Introduction
Water is arguably the most critical nutrient, as its absence can be lethal within a few days [
1‐
3]. To promote adequate water intake at the population level, several countries and transnational authorities have developed water intake recommendations based on national estimates of water intake [
4‐
6]. In the UK, data on total water intake (TWI) is not normally published in national survey reports and there is currently no Dietary Reference Value. The main purpose of this study was to quantify TWI and its relation to patterns of beverage consumption and then to explore associations between types of beverage consumed and the intake of water and energy. It has been estimated from studies in Europe and the USA that around 70% -80% of TWI comes from beverages of various types (including water, tea and coffee, milk, soft drinks, juice and alcoholic drinks), with the remainder contributed by water in food [
4,
7], and we expected the British population would have a similar pattern. Beverages are widely available in developed societies, where tap water is essentially free, but there are nevertheless concerns that some people may not be consuming sufficient fluid for optimal health and some people may be over-consuming. Children, elderly or infirm people, and those working in hot environments, are some of those most vulnerable to the effects of dehydration, but many adults may also be inadequately hydrated at some time. Unfortunately, hydration status is rarely measured in epidemiological studies and this hampers attempts to assess the adequacy of water intakes at a population level. However, guidelines have been established to determine how much water humans require (on average) to avoid dehydration and to optimise physical and psychological function.
In 2005, the Food and Nutrition board of the Institute of Medicine (IOM) published adequate intake (AI) values for TWI in temperate climates [
5]. The AI for total water (from a combination of drinking water, beverages, and food) was set based on the median total water intake from US survey data. For young men and women (19–30 years) this is 3.7 L and 2.7 L per day, respectively. These recommended intakes are based on median intakes of generally healthy individuals who are adequately hydrated, but the report pointed out that individuals can be adequately hydrated at levels below, as well as above, the AIs provided. The American AIs exceed those of other authorities, while the recent recommendations produced by the European Food Safety Authority (EFSA) in 2010 are the most conservative to date, at 2.0 L per day for adult females and 2.5 L per day for adult males [
4].
Limited data exist on daily water intake in other countries, and comparison is sometimes hampered by different methods of definition and data collection [
8]. In the UK, TWI is not currently quoted in the published reports but the mean value has been calculated as 2494 g/day among adults aged 19–64 y in 2008/9 [
9]. Average water intakes in other European countries appear to be broadly similar to those in the UK (e.g. mean 2461 ml/day in Sweden [
10]), or else lower than the UK, (e.g. mean intake 1984 ml/d in France [
11]; 2039 ml/day in Germany [
12]; 2222 ml/d in the Netherlands [
13]). Reported intakes of total water in North America are considerably higher than in Britain and Europe. In 2005–2006, American adults participating in the National Health and Nutrition Examination Surveys (NHANES) reported consuming 3.18 L of total water within the previous 24 hours [
7], slightly less than the 3.35 L reported in 1999–2004.
While all beverages support hydration by virtue of their high water content, many also supply calories. Excessive consumption of caloric beverages has been widely viewed as contributing to the obesity epidemic [
14,
15], although systematic reviews have highlighted the need for better randomised controlled trials, in order to demonstrate a causal effect [
16,
17]. The main putative mechanism involves reduced satiety and incomplete compensation for calories ingested in liquid form [
18]. According to this theory, so-called “liquid calories” are more likely than solid calories to result in passive overconsumption and excess energy intake [
19]. There are, however, few studies that have examined relationships between beverage consumption patterns and energy intake in the British population, apart from the recent paper by Ng et al. [
9], which reported that the proportion of energy from beverages changed very little between 1986 and 2008/9, although there were some shifts between sources. Our paper differs in scope, focusing on consumption over 24 hours and 7 days of the week and comparing water intakes, in men and women with reference values.
The National Diet and Nutrition Survey database provides what is probably the best source of detailed information on the diets of normal individuals in Britain [
20,
21]. In 2008 the survey adopted a new method, collecting data via a (non-weighed) diet record over 4 days, in place of the former weighed record over 7 days, in 2000/2001. The latter, slightly older, data provide the opportunity to study variation over days of the week and may give a better indication of participants’ usual intake. The food records list the weight in grams of each item of food and drink consumed (including tap water) for each of the 1724 participants (12,068 person-days of data), while the nutrient database includes the water, energy and nutrient content of each item. Time of consumption is also recorded, providing a rich resource for exploring patterns of consumption (timing, frequency, variety etc.) and linking this with personal data. Using raw data from the NDNS we attempted to address the following questions:
1)
Is the UK adult population consuming adequate amounts of total water?
2)
How does beverage consumption vary by age and gender, day of the week and time of day and is this related to total water intake?
3)
Is the variety of beverages consumed a positive predictor of total water intake?
4)
How much energy do beverages contribute to the UK diet and in what proportion?
