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Review
. 2024 May 30;16(11):1709.
doi: 10.3390/nu16111709.

Trends and Missing Links in (De)Hydration Research: A Narrative Review

Alexandre Rebelo-Marques  1   2   3 , Bruna Coelho-Ribeiro  4   5 , Adriana De Sousa Lages  6 , Renato Andrade  7   8   9 , José Afonso  10 , Rogério Pereira  7   8   11 , Ana Sofia Batista  1 , Vitor Hugo Teixeira  12   13   14 , Cristina Jácome  15
Affiliations
Review

Trends and Missing Links in (De)Hydration Research: A Narrative Review

Alexandre Rebelo-Marques et al. Nutrients. .

Abstract

Despite decades of literature on (de)hydration in healthy individuals, many unanswered questions remain. To outline research and policy priorities, it is fundamental to recognize the literature trends on (de)hydration and identify current research gaps, which herein we aimed to pinpoint. From a representative sample of 180 (de)hydration studies with 4350 individuals, we found that research is mainly limited to small-scale laboratory-based sample sizes, with high variability in demographics (sex, age, and level of competition); to non-ecological (highly simulated and controlled) conditions; and with a focus on recreationally active male adults (e.g., Tier 1, non-athletes). The laboratory-simulated environments are limiting factors underpinning the need to better translate scientific research into field studies. Although, consistently, dehydration is defined as the loss of 2% of body weight, the hydration status is estimated using a very heterogeneous range of parameters. Water is the most researched hydration fluid, followed by alcoholic beverages with added carbohydrates (CHO). The current research still overlooks beverages supplemented with proteins, amino acids (AA), and glycerol. Future research should invest more effort in "real-world" studies with larger and more heterogeneous cohorts, exploring the entire available spectrum of fluids while addressing hydration outcomes more harmoniously.

Keywords: athlete; dehydration; fluid intake; health; hydration; performance; research gaps; sports; sports drinks; water.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Publication-level distributions: (A)—publication dates; (B)—country of origin; (C)—world map: countries producing the studies; (D)—study design; (E)—hydration strategy of the articles. Legend: k—number of articles.
Figure 2
Figure 2
Participants related information: (A)—sex; (B)—age pyramid; (C)—level of competition based on the Participation Classification Framework (PCF) (218). Legend: NR—non-reported; Tier 1—recreationally active; Tier 2—trained/developmental; Tier 3—highly trained/national level; Tier 4—elite/international level.
Figure 3
Figure 3
Intervention-related information: (A)—exercise types; (B)—environmental conditions: temperature; (C)—environmental conditions: relative humidity; (D)—intake protocol: beverages type; (E)—intake protocol: beverages composition; (F)—within-session timing; (G)—within-season timing. Legend: AA—amino acids, CHO—carbohydrate, E—electrolytes, NR—non-reported, ORS—oral rehydration solution, PRO—proteins, RT—Room temperature.
Figure 4
Figure 4
Outcome-related information: (A)—outcomes domains; (B)—performance outcomes; (C)—health outcomes; (D)—hydration outcomes. Legend: 3MH-I—3-methylhistidine, ADH—antidiuretic hormone, BHI—beverage hydration index, Hct/hb—hematocrit/hemoglobin concentrations, PCr—phosphocreatine, PO—power output, RPE—rate of perceived exertion, USG—urine specific gravity, VCo2—volume of carbon dioxide breathed out, Vo2—oxygen consumption.
Figure 5
Figure 5
Evidence gap map. Interpretation guide: the area of the circles is proportional within cells but not between cells and is directly related to the proportion of the number of studies for that category. Each trial may be plotted multiple times, as the same trial may fit various categories (e.g., one trial may have analyzed multiple outcomes domains and include multiple exercise types). The number between different categories may differ, as the studies may have provided sufficient information to assess one field but not another. Legend (ordered alphabetically): AB—above, and post-set, BL—below, BLAB—below and above, BLN—below and normal, Blood—blood measures, BS—both sexes, BW changes—body weight changes, CHO—carbohydrates, CHO-E—carbohydrate-electrolytes, COMP—competitive phase, CRT—cool and RT, CYC—cycling, E—electrolytes, HCT/HB—hematocrit and hemoglobin concentrations, HR—heart rate, HRT—hot and RT, Hydr—hydration, INTER—inter-set, Inter-set, IP—inter-set and post-set, K+—potassium, N—normal, Na+—sodium, NAB—normal and above, PCF—performance competitive framework, PI—pre-set and inter-set, PIP—pre-set, Plasma osmo.—plasma osmolality, Plasma vol.—plasma volume, PO—power output, POST—post-set, POST-S—post-season, PP—pre-set and post-set, PRE—pre-set, PRE-S—pre-season, PRF—performance, PS—performance skills, PT—performance time, RNN—running, ROP—respiratory outputs, RPE—rate of perceived exertion, RT—room temperature, SCC—soccer, SD—sports drinks, Subjective—subjective feelings, SUG—sugars, Sweat out.—sweat outputs, T1—Tier 1, T1 + 2—mixed Tiers 1 and 2, T1 + 3—mixed Tiers 1 and 3, T2—Tier 2, T2–5—mixed Tiers from 2 to 5, T3—Tier 3, T4—Tier 4, Taste—taste perceptions, Temp—temperature, TRD—treadmill, U—unreported or unclear, UP—unknown placebo, Urine out.—urine outputs, USG—urine-specific gravity, VCo2, VO2, W—water, ≥T1—at least Tier 1. * Exercise types represented in less than 8% of studies, ** beverage types represented in less than 14% of studies, *** beverages composition represented in less than 19% studies, **** performance outcomes represented in less than 9% of studies, ***** health outcomes represented in less than 9% of studies, ****** hydration outcomes represented in less than 14% of studies.
Figure 6
Figure 6
Research priorities regarding hydration in healthy individuals and athletes.
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Grants and funding

The publication fees were supported by National Funds through FCT—Fundação para a Ciência e a Tecnologia, I.P., within CINTESIS, R&D Unit (reference UIDB/4255/2020). CIFI2D is financed by the Portuguese Foundation for Science and Technology, under the https://doi.org/10.54499/UIDB/05913/2020. Bruna Ribeiro was funded by 2023.04302.BD.