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Assessing the fluid intake level of different populations has, to date, attracted very little interest. The comparison of existing data based on food surveys reveals notable differences between countries and within different surveys in 1 country. Methodological issues seem to account to a large extent for these differences. Recent studies conducted using specifically designed diaries to record fluid and water intake over a 7-day period tend to give more accurate results. These recent studies could potentially lead to the revision of the values of adequate intakes of water in numerous countries.
The importance of euhydration in maintaining physical and mental health is increasingly being recognized.1,2 There is also increasing concern regarding the role of liquid sugars, that is, sugars from soft drinks such as carbonated soft drinks, still soft drinks (ice tea for example), and also fruit juice, in the increasing prevalence of obesity and associated metabolic diseases including type 2 diabetes mellitus and cardiovascular disease.3 Despite these important public health issues, few countries have developed scientifically based recommendations for the consumption of water or other fluids. In France, there are clear recommendations for every essential nutrient except water. Although several pages of the recommendations are dedicated to fluids, there are no conclusions regarding what constitutes an adequate intake (AI) of water.4 The United States is one of only a few countries to make clear recommendations on the AI of water.5 There is clearly a need for more countries to set clear recommendations on water intake that are derived from scientific evidence.
When recommendations for the AI of water have been made, they have been based on several factors including intake data from national surveys. However, the methodology used to collect these data varies, and the way these data are used is variable. In addition, when recommendations are made for a group of countries, such as the European Union, these disparities become extremely important and raise concerns about the validity of the recommendations.
In 2010, the European Food Safety Authority (EFSA) published its scientific opinion on AIs for water, making recommendations for different age and gender groups.6 The EFSA panel considered many factors before making its recommendations including physiochemical data on water, body water losses and their regulation, body water balance and turnover, hydration status, physiology of hydration, and the relationship between energy and water intake data. The intake data were obtained from observational studies in healthy populations in different European countries. Figure 1 shows the data from 13 countries that were used in the formulation of the EFSA recommendations. There were marked differences between countries with the total intake varying from 720 mL/d in Hungary to 2621 mL/d in Denmark.
In addition, differences between national surveys within individual countries have also been observed. In France, the first Individual National Survey on Dietary Consumption (INCA1)8 reported an average daily beverage intake of 1.2 L/d in 1999, which increased to 1.6 L in 2004 as reported in the INCA2 survey.8 The same year (2004), the Comportement et Consommations Alimentaires en France (CCAF) survey9 reported an average daily intake of 1.2 L. The 2007 CCAF9 report showed a small increase (1.3 L) in intake compared with the previous CCAF survey. In 2006, a survey conducted by Danone, in collaboration with TNS-SOFRES,10 reported the highest intake levels to date, with an average daily beverage intake of 1.8 L. There is clearly a great deal of variation in the total intake between all the reported surveys, and this variability is undoubtedly due to the differences in the methodologies used.
The INCA and CCAF surveys in France were food surveys designed to capture energy and nutrient intake rather than fluid intake, unlike the 2006 survey that was solely concerned with fluid intake. It appears that the emphasis on recording food may have led to underestimations in fluid intake. The EFSA panel used the INCA1 survey results when considering intake data. Given the comparisons above, it is likely that the panel used intakes that were much lower than other surveys. Concerns about the intake data were raised at the time, and the panel commented that the data from European countries were often not comparable because of the use of different assessment methods and categorization of fluids. This and the paucity of scientific data on water or fluid intake per se undoubtedly introduced difficulties for the panel when considering the available data. It is therefore essential to develop and use fluid-specific methodologies such as the 2006 French survey that captures all fluid intakes as such studies will provide the best possible estimate of water intake, which should be used in future revisions of the recommendations.
When evaluating the quality of survey data, it is important to consider many factors including (i) accuracy, (ii) duration representativeness, (iii) people representativeness, and (iv) specificity for fluids.
