Total Water Intake and Total Fluid Intake Worldwide: A Systematic Literature Review in Children and Adolescents

Abstract

Background: Investigating fluid consumption among children and adolescents poses a challenge due to varying dietary behaviors and hydration needs. This systematic review aims to assess total water intake (TWI) and total fluid intake (TFI) in children and adolescents, focusing on gender differences and adherence to reference values proposed by the European Food Safety Authority (EFSA) and the Institute of Medicine (IOM). Methods: A systematic literature search of PubMed, Scopus, and Scholar was conducted to identify studies published between 2004 and 2024 reporting on TWI and TFI for children and adolescents. Studies were included if they reported fluid intake data in healthy populations, provided quantitative measures of TWI or TFI, and aligned with the EFSA or IOM reference values. From 8731 initial articles, 24 studies met the inclusion criteria. Data were synthesized narratively, and compliance with hydration guidelines was assessed. Results: The review included 24 studies, encompassing 16,254 children and 15,367 adolescents. The majority of participants failed to meet the recommended guidelines. Only one study reported compliance with the recommended TWI values, while four studies in children and four in adolescents showed adherence to the recommended TFI values. Conclusions: The results underscore a widespread inadequacy in achieving the hydration guidelines among children and adolescents, emphasizing the need for targeted interventions to improve fluid intake. Public health interventions are needed to promote adequate fluid intake, particularly in populations at risk of dehydration-related health outcomes. Future research should focus on identifying barriers to adequate hydration and developing targeted strategies to improve fluid intake behaviors.

1. Introduction

Water is an essential nutrient for children and adolescents, playing a crucial role in vital body functions as there are many systems and processes that rely on hydration status, such as cellular homeostasis, body temperature regulation, and proper circulation [1]. For this reason, EFSA stated that water is the most essential ingredient for humans as it is responsible for maintaining the physiological functions of the body [2]. However, according to the IOM [3], determining specific water intake values for each population group is rather challenging, and it employs the term “adequate” based on the median values derived from the National Health and Nutrition Examination Survey (NHANES) III [4]. The proposed adequate values from the EFSA and IOM are summarized in

Table 1. Total water intake (TWI) and total fluid intake (TFI) guidelines for children and adolescents from EFSA and IOM.

Age Groups	Age	European Foods Safe Authority, 2010 mL/day		National Academy of Medicine, 2004 mL/day
		Total Water Intake (TWI) 1	Total Fluid Intake (TFI) 2	Total Water Intake (TWI) 1	Total Fluid Intake (TFI) 2
Children (male)	9–13	2100	1680	2400	1920
Children (female)	9–13	1900	1520	2100	1680
Adolescents (male)	14–18	2500	2000	3300	2640
Adolescents (female)	14–18	2000	1600	2300	1840

¹ TWI = plain water + beverages + water from food moisture; ² values refer to the assumption that water from solid food contributes 20% to the TWI. TFI = TWI − 20%, TFI calculated based on Kavouras et al., 2017 [5].

.

The investigation of fluid consumption in the population group of children and adolescents presents a significant challenge. On the one hand, children are prone to recall bias and heavily rely on their parents for nutritional guidance [6]. On the other hand, adolescents experience rapid changes in their dietary habits in association with increased demand for essential nutrients as a result of hormonal and growth changes, which significantly influence water intake both from beverages and plain water, as well as from food [7].

In recent years, there has been a notable increase in the number of studies investigating fluid consumption, reflecting its growing importance in the context of children and adolescents. Interestingly, it was found that only 50% of the 38 studies included in the review reported data on water consumption [8] and only 39% of the children and 25% of the adolescents exhibited compliance with the EFSA recommended intake levels [9]. However, there is a lack of data that specifically aggregates the TWI and TFI values for children and adolescents which highlights the necessity for thorough research into the hydration patterns of these age groups. This may facilitate the evaluation of adherence to guidelines and the provision of a more precise overview of water intake.

The lack of comprehensive, aggregated data on TWI and TFI for children and adolescents underscores the need for a systematic examination of hydration patterns. This systematic literature review addresses these gaps and aims to evaluate and compare TWI and TFI values for children and adolescents, taking into account gender and adherence with the reference values proposed to provide insights into hydration adequacy and identify potential disparities or deviations from established recommendations.

2. Materials and Methods

2.1. Search Strategy

A systematic literature search was conducted on 10/11/24 in PubMed, Scopus, and Scholar following the PRISMA guidelines [10]. To systematically review the relevant literature, we employed a search strategy utilizing a combination of keywords: adolescent* OR youth* OR child* OR student* AND “water intake” OR “fluid intake” OR “hydration status” OR dehydration). The primary outcomes were about hydration status assessed by mean TWI and mean TFI, while the secondary outcomes were the proportion of participants dehydrated. No funding was received for this study and this review was not registered.