5)
Is energy in liquid form (i.e. from beverages) positively associated with total energy intake?
Methods
The survey
The
National Diet and Nutrition Survey: adults aged 19 to 64 years, (NDNS) is a nationally representative survey of the diet and health of adults living in private households in Great Britain in 2000/2001 [
21]. It was commissioned jointly by the Department of Health and the Food Standards Agency and ethical approval was obtained via the normal channels. Fieldwork was conducted over a 12-month period in 2000/2001 to cover any seasonality in eating behavior and in the nutrient content of foods. Overall, 61% of the eligible sample (n = 3704) completed the dietary interview (responding sample, n = 2,251) and 77% of those who completed the dietary interview completed a full seven-day weighed dietary record (diary sample, n = 1724, representing 12068 person-days of data) [
21]. There was no evidence of serious non-response bias, although inevitably surveys of this type favour willing participants whose diets may be less extreme, or less variable day to day.
Respondents were asked to keep a weighed record of all food and drink consumed, both in and out of the home. Each respondent was issued with a set of digital food scales and instructed how to weigh and record items in two diaries, “Home” and “Eating out”. A description of each item was recorded, including the brand name of the product and if appropriate, the method of preparation. Also recorded was the time (to the nearest 5 minutes) and location. Everything consumed by the respondent had to be recorded, including drinks of water and medications and supplements. Respondents were asked to weigh everything they could as separate items in order that the nutrient composition of each could be calculated. Recipes for all home-made dishes were collected. Where it was not possible to weigh the food, respondents were asked to record as much information as possible, particularly the portion size and an estimate of any leftover and, for meals out, price and place of purchase. In certain circumstances duplicate portions were purchased for weighing. Interviewers visited workplace canteens to collect information on portion sizes, cooking methods and ingredients.
Each interviewer called back 24 hours after placing the diaries to check that the items were being recorded correctly, to give encouragement and re-motivate where appropriate. Further visits were included as necessary and any apparent omission of meals or snacks was probed. Interviewers were trained in coding the diaries and could therefore identify the level of detail needed for different items. As fieldwork progressed, new codes were added for homemade dishes and items appearing in the diaries. Interviewers were asked to assess the quality of the dietary record and how far they thought it reflected the respondent’s normal diet. Further detailed checks were carried out for completion and consistency at the Office for National Statistics and then information from the dietary records was linked to the Nutrient Databank. This holds information on 56 nutrients, including water content, for each of the 6000+ food codes.
Respondents also completed a diary detailing the time spent on specified activities, and the intensity of activity, over the same 7 days as the dietary record. A physical activity score was calculated by multiplying the duration of each activity by the metabolic equivalent value (MET) for activities of that intensity. The average MET score represents an estimate of hourly energy expenditure per kg of body weight. Body weight was measured by trained personnel at the nurse visit after completion of diaries. Further details are given in Appendix D of the published report [
22].
Data preparation and analysis
Computerised raw data files and documentation from this survey, comprising weighed food records, questionnaires and biochemical and medical data, were obtained under license from the UK Data Archive (
http://www.data-archive.ac.uk) and analysed using IBM® SPSS statistics v19 (SPSS IBM Inc Chicago, Illinois, USA).
Data were first analysed at the level of the individual food diaries, comprising weighed records for 12068 days (7 days for each survey respondent). Individual food items are recorded both with their original food codes and a higher level classification into 115 food groups for the published reports. The present analysis focused on total water intake (TWI) of all food and drink, as determined from food composition tables in the database. Metabolic water (water derived from oxidation of substrates) was not included as the focus was on comparison with dietary water requirements. Water in coffee or tea was coded as coffee/tea, while water for diluting concentrated soft drinks was coded as soft drink. Bottled water was coded separately from tap water. Milk includes all liquid milk, including that in tea and coffee, and milk consumed with cereal. Discretionary table sugar added to beverages was counted as food, rather than drink, as it was not possible to differentiate sugar use in drinks from sugar sprinkled on cereal or fruit consumed at the same time. For this project beverages were combined into 8 categories for further analysis: hot beverages (tea/coffee); milk; 100% fruit juice; caloric soft drinks (including sodas, juice drinks, sports and energy drinks); diet soft drinks; alcoholic drinks (including beer, cider, wine, spirits, alcopops); bottled water; and tap water. To investigate trends over the day, consumption occasions were aggregated into hourly intervals and into 5 periods, approximately corresponding to breakfast (<1000 hrs), lunch (>1000 to 1400 hrs), afternoon (>1400 to 1800 hrs), evening (1800 to 2200 hrs) and night (>2200 to 0200). Mean daily consumption, calculated over 7 days, was used to compare between individuals. TWI was compared with EFSA Dietary Reference Values (DRV) for Adequate Intake of Water (AI) for adult men and women of 2.5 L and 2.0 L, respectively [
4]. The EFSA value for adequate intake is more conservative than other recommendations from WHO or IOM [
5]. Nordic and German speaking countries take the approach that water intake is considered inadequate when it is less than 1 g per kilocalorie of energy requirement [
4]. Therefore a combined classification (TWI-2), based on dual criteria, (i) the AI, and (ii) the ratio of water intake in grams to energy intake in kcal [
4] was used to provide a more conservative estimate of the proportion of adults consuming low amounts of water (although with no implication of inadequate hydration status).