(i) Analysis of the most recent CCAF survey showed that, in France, most fluid intake occurs with meals and is consumed at home.11 However, the reason for this is that the questionnaires used in the survey were based on meals. Indeed, dietary survey methods concentrate on recording eating occasions rather than drinking occasions. A recent survey conducted in the United Kingdom (personal communication) reported significantly higher fluid intakes compared with national UK dietary surveys and showed that up to 70% of fluids were consumed outside meals. It is therefore likely that, in some dietary surveys, drinking-only occasions may not be recorded. However, it is important to consider that there may be cultural differences between countries in the amount drunk with and outside meals.
(ii) Random errors inherent in intake assessment vary with the method used, nutrient assessed, and the population studied. Increasing the number of measurement days will reduce the random errors that affect the precision of the method.12 The need to evaluate fluid intake over a weekly period was demonstrated in a recent study. Fluid intake was recorded over a week, and the average intakes for each day were compared. There was statistically significant variation across the week, with the lowest amount being drunk during the working week and the average intake gradually increasing to a maximum on the Saturday. This variation suggests that a period of 7 consecutive days is necessary to provide an average fluid intake that is more representative of daily behavior.
(iii) Behavior for fluid consumption varies according to an individual's physiologic variables and some factors from the overall environment, including climate, physical activity, and also education level, social class, and nutritional knowledge. For population surveys, it is essential to represent all kinds of people. Quota methods based on the following parameters: age, gender, geographical area, education level, and socioeconomic class, appear to be the most suitable way to recruit subjects.
(iv) The methodology used for recording fluid intake varied between the French surveys. In the INCA1 survey, participants were asked only 1 question per day about fluid in the entire questionnaire. This was increased to 1 question per meal in the INCA2 survey, whereas our survey concentrated entirely on fluid intake. This illustrates the importance of being specific in nutritional surveys to track fluids specifically.
Clearly, the best methodology to collect complete fluid consumption appears to be a 7-day, real-time, and fluid-specific diary.
The 7-day fluid diary methodology described in this article has been used in several countries including Poland (2006), Indonesia (2008), Mexico (2008), and Japan (2010). The findings from these surveys show that significant proportions of the survey populations drank less than the EFSA recommends. The EFSA panel recommended an AI for adult women of 2 L of water from fluids, which may be used as a conservative marker of intake adequacy. In Poland, the average intake was 1.66 L/d, and 75% (918/1225) of the survey population drank less than water AIs proposed by the EFSA. In Indonesia, Mexico, and Japan, the average intakes were 1.84, 2.38, and 1.50 L/d respectively. The percentages of people not drinking 2 L/d were 64% (1660/2594) in Indonesia, 42% (406/966) in Mexico, and 78% (1077/1381) in Japan.
Although the inadequate intakes of significant percentages of these populations are a cause for concern, it is also important to also consider the type of fluids being consumed. Figure 2 shows the types of fluids consumed by the survey populations in Japan, Poland, Indonesia, and Mexico. Significant amounts of soft drinks (carbonated and still) were consumed in Poland and Mexico. With increasing evidence of a link between consumption of sugars from soft drinks in the development of obesity and associated morbidities,13-15 strategies are urgently being developed to address this escalating public health problem. The high consumption of energy-containing fluids in Mexico was the stimulus for the formation of an expert panel.16 The expert panel considered the scientific evidence and formulated recommendations on the amount of water and other fluids that should be consumed. These recommendations were further developed into education and public health programs that are aimed at increasing the consumption of water and reducing the intake of soft drinks.
Recommendations on the AI of water are partly based on existing survey data. However, these data were collected using methodologies that aimed to capture the intake of energy and nutrients and may therefore have underestimated water and total fluid intake. Using the appropriate methodology is the key to producing complete fluid intake data. A 7-day fluid-specific diary appears to be the most appropriate method. In order to set meaningful and accurate recommendations for AIs of water and other fluids, surveys are needed using fluid-specific methodologies in more countries.
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