2.2. Selection Procedure

Duplicates of records found in the search were removed. Two independent researchers (GP and KA) screened the remaining records for titles and abstracts to identify studies meeting the inclusion criteria. Any disagreements arising at this stage were resolved through discussion or consultation with a third reviewer. Full-text screening was performed by the same researchers and any disagreements were resolved similarly. No automation tools were used in the process. The review authors declare no competing interests. To minimize bias and improve the robustness of our synthesis, we systematically examined the following factors in each study:

• Sample Selection: We reviewed whether the studies clearly defined their inclusion and exclusion criteria plus whether they provided details on participant recruitment or not. Only the studies with well-defined populations and representative samples were considered in our systematic review as they are far more reliable.
• Measurement Methods: The assessment tools used to quantify total water intake (TWI) and total fluid intake (TFI) were closely evaluated. The studies employing validated dietary assessment methods, namely 24 h recalls, food frequency questionnaires (FFQs), or hydration-specific intake logs, were included, as they are considered methodologically valid. The studies relying on self-reported data without any form of further validation were noted as having a potential risk of measurement bias.
• Potential Confounders: We assessed whether the studies accounted for the key variables influencing hydration, such as physical activity levels, climate conditions, and dietary patterns. The studies that did not adjust for these factors were noted as having a higher risk of confounding bias.
• Data Completeness and Reporting: We examined whether the studies reported the key hydration outcomes (mean TWI/TFI values with standard deviations) and whether they had significant missing data. The studies that did not provide sufficient detail for data synthesis were excluded from the review.

2.3. Inclusions and Exclusions

The criteria for including the articles for this review were the following: (1) studies with child boy and girl (9–13 years) and adolescent boy and girl (14–18 years) participants; (2) studies analyzing TFI and TWI; (3) articles providing information on water/fluid intake (mL/day); and (4) articles published between 2004 and 2024.

Studies were excluded if they (1) involved participants under 8 years old and over 18 years old; (2) were clinical studies; (3) focused on chronic diseases or disabilities; or (4) were irrelevant to the review’s purpose; (5) were gray literature; or (6) were not in English language.

Figure 1 presents the research strategy. Of the 8731 studies screened, only 24 were found relevant for the final review. In this review, data for TWI and for TFI was stratified by age groups: (children (9–13 years) and adolescents (14–18 years)), and by gender. These divisions were made to align the data with the recommendations of the EFSA and the IOM.

2.4. Data Extraction

A standardized data extraction was carried out by the study team. All the data passed to this database were examined by one of the researchers (KA). The data were as follows: year, authors, type of article, sample size, country, study design, assessment tool, and values on both children and adolescents (boys and girls). In the presentation of results, the mean and standard deviation (SD) of the total water intake (TWI) and total fluid intake (TFI) values were used to summarize hydration patterns, and the proportion (%) of participants classified as dehydrated based on the established thresholds were reported to assess hydration status in each group. If the studies reported missing or incomplete summary statistics (e.g., means or SDs) and sufficient data could not be extracted from the publication, no further action was taken to obtain or impute the missing information. Such studies were excluded from the synthesis but were mentioned in the narrative of the systematic review when relevant. No sensitivity analyses were conducted in this review, as all the included studies met the eligibility criteria and provided adequate data for synthesis.

The definitions of TWI and TFI have been established by EFSA and IOM. TWI is the total fluid intake including water from food (i.e., water from vegetables or fruit), while TFI is the total fluid intake from beverages and water sources only. The guidelines for children and adolescents for TWI and TFI can be found in Table 1.

3. Results

Of the 24 included studies, 17 (71%) utilized validated dietary assessment tools, increasing confidence in the reported hydration data. Six studies (25%) explicitly considered climate factors when analyzing hydration patterns, reducing confounding bias. Five studies (21%) had small sample sizes (<200 participants), which may limit generalizability but were retained for their regional relevance. While variations in study design and assessment methods introduced considerable heterogeneity, the overall quality of the included studies was considered moderate, given the consistency of findings across different populations. The review includes 24 articles (Ν = 24) around the world (

Table 2. Summarized data for water intake in children and adolescents, by gender.