Statistical analyses
Univariate analyses were carried out for men and women separately because of known differences in water consumption and recommended intakes. Crude differences in TWI and beverage consumption between groups were assessed using T-tests with Bonferroni correction for multiple comparisons. Non-parametric tests were used for variables that were markedly non-normal. Chi-square tests were used for categorical variables. All tests were 2-tailed and P < 0.05 taken as indicating statistical significance.
Multivariate analyses included adjustment for sex, age, bodyweight, smoking, dieting, physical activity level and mis-reporting (a recognized problem in dietary surveys with the potential to affect results) [
23]. We attempted to adjust for implausible energy reporting using individuals’ ratio of energy intake (EI) to basal metabolic rate (BMR) [
24] and applied Goldberg cut-offs corresponding to confidence limits of plausible intake based on a 7 day diet record at a physical activity level (PAL) of 1.55 [
25]. Adults whose reported EI was <1.05 times their BMR were classified as under-reporters, while those with reported EI >2.28 times BMR were classified as over-reporters. Rather than exclude misreporters entirely, which can introduce bias, we included the classification variable in the analysis.
Analysis of covariance was used initially to examine how energy intake varied as the proportion of “liquid calories” increased (compared with food calories); covariates in the model were: sex, age, bodyweight, smoking, dieting, mis-reporting and physical activity. Multiple linear regression was used to estimate the effect on energy intake of varying the type of beverages consumed, whilst controlling for the effect of confounders. The effect of substituting non-caloric beverages by caloric beverages was estimated by including caloric beverages (as percentage of total beverage weight) as the main independent variable, with total beverage weight (g) held constant. This necessarily implies an equal and opposite change in other beverages. A further model included energy from food, thus disallowing compensation (reduction of food calories). Finally, we used the within-person daily consumption data to explore the effect of changes in consumption day-to-day, with each person acting as their own control. The independent variables in this analysis were all difference variables (deviation from the subject’s 7 day mean, deviation in total beverage consumption from subject mean) with deviation in energy intake as the outcome.
Discussion
Using 12068 days of weighed dietary records from a nationally-representative sample of British adults aged 19 to 64 years, we have quantified total water intakes, and investigated relationships between beverage consumption patterns and water and energy intakes.
Mean TWI was almost identical to the European (EFSA) reference or “adequate intake” (AI) of 2 L for women and 2.5 L for men [
4]. EFSA determined their AIs from a combination of observed intakes in population groups, with desirable urine osmolarity values and desirable water volumes per energy unit consumed. These AIs only apply to conditions of moderate environmental temperature and moderate physical activity levels; physically active individuals or those working in hot conditions may require more. However, AIs are likely to exceed requirements and cannot predict which individuals are under- or over- hydrated in practice. Using more conservative criteria (TWI below AI
and TWI <1 g per kcal of energy intake) approximately 33% of men and 23% of women were classified as having low water intakes. These may be at greater theoretical risk of poor hydration, but may actually be appropriately hydrated if they have low volumes of water loss due to their individual physiology, environment and physical activity. In our opinion a value of 1 g water per kcal energy requirement (rather than energy intake) would seem an appropriate basis for a UK recommended mean water intake, and has the utility of broad applicability across age/sex groups. However, it would be helpful to attempt to specify a confidence interval around this mean to avoid confusion and misuse.
Beverages supplied 75% of TWI for adults in this British study. This is consistent with other estimates [
4,
7,
8], but the proportion may vary considerably between individuals and between populations. As we have shown, beverage consumption (and water intake) is not evenly spread throughout the day, but tends to be concentrated in the evening. This leaves the possibility that some adults may be relatively overhydrated in the evening/nighttime and under-hydrated in the morning. As alcoholic beverages were a significant source of water and energy, especially among men, and were strongly associated with evening consumption, men who drink alcohol may be particularly vulnerable. TWI can be increased by offering more variety in beverages [
26]. However, if such an intervention is to benefit public health, it is important that the additional fluid is consumed when needed, and does not result in excessive water intake, unwanted effects on energy balance, or inadequate nutrition.
