between 19 and 70 years the AI is 2.8 l/day of total water, 2.1 l of which should be consumed as fluids (NHMRC 2006). Separate recommendations are also made for children, pregnancy and lactation. The World Health Organization states that sedentary men and women under average conditions require 2.9 and 2.2 l water each day, respectively, for adequate hydration (WHO 2005). Separate recommendations have also been made for children, pregnancy and lactation. Physically active men, women and children at high temperatures may require up to 4.5 l of water each day. The most recent survey of food and beverage consumption in the UK, the National Diet and Nutrition Survey, was used to gather information about the dietary habits and the nutritional status of the British population. Unfortunately, the survey does not report data on total water intake, so instead we have to estimate intake by looking at the proportion of the population that reported consuming various different beverages and the average amounts consumed (Table 4). In relation to fluid intake, the following information was collected: 52% of men and 42% of women drank carbonated soft drinks and 43% of men and 47% of women drank fruit juice. Both tea and coffee were drunk by nearly three-quarters of respondents, although women were more likely to have drunk herbal tea, tap water and other beverages. Tap water was drunk by 73% of women, in comparison with 59% of men (Henderson et al. 2002). The European Food Safety Authority (EFSA) has been collating information on fluid intakes across Europe and has recently pooled the diverse data sets collected by a small selection of European countries (Table 5). Because of national differences in the way the data have been collected, the figures are not necessarily comparable, but they do give an idea of the volume of fluid consumed on average each day. Recommendations encompassing the requirement for fluids most often take the form of adequate intakes for specific population groups, based on observations of hydration status in healthy individuals. Hydration can be assessed in a number of different ways including measurements of: body weight; osmolarity of plasma; sodium concentration; urine volume and colour; thirst; or plasma volume. This has resulted in a fairly diverse evidence base that makes setting recommendations for ensuring adequate hydration difficult, and it is even harder if you want to take into account factors associated with the so-called optimal health. At a European level, and as part of ongoing work to ensure that community action in the area of nutrition is underpinned by the latest scientific evidence, the European Commission has asked EFSA's Panel on Dietetic Products, Nutrition and Allergies to deliver an opinion on reference intakes for nutrients and energy for the European population (EFSA 2008). These figures will replace the nutrient and energy intakes for the European Community provided by the Scientific Committee for Food in 1992. Although water was not specifically mentioned in the terms of reference, the EFSA Panel has decided that it should be included as ‘water and adequate hydration of the body is essential for health and life’. The Opinion (Question No, EFSA-Q-2005-015a), summarised in Table 6, was agreed upon in April 2008 and was released for a public consultation, running until mid-December 2008. It is anticipated that EFSA will establish the reference values in stages. The work, which first looked at energy, water and macronutrients, is expected to be completed by 2009. Work on micronutrients will start in mid-2009. What is clear from the evidence presented by the EFSA expert panel, along with the comments made by Negoianu and Goldfarb in their recent editorial, is that increasing the amount of water consumed appears to be of no additional benefit to health. Provided the body is not dehydrated, there is no evidence that organs function more efficiently, concentration improves or skin looks notably healthier (see Kleiner 1999; Manz & Wentz 2005; Ritz & Berrut 2005). Additionally, the EFSA panel reiterates the fact that water can be obtained from all drinks and food consumed. Water is the most abundant constituent of the human body and regular fluid intake is essential for our bodies to work properly. The amount of fluid needed varies between people and according to age, time of year, climatic conditions, diet and levels of physical activity, so it is difficult to set DRVs. Public health recommendations that convert the volume of fluid required into cups or glasses may be of use when it comes to communicating the message, but such recommendations are an over-simplification. The ‘water’ can be obtained from all food and drink sources and it is important that the recommendation is not misinterpreted; public health bodies are not suggesting that individuals should consume 6 to 8 glasses of pure water in addition to other drinks and food. However, provided that such recommendations are accompanied by additional information explaining what the figures mean for the daily diet, communicating the fluid requirement as a more convenient household measure can help people to picture how much fluid their bodies actually require.