	Studies	Type of Article	Sample	Country	Hydration Outcome	Assessment Tool	Total Mean Score mL/day
1	[24]	Cross-sectional	230 Adolescents	Malaysia	TFI	15-item beverage intake questionnaire	TFI 1854 ± 895
2	[25]	Cross-sectional	279 Children and 370 Adolescents	China	TFI	7-day fluid-specific record (Liq.In7 record)	TFI children 966 ± 30; adolescents 1177 ± 31
3	[26]	Cross-sectional	210 Children	Greece	TFI	Fluid intake for 2 days	TFI 1771 ± 634
4	[5]	Cross-sectional	150 Children	Greece	TFI	Fluid intake for 2 days, 24 h urine collection	TFI Boys 1729 ±756; TFI Girls 1550 ± 573
5	[28]	Cross-sectional	3611 Children and 8109 Adolescents	Global	TFI	7-day fluid record in school or online	Children TFI Boys 651 ± 535 Girls 661 ± 525; Adolescent TFI Boys 813 ± 600 Girls 740 ± 556
6	[27]	Cross-sectional	760 Children	Belgium	TFI	Total liquid intake during 7 consecutive days	TFI 809 (513–1089)
7	[29]	Cross-sectional	207 Children	Malaysia	TFI	Questionnaire BEVQ-15	TFI 1335 ± 550
8	[30]	Cross-sectional	388 Children and 478 adolescents	Indonesia	TFI	7-day fluid-specific record	TFI 4–9 years 2165 ± 45; TFI 10–17 years 2488 ± 49
9	[31]	Cross-sectional	2710 Children and 1541 Adolescents	Turkey	TFI	Questionnaire	TFI 1611 ± 688 Children TFI 1603 ± 682; Adolescents TFI 1744 ± 710
10	[32]	Cross-sectional	907 Children and 566 Adolescents	France	TFI	Based on a 7 day food record	TFI 11–14 years 1883 ± 473, 15–17 years 1955 ± 595
11	[33]	Cross-sectional	1551 Adolescents	Spain	TFI	Two non-consecutive 24 h diet recalls period.	TFI 1004
12	[13]	Cross-sectional	358 Children	Lebanon	TWI	Questionnaire	TWI 1698 ± 17 9–13 years old
13	[14]	Cross-sectional	372 Children	Spain	TWI	Questionnaire HSQ-AY	TWI Boys: 2649 (2217–3353); TWI Girls: 2509 (1924–3316)
14	[15]	Cross-sectional	835 Children	France	TWI	7-day food record	TWI 1324 (1295, 1353)
15	[16]	Cross-sectional	563 Children and 517 Adolescents	Australia	TWI	Two days of dietary recall.	TWI Children 1700 ± 500 Adolescents 2100 ± 600
16	[17]	Cross-sectional	845 Children	UK	TWI	4-day food records.	TWI 1338 ± 401
17	[11]	Cross-sectional	242 Children	Spain	TWI	3 d weighed dietary records and 24 h urine collection	TWI 1412 ± 431
18	[12]	Cross-sectional	373 Children	Spain	TWI	Two 24 h recalls	TWI 1483 ± 507
19	[18]	Cross-sectional	588 Children and 417 Adolescents	Mexico	TWI	Two 24 h recall surveys and 12-month FFQ	TWI children 1605 ± 644; TWI adolescents 1922 ± 650
20	[19]	Cross-sectional	65 Children and 81 Adolescents	Spain	TWI	Liquid intake 7-day record (Liq.in7).	TWI children 1184 ± 67; TWI adolescents 1321 ± 68
21	[20]	Cross-sectional	212 Children and 210 Adolescents	Spain	TWI	three-day recording period	TWI children 1387 (41); TWI adolescents 1366 (46)
22	[21]	Cross-sectional	449 Children	Guatemala	TWI	FFQ	TWI 1837 ± 484
23	[22]	Cross-sectional	1004 Children and 1997 Adolescents	USA	TWI	Complete and reliable 24 h dietary recall	All groups 1920 ± 36; TWI Children 1644 ± 39; TWI Adolescents 2397 ± 66
24	[23]	Longitudinally	1083 Children	Germany	TWI	3 day Weighted dietary records	TWI 1784 ± 405

) that contain either information on TWI or TFI. In particular, 13 studies [11,12,13,14,15,16,17,18,19,20,21,22,23] reported TWI in children (9–13 year) and adolescents (14–18 year), whereas 11 studies [5,24,25,26,27,28,29,30,31,32,33] provided a more detailed description of the TFI in both children and adolescents.

The findings of the 24 studies retrieved were further elaborated and stratified by sex in order to assess any differences between sexes. This review managed to aggregate data from 16.254 (52%) children and 15.367 (48%) adolescents (total 31,621). As illustrated in Figure 2, 56% of the studies originate from Europe, while 24% are from Asia.