Since our analysis was conducted, Ng et al. [
9] have reported on UK trends in beverage consumption over the past 25 years. There are some differences in methodology between our studies, but their calculation for adults in 2000/2001 (18% of energy obtained from beverages) compares with our estimate of 16% (which excluded discretionary sugar). Had we been able to include table sugar added to drinks, we would have seen a small contribution of tea and coffee to energy intake but our overall findings would not have been affected. Between 2000 and 2008/9 the energy intake contributed by beverages (in total) did not change, while sweetened tea and coffee declined and consumption of alcoholic beverages and caloric soft drinks rose, although not significantly [
9]. In 2008/9 alcoholic beverages provided 770 kJ/d (184 kcal/d) per capita, and caloric soft drinks (soda and juice drinks) 209 kJ (50 kcal/d) per capita; hence our conclusions concerning the dominance of alcohol appear to remain valid. According to Ng et al., TWI increased by approximately 9% between 2000/2001 and 2008/2009, as people consumed more water as a beverage and more water in food (fruit and vegetables) [
9], a trend that may have been influenced by healthy eating campaigns and marketing of bottled water. However, consumption of water, tea and coffee, soft drinks and juice among adults has changed little over the last 3 years (NDNS 2008–2011) [
27,
28], and plain water consumption in the UK remains lower than among adults in France [
11], Canada [
29] or America [
30]. Consumption habits are particularly well- documented in the US, and NHANES surveys show alcohol providing (only) 115 kcal/d for adults, while soda /fruit drinks provided 141kcaL/d in 2005/2006 [
30]. It appears that the main increase in soft drink consumption in the US occurred between the 1970s and the 1990s, and a new study confirms that liquid calories have been falling in the US diet over the past decade, although consumption remains high [
31].
The present study demonstrates that well-conducted national surveys such as the NDNS have the potential to yield rich contextual data that can be linked with nutrition and health measures. We found significant variation in consumption by day of the week and by time of day (see Additional file
1: Appendix for supplementary figures), which few other studies have been able to describe in detail. Alcoholic drinks in particular were a major contributor to evening and weekend peaks in beverage consumption. The extent to which timing of drinking occasions relates to overall TWI and EI requires further study. Associations have also been demonstrated between meal patterns and EI, with the suggestion that evening consumption of calories may be more conducive to total energy excess [
32]. However these findings may merely reflect the cultural reality of the evening being a time for eating and drinking. Significantly, and possibly for the first time using such data, we have estimated the effect on EI of switching beverage consumption between caloric and non-caloric sources using within-person daily records.
In terms of the limitations of this study we draw attention to the usual caveats on making causal inferences from observational data. Whilst the data are of the highest quality obtainable, and respondents were revisited to check records and probe for missing items and weights, dietary assessment is an imperfect science. Most errors are likely to be in the direction of omission or underestimation (perhaps especially where alcohol is concerned). Errors of overestimation may also have occurred where a drink was spilt or unfinished, although efforts were made to prevent this by asking respondents to weigh leftovers and estimate spills. There is evidence from validation studies that true EI is underestimated by about 20-25% in the NDNS [
33], but to our knowledge no studies have tried to quantify under-reporting of water intake or beverages specifically. Estimates of water intake adequacy based on survey data are likely to suffer from under-estimation also.
In conclusion, a significant proportion of British adults surveyed in 2000/2001 had low TWI, although in the absence of clinical measures this could not be equated with poor hydration status. Further work may be warranted to assess current intakes and to explore correlations with urine volumes, as recommended recently [
3]. Soft drinks were not a major source of liquid calories among these British adults, whereas alcohol was more significant. All beverages supplying energy can contribute to higher total EI, but in practice some degree of compensatory under-consumption of other foods or beverages may occur. Mechanistic and experimental studies are required to address directly the satiety effects of beverages, while work is also needed to understand the drivers of beverage consumption, which are not merely physiological but also psychological, social and environmental. Further research based on dietary patterns rather than singular foods or dietary components may help identify unhealthy behaviors and provide better evidenced-based recommendations for adequate fluid intake and optimal beverage consumption.
Acknowledgement
This study followed ILSI guidelines relating to financial conflicts and scientific integrity [
34]. The sponsors (Coca Cola) had no part in the study design, analysis, interpretation, writing or editing of the manuscript. Conclusions and opinion expressed remain the responsibility of the authors and not of the sponsors or of the Survey funders, contractors or depositors.
We thank Dr Graham Horgan for providing statistical advice.
Funding of survey
UK government (Food standards Agency, Department of Health).
Funding of secondary analysis
Industry (Coca Cola).
Competing interests
SG has received funding for research and consultancy from a number of food and beverage companies. SS has received funding for research and consultancy from a number of companies that produce drinks.
Authors’ contributions
SG conceived the project and was responsible for analyzing and interpreting data, drafting and finalizing the paper. SS contributed to interpretation of results and discussion. Both authors read and approved the final manuscript.