. 2019 Jan 1;11(1):70. doi: 10.3390/nu11010070

Table 1.

Summary of key hydration reviews.

Citation	Review Type/ Reporting Quality	Description	Key Findings/ Conclusions
Skin Health
Akdeniz et al., 2018 [24]	Systematic review Met 17 of 19 required PRISMA items for systematic review. ¹ Fulfilled 4 of 13 required AMSTAR 2 items for systematic review.²	Assessed fluid intake and skin hydration and/or barrier function. Included human intervention studies published through 2016. Met PRISMA reporting criteria except for absence of explicit statement of questions being addressed. Risk of bias for each study assessed using Cochrane Risk of Bias tool. 5 intervention studies included.	Additional dietary water intake is associated with increased skin hydration and reduced skin dryness. Evidence is weak overall in terms of quantity and methodological quality and the clinical relevance is unclear.
Neurological Function
Benton and Young, 2015 [25]	Narrative review	Assessed relationship of hydration and mood and/or cognition. Information on search strategy, selection process, and data extraction were not provided. 30 intervention studies included.	Data consistently show a reduction of >2% in body mass due to dehydration results in effect on mood, fatigue, and alertness. Effects on cognition have been less consistent. Only a few studies have looked at females, and due to the effect of sex hormones on kidney function, this is a gap in knowledge. Four intervention trials on cognition in children show an effect of hydration. Lack of studies in sedentary adults living in temperate climates, as well as older adults.
Masento et al., 2014 [26]	Narrative review	Assessed relationship of hydration and cognition. Search strategy not provided in full detail and no information on selection process and data extraction were provided. 22 intervention and 4 observational (prospective cohort and cross-sectional) studies included.	Data suggests hydration is important for supporting cognition and mood. Hydration is particularly important in those with poor fluid regulation, which includes children and elderly. Challenges in quantifying effects across studies include a lack of standardized approaches to assessing cognition, as well as difficulties in assessing hydration state.
Renal Function
Cheungpasitporn et al., 2016 [27]	Systematic review/Meta-analysis Met 22 of 24 required PRISMA items for meta-analysis.¹ Met 10 of 16 required AMSTAR 2 items for meta-analysis. ²	Assessed fluid intake and kidney stones, as well as adherence and safety of high fluid intake to prevent kidney stones. Included RCTs and observational studies published from 1980 through 2014. Met PRISMA reporting criteria except for absence of explicit statement of questions being addressed. Risk of bias for each study assessed using Jadad score. 2 RCT, 6 prospective cohort, and 1 cross-sectional studies included.	Significant association between high fluid intake and a lower risk of incident kidney stones based on pooled risk ratio (RR) for RCT only [0.40 (95% CI 0.20–0.79)] and observational studies only [0.49 (95% CI 0.34–0.71)]. A subgroup analysis found that high fluid intake was associated with decreased kidney stone risk in both men [RR 0.67 (95% CI 0.58–0.79)] and women [RR 0.65 (95% CI 0.56–0.76)]. RCTs were graded as low-quality and observational studies were graded as moderately high, with detectable heterogeneity among observational studies.
Gastrointestinal Function
Boilesen et al., 2017 [28]	Narrative review	Assessed the role of water and fluid intake in the prevention or treatment of functional intestinal constipation in children and adolescents. Information on search strategy, selection process, and data extraction provided, albeit not in full detail. Included epidemiological and clinical studies published from 1966 through 2016. 5 intervention, 5 cross-sectional, and 1 case-control studies included.	Studies with a sample of the general population suggest that a lower intake of water and/or fluids is associated with intestinal constipation; however, those with existing constipation did not show an advantage of greater fluid intake.
Body Weight and Body Composition
Stookey, 2016 [29]	Qualitative review	Assessed totality of evidence on hydration and weight-related outcomes. Included human intervention studies published through 2014. Search strategy not provided in full detail but selection process and data extraction were fully explained. 134 RCT included on hydration and energy intake, energy expenditure, fat oxidation, and weight change.	Drinking water increases energy expenditure in metabolically-inflexible, obese individuals. Drinking water increases fat oxidation when blood sugar and/or insulin are not elevated and when it is consumed instead of caloric beverages.

Abbreviations: AMSTART, A MeaSurement Tool to Assess systematic Reviews; CI, confidence interval; PRISMA, Preferred Reporting Items for Systematic Reviews; RCT, randomized controlled trial; RR, relative risk. ¹ PRISMA is an evidence-based minimum set of items for reporting in systematic reviews and meta-analyses and has been used to assess reporting quality. For meta-analysis, the PRISMA checklist contained 24 required reporting items that were used to assess quality. For systematic reviews, 19 items remained after exclusion of items specific to meta-analyses (i.e., item 13, 14, 15, 21, and 22 which are related to data analysis and overall risk bias assessment). ² AMSTAR 2 is an instrument used to assess the methodological quality of systematic reviews and meta-analysis. It has 16 items in total, whereby three of these are specific for meta-analysis. AMSTAR 2 is not intended to generate an overall score and thus, none is provided.