Table 3. Comparative evaluation of the TWI in children and adolescents, in accordance with the criteria set forth by EFSA and IOM.

	TWI
	Children (8–14 year)
	Boys		Girls
	EFSA 1	IOM 1	EFSA 1	IOM 1
	Adequate Intake (mL/day)	Adequate Intake (mL/day)	Adequate Intake (mL/day)	Adequate Intake (mL/day)
	2100 mL/day	2400 mL/day	1900 mL/day	2100 mL/day
Studies
[11]	1465 ± 473	1465 ± 473	1357 ± 378	1357 ± 378
[12]	1443 ± 489	1443 ± 489	1520 ± 521	1520 ± 521
[13]	1739 ± 17	1739 ± 17	1554 ± 9	1554 ± 9
[14]	2649 (2217–3353)	2649 (2217–3353)	2509 (1924–3316)	2509 (1924–3316)
[15]	1506 (1443, 1570)	1506 (1443, 1570)	1313 (1249, 1377)	1313 (1249, 1377)
[16]	1700 ± 500	1700 ± 500	1300 ± 300	1300 ± 300
[17]	1359 ± 390	1359 ± 390	1316 ± 410	1316 ± 410
[23]	1891 ± 428	1891 ± 428	1676 ± 386	1676 ± 386
[22]	1880 (1730, 2030)	1880 (1730, 2030)	1790 (1640, 1940)	1790 (1640, 1940)
[21]	1845 ± 555	1845 ± 555	1919 ± 508	1919 ± 508
[20]	1440 ± 46	1440 ± 46	1334.55 ± 46.58	1334.55 ± 46.58
[18]	1831 ± 713	1831 ± 713	1768 ± 664	1768 ± 664
[19]	1220 ± 99	1220 ± 99	1133 ± 85	1133 ± 85
	Adolescents (14–18 year)
	Boys		Girls
	EFSA 1	IOM 1	EFSA 1	IOM 1
	Adequate intake (mL/day)	Adequate intake (mL/day)	Adequate intake (mL/day)	Adequate intake (mL/day)
	2500 mL/day	3300 mL/day	2000 mL/day	2300 mL/day
Studies
[16]	2100 ± 600	2100 ± 600	1700 ± 500	1700 ± 500
[18]	2032 ± 650	2032 ± 650	1812 ± 650	1812 ± 650
[19]	1294 ± 80	1294 ± 80	1374 ± 128	1374 ± 128
[20]	1398 ± 43	1398 ± 43	1236 ± 40	1236 ± 40
[22]	2890 (2650, 3130)	2890 (2650, 3130)	1970 (1840, 2100)	1970 (1840, 2100)

¹ desirable values according to EFSA and IOM.

and

Table 4. Comparative evaluation of the TFI in children and adolescents in accordance with the criteria set forth by EFSA and IOM.

	TFI
	Children (8–14 year)
	Boys		Girls
	EFSA 1	IOM 1	EFSA 1	IOM 1
	Adequate Intake (mL/day)	Adequate Intake (mL/day)	Adequate Intake (mL/day)	Adequate Intake (mL/day)
	1680 mL/day	1920 mL/day	1520 mL/day	1680 mL/day
Studies
[25]	981 ± 38	981 ± 38	949 ± 47	949 ± 47
[26]	1870 ± 702	1870 ± 702	1673 ± 544	1673 ± 544
[27]	920 (671–1154)	920 (671–1154)	836 (606–1061)	836 (606–1061)
[28]	651 ± 535	651 ± 535	661 ± 525	661 ± 525
[5]	1729 ± 756	1729 ± 756	1550 ± 573	1550 ± 573
[29]	1338 ± 556	1338 ± 556	1283 ± 540	1283 ± 540
[30]	2169 ± 57	2169 ± 57	2159 ± 71	2159 ± 71
[31]	1658 ± 690	1658 ± 690	1560 ± 670	1560 ± 670
[32]	2019 ± 474	2019 ± 474	1746 ± 441	1746 ± 441
	Adolescents (14–18 year)
	Boys		Girls
	EFSA 1	IOM 1	EFSA 1	IOM 1
	Adequate intake (mL/day)	Adequate intake (mL/day)	Adequate intake (mL/day)	Adequate intake (mL/day)
	2000 mL/day	2640 mL/day	1600 mL/day	1840 mL/day
Studies
[24]	2149 ± 1070	2149 ± 1070	1680 ± 725	1680 ± 725
[25]	1240 ± 46	1240 ± 46	1113 ± 42	1113 ± 42
[28]	813 ± 600	813 ± 600	740 ± 556	740 ± 556
[30]	2499 ± 65	2499 ± 65	2472 ± 74	2472 ± 74
[31]	1854 ± 714	1854 ± 714	1654 ± 697	1654 ± 697
[32]	2150 ± 565	2150 ± 565	1769 ± 581	1769 ± 581
[33]	1004 ± 47	1004 ± 47	1004 ± 29	1004 ± 29

¹ desirable values according to EFSA and IOM.

present a comprehensive overview of the comparative analysis of the values, as evaluated by both the EFSA and the IOM in TWI and TFI by sexes. The total number of males was 15,656 and 15,965 for females (

Table 5. Characteristics of the studies included in this systematic review N = 24.

Country	(N) (%)
Europe	13(56%)
South America	1(4%)
North America	2(8%)
Australia	1 (4%)
Asia	6 (24%)
Global	1 (4%)
Group age
Children 9–13 year boys	8138 (50%)
Children 9–13 year girls	8116 (50%)
Total Children	16,254 (100%)
Adolescents 14–18 year boys	7518 (49%)
Adolescents 14–18 year girls	7849 (51%)
Total adolescents	15,367 (100%)
Publication year
2004–2014	6 (25%)
2014–2024	18 (75%)
Number of participants
TWI children	7032 (23%)
TWI adolescents	2522 (8%)
TFI children	9222 (29%)
TFI adolescents	12,845 (40%)
Total children and adolescents	31,621 (100%)

).

3.1. TFI in Children by Gender

In a study including 279 children (9–13 year) from China, the mean values of fluid intake in boys and girls are 981 (38) mL/day and 949 (47) mL/day, respectively [24]. Furthermore, in a study with 3611 children from around the world where the fluid intake was assessed with a validated 7-day questionnaire, boys had a mean fluid intake of 651 (535) mL/day and girls 661 (525) mL/day [28], values much lower than the ones proposed in the IOM guidelines. In addition, 207 children from Malaysia had an average fluid intake of 1335 (550) mL/day [29]. In the studies by Bougatsas et al. [26] and Kavouras et al. [5], in Greece, it was observed that the mean fluid intake values exceed those of the EFSA guidelines for fluid consumption with 1771 (634) mL/day and 1700 (573) mL/day, respectively. The desired EFSA and IOM values are met by only two studies. The first involves 388 children from Indonesia with values in boys and girls of 2169 (57) mL/day and 2159 (71) mL/day, respectively [30]. The second is on 789 children from France with values in boys and girls of 2019 (474) mL/day and 1746 (441) mL/day, respectively [32].

3.2. TFI in Adolescents by Gender

Likewise, similar results are found in adolescents (14–18 year). Specifically, a study including 370 adolescents from China reported that boys had an average fluid intake of 1240 (46) mL/day and girls 1113 (42) mL/day [25]. Much lower values were observed in 8109 adolescents worldwide, with boys and girls having a mean fluid intake of 813 (600) mL/day and 740 (556) mL/day, respectively [28]. In the study by Somuncu et al. [31], 1541 adolescents did not meet the desired values except for girls with a mean of 1654 (697) mL/day. In the study by Lioret et al. [32], only 535 adolescents exceeded the EFSA values with 2032 (650) mL/day and 1812 (650) mL/day in boys and girls, respectively. Finally, in 1988 adolescents from Spain showed a mean value of 1004 (47) mL/day in boys and 1004 (29) mL/day in girls, values that were much lower than the desirable values of the EFSA and the IOM [33].

3.3. TWI in Children by Gender

In a study examining TWI in Spanish children, a 3-day food record revealed that boys consumed 1465 (473) mL/day and girls consumed 1357 (378) mL/day. These data are presented in a report by Perales-García et al. [11], published in 2018. In the same country, after two consecutive recalls, the values were approximately the same for both boys and girls, at 1443.00 (489) mL/day and 1520 (521) mL/day, respectively [12]. In a study conducted by Jomaa et al. [13] involving 358 children from Lebanon, the mean TWI was found to be 1739 (18) mL/day in boys and 1554 (9) mL/day in girls. In a separate study involving 372 children from Spain [14], the median TWI was 2649 (2217–3353) mL/day in boys and 2509 (1924–3316) mL/day in girls, as determined through a validated questionnaire for children [34]. It is notable that only these values exceed the guidelines set forth by both the EFSA and the IOM. In France, Vieux et al. [15] reported median values of 1506 (1443, 1570) mL/day for boys and 1313 (1249, 1377) mL/day for girls. In the UK, Vieux et al. [17] reported mean values of 1359 (390) mL/day and 1316 (410) mL/day, respectively. In a sample of 588 children from Mexico, the use of a two-consecutive-recall method revealed higher TWI values, with boys averaging 1831 (713) mL/day and girls 1768 (664) mL/day [18]. The mean TWI for 449 Guatemalan children was 1837 (484) mL/day, as reported by Montenegro-Bethancourt et al. [21]. Conversely, Iglesia-Altaba et al. [19] found that the mean TWI for boys was 1220 (99) mL/day and for girls was 1133 (85) mL/day, values that fall below the recommended levels proposed by the EFSA and IOM guidelines. The median TWI for 1047 children in the USA was 1.88 (1.73, 2.03) L/d, as reported by Kant and Graubard [22]. In Germany, the mean TWI for 3736 children was 1891 (428) mL/day for boys and 1676 (386) mL/day for girls, as reported by Sichert-Hellert et al. [23].

3.4. TWI Adolescents by Gender

Regarding TWI in adolescents, the findings were similarly noteworthy, as this age group also appeared to be unable to meet the recommended daily intake (RDI) guidelines. In a study conducted by Sui et al. [16], 269 adolescents from Australia reported an average daily intake of 1.7 (0.5) L, with boys consuming 2.1 (0.6) L and girls consuming 1.7 (0.5) L. In Mexico, following two consecutive recalls, 417 adolescent boys and girls were found to consume 2032 (650) mL/day and 1812 (650) mL/day water, respectively [18]. In Spain, 81 adolescents were observed to consume an average of 1321 (68) mL/day [19]. In the same country, the results from another study indicated a similar consumption pattern, with an average of 1366 (46) mL/day [20]. In a study conducted by Kant and Graubard [22], it was observed that the median TWI values for adolescent boys were 2809 L/day (2.65, 3.13), while the median TWI values for adolescent girls were 1.97 L/d (1.84, 2.10). It is noteworthy that only the TWI values of the male subjects in the study by Kant and Graubard [22] exceeded the EFSA guidelines (Table 3).

The included studies demonstrated substantial heterogeneity, which was explored by examining differences in study methodologies, populations, and outcome measures. Variations in fluid intake assessment tools (e.g., FFQs, 24 h recalls, water balance questionnaires) contributed to inconsistencies in the reported results. Additionally, demographic factors such as age, gender, and geographic region influenced fluid consumption patterns, further contributing to heterogeneity. Environmental factors, such as temperature and physical activity levels, were inconsistently reported and analyzed, limiting the ability to account for their role in hydration status.

No sensitivity analyses were conducted in this review. All the included studies met the predefined eligibility criteria and provided sufficient data for synthesis, reducing the need for additional analyses to test the robustness of the findings. An assessment of reporting bias revealed that several studies did not report key data, such as means and standard deviations for TWI and TFI or specific thresholds used for dehydration classification. While these studies were excluded from the synthesis, they were mentioned in the narrative review. The exclusion of studies with incomplete data may have introduced selection bias. Additionally, the lack of pre-registration for some studies and limited reporting of non-significant findings indicate a potential risk of publication bias. The certainty of the evidence was assessed qualitatively due to the heterogeneity of the included studies. The evidence for hydration inadequacy in children and adolescents was considered moderate, as most studies consistently reported low compliance with recommended fluid intake levels. However, the lack of standardized assessment tools and universal guidelines reduced the overall confidence in the body of evidence.

4. Discussion

During childhood and adolescence, maintaining water balance in the body is crucial. This is because the appropriate functioning of the brain is maintained, and the body is able to retain the optimal levels of fluids necessary for physical and sporting activities [35]. The purpose of this review was to evaluate and compare the TWI and TFI values, disaggregated by gender, for children and adolescents against the reference values proposed. Our findings are in line with the prevailing scientific consensus that boys and girls during childhood and adolescence tend to consume lower amounts of water, both in total (including water from food) and in beverages including water [6].

4.1. Global Variations in Water Intake

According to the literature, two major categories of total consumption are considered: TWI and TFI. TWI, which also includes water from foods and beverages, refers to the total volume of fluid consumed, while TFI refers to the total volume of all beverages, including water.

Both EFSA and IOM agree that water from food constitutes approximately 20% of the TWI, with the remaining 80% being derived from beverages, including water [5]. The recommended adequate TWI quantities based on IOM for children, which refers to the total volume of fluid consumed, are 2400 mL/day and 2100 mL/day for boys and girls, respectively, while for adolescents, the recommended quantities are 3300 mL/day and 2300 mL/day for boys and girls, respectively [3]. In contrast, EFSA employs a different approach, utilizing biomarkers of osmolality in combination with desirable water intake data derived from scientific studies [36]. Consequently, the recommended TWI values for children (9–13 years) are 2100 and 1900 mL/day for boys and girls, respectively, while for adolescents (14–18 years), the values are 2500 and 2000 mL/day for boys and girls, respectively [2]. The discrepancy in values (EFSA vs. IOM) can be attributed to the fact that IOM considers daily physical activity and climate variations [36]. The challenge of making a comparison between these values is compounded by the fact that the two organizations have proposed different values, up to 800 mL/day, for the TWI of adolescent males [36].

There is also a discrepancy in the way in which TWI is calculated across different regions of the world. In countries such as Japan and China, the intake of water from food items accounts for up to 51% of the total water intake [37]. Conversely, in countries with a warm climate, the suggested values are considerably higher [7]. For example, South American countries have been observed to significantly increase the distribution of TFI by consuming the traditional mate drink [28]. This results in a lack of consensus on global guidelines for water intake and consequently, some regions have established their own guidelines. For instance, China has set four recommendations [25], as have Australia [16] and Indonesia [30].

In the majority of studies involving subjects aged 4–8 years, no sex separation was observed. Consequently, these subjects were excluded from the analysis, with the exception of four studies [19,25,28,30], which assessed both boys and girls.

All the studies analyzed (N = 24) include values for boys and girls in the specified age groups, either in TFI or TWI, depending on the specific aims of each study. The results indicate that the percentage of boys and girls in the study population is nearly equivalent, with 49% of the subjects identified as male and 51% identified as female.

From the results, it should be noted that in only one study participants managed to meet the TWI values recommended by EFSA and IOM (Table 3). Regarding the TFI values, there were four studies in children and four in adolescents where the participants managed to meet the TFI values recommended by EFSA and IOM (Table 4). Furthermore, in adolescents, only 654 boys (22%) [22] out of a total of 2522 met the TWI criteria, which is a cause for significant concern regarding euhydration. One potential explanation is that the dietary sources of water are insufficient to reach the body’s needs. This may be due to the absence of fruits and vegetables from the diet or the lack of water-rich foods in the diet [22]. It is important to note that children are significantly influenced by their parents in terms of their drinking habits. Conversely, parental knowledge of appropriate fluid consumption has been positively associated with their children’s drinking habits and, subsequently, their water balance [29,38].

4.2. Regional and Climate Variations

The climate seems to affect preferences regarding fluid consumption. For instance, the lowest mean TFI was observed in 760 Belgian children, with a mean value of 809 (513–1089) mL/day. When stratified by sexes, the mean TFI was 920 (671–1154) mL/day in boys and 836 (606–1061) mL/day in girls [27]. In Indonesia, the mean TFI value in 388 children was 2165 ± 45 mL/day, which was approximately three times higher than that observed in Belgian children [30]. This considerable difference may be attributed to the fact that the two countries have markedly different climates and also disparate preferences regarding fluid consumption. These values from Indonesia exceed the EFSA and IOM guidelines as well as the Indonesian guidelines [30].

Regarding TWI in children, the only study meeting the EFSA guidelines is a study including 372 children from Spain with boys and girls showing mean values of 2649 (2217–3353) mL/day and 2509 (1924–3316) mL/day, respectively [13]. It is remarkable as it is the only study using a validated water balance questionnaire in Spanish children. In a global study with 13 countries from 3 continents (3611 children and 8109 adolescents using the same total fluid assessment tool), a low TFI was observed, with a mean of 651 (535) mL/day and 661 (525) mL/day for boys and girls of childhood age, respectively, and 813 (600) mL/day and 740 (556) mL/day for boys and girls of adolescent age, respectively [28]. It is evident that the values are relatively low due to the disparate age groupings, with children classified as 4–9 years old and teenagers as 10–17 years old. Despite that, the values in children and adolescents are considerably lower than those recommended by the EFSA. These values are not unusual, as similar results can be found in the review by Özen et al. [8] in terms of consumption and values per beverage.

4.3. Impact of Sugar-Sweetened Beverages (SSBs)

Given the rapid increase in the consumption of sugar-sweetened beverages (SSBs) which contain free sugars and calories among children and adolescents, it is essential to assess their impact on hydration and overall health. This is particularly important given the evidence that higher water intake is associated with lower BMI, while increased consumption of SSBs can contribute to higher BMI, metabolic disturbances, and adverse health outcomes, including tooth decay [39]. Excessive intake of SSBs not only provides empty calories with minimal nutritional value but also displaces healthier beverage options like water and milk, which are essential for growth, hydration, and bone health during childhood and adolescence. Moreover, habitual SSB consumption has been linked to an increased risk of developing type 2 diabetes, hypertension, and cardiovascular diseases later in life, highlighting the long-term implications of early dietary patterns. In contrast, higher water intake has been associated with improved satiety, better dietary quality, and the substitution of calorically dense beverages, all of which contribute to healthier body weight regulation. Encouraging children and adolescents to consume more water and reduce SSB intake can, therefore, serve as a simple yet effective strategy to promote better hydration, support healthy weight management, and reduce the risk of chronic diseases.

Schools and community programs can play a critical role in shaping these behaviors by promoting water availability, discouraging the sale and consumption of SSBs, and incorporating education about the benefits of proper hydration into health curricula. Policy-level interventions, such as implementing taxes on SSBs and mandating clearer nutritional labeling, can further help reduce consumption. For instance, in 672 children following a communication intervention, awareness of the importance of water over SSBs increased statistically significantly from other schools [39]. Combined with ongoing parental guidance and public health campaigns, these measures can foster healthier hydration habits in children and adolescents, creating a foundation for improved lifelong health. Educational initiatives targeting schools and communities are necessary to encourage the reduction or avoidance of SSBs, as suggested by Guelinckx et al. [28].

With regard to the IOM guidelines, only two studies reported achieving the specified values for TFI. One study, conducted in Indonesia, found TFI values of 2169 (±57) mL/day and 2159 (±71) mL/day for boys and girls, respectively, and 2472 (±74) mL/day for adolescent girls [30]. The second study, by Lioret et al. [32], reported mean TFI values of 2019 (±474) mL/day for boys and 1746 (±441) mL/day for girls within the childhood age group. To address the challenges in accurately assessing fluid intake, mobile health (mHealth) tools and smart applications could play a transformative role. These technologies enable the real-time tracking of dietary and fluid intake, including hydration indices, offering a more efficient and reliable alternative to traditional methods such as food frequency questionnaires (FFQs) or 24 h dietary recalls. Tools of mHealth can also facilitate large-scale studies and interventions, helping to identify hydration gaps and promoting better fluid intake behaviors in children and adolescents. It is possible that this globally smart tool could be of benefit in the event of any future modifications in adequate intakes proposed.

This review took into consideration data from over 31,500 children and adolescents assessing water consumption both as TWI and TFI. In addition, studies were systematically evaluated and compared not only with the EFSA, but also with the IOM guidelines, examining hydration patterns separately by gender in childhood and adolescence.

4.4. Limitations

The studies included in this review utilized diverse tools for assessing fluid intake, such as food frequency questionnaires (FFQs), recalls, and water balance questionnaires. These varying methodologies introduce heterogeneity and potential inaccuracies in estimating adequate fluid intake. Additionally, the lack of universal, standardized fluid intake guidelines tailored to age-specific needs and environmental factors, such as temperature and physical activity levels, complicates comparisons across studies. Validated tools for fluid intake assessment remain limited, with commonly used methods, such as food diaries and 24 h recalls, being time-intensive, prone to recall bias, and particularly challenging for children and adolescents to complete with accuracy. Although this review employed a systematic approach to identify and synthesize relevant studies, potential limitations include reliance on three databases for study identification, which may have excluded relevant studies published in other databases. Additionally, studies with missing or incomplete data on key outcomes were excluded from the synthesis, potentially introducing selection bias. The decision not to contact authors for missing data or conduct sensitivity analyses may have limited the robustness of the findings.

5. Conclusions

This review underscores the widespread inadequacy of meeting hydration guidelines among children and adolescents worldwide. While some populations achieve recommended fluid intake levels, the majority fall short, highlighting the need for targeted public health interventions. Efforts should focus on promoting water consumption, reducing sugar-sweetened beverage intake, and educating communities about proper hydration. The provision of free drinking water in public parks and the implementation of information campaigns promoting hydration could be implemented by the governments. Parents should also be mindful of adequate water supplies at school or during sports, as well as establishing a hydration routine. Furthermore, schools should educate students about the benefits of water and create proper water facilities that are easily accessible, as well as implement reminders from teachers about the importance of water consumption.

The findings highlight the urgent need for the development and adoption of standardized, validated tools for assessing fluid intake in children and adolescents. To improve the accuracy of hydration assessment, adopting mobile health (mHealth) tools and smart applications offers promising solutions. These technologies can overcome the limitations of traditional methods, providing more precise data and enabling scalable interventions. Future research should aim to standardize age- and environment-specific hydration guidelines and develop validated assessment tools tailored for young populations. Addressing these gaps is crucial to fostering optimal hydration practices and supporting the health and well-being of children and adolescents globally.