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. 2020 Aug 24;12(9):2554.
doi: 10.3390/nu12092554.

Inputs to Thirst and Drinking during Water Restriction and Rehydration

Lawrence E Armstrong  1   2 , Gabrielle E W Giersch  1 , Leslie Dunn  1 , Aidan Fiol  1 , Colleen X Muñoz  3 , Elaine C Lee  1
Affiliations

Inputs to Thirst and Drinking during Water Restriction and Rehydration

Lawrence E Armstrong et al. Nutrients. .

Abstract

Current models of afferent inputs to the brain, which influence body water volume and concentration via thirst and drinking behavior, have not adequately described the interactions of subconscious homeostatic regulatory responses with conscious perceptions. The purpose of this investigation was to observe the interactions of hydration change indices (i.e., plasma osmolality, body mass loss) with perceptual ratings (i.e., thirst, mouth dryness, stomach emptiness) in 18 free-living, healthy adult men (age, 23 ± 3 y; body mass, 80.09 ± 9.69 kg) who participated in a 24-h water restriction period (Days 1-2), a monitored 30-min oral rehydration session (REHY, Day 2), and a 24-h ad libitum rehydration period (Days 2-3) while conducting usual daily activities. Laboratory and field measurements spanned three mornings and included subjective perceptions (visual analog scale ratings, VAS), water intake, dietary intake, and hydration biomarkers associated with dehydration and rehydration. Results indicated that total water intake was 0.31 L/24 h on Day 1 versus 2.60 L/24 h on Day 2 (of which 1.46 L/30 min was consumed during REHY). The increase of plasma osmolality on Day 1 (297 ± 4 to 299 ± 5 mOsm/kg) concurrent with a body mass loss of 1.67 kg (2.12%) paralleled increasing VAS ratings of thirst, desire for water, and mouth dryness but not stomach emptiness. Interestingly, plasma osmolality dissociated from all perceptual ratings on Day 3, suggesting that morning thirst was predominantly non-osmotic (i.e., perceptual). These findings clarified the complex, dynamic interactions of subconscious regulatory responses with conscious perceptions during dehydration, rehydration, and reestablished euhydration.

Keywords: brain; dehydration; drinking; oropharyngeal; osmolality; sensation.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Perceptual ratings were recorded by test participants at the beginning of every waking hour. All ingested food and fluid was recorded in a diary. A single small urine sample was collected upon waking each morning; all other urine was collected in a 24-h container. Abbreviations: #, visited laboratory for body weight, blood and urine samples; AL, consumed water and food ad libidum for 24 h; H, evening hydration procedure (500 mL water consumed above ad libidum intake); E, experimental intervention began (gray shaded zone; ~0730 h on Day 2 to ~0730 h on Day 3), subjects drank no fluids and ate dry foods; T, when thirst was first sensed, the subject notified investigators and returned to the laboratory; REHY, consumed water ad libidum during 30-min seated laboratory observation period; C, conclusion of research participation.
Figure 2
Figure 2
Visual Analogue Scale (VAS) ratings for thirst, desire for water, mouth dryness, and stomach emptiness during water restriction and rehydration. Values are means (n = 14–18) and representative error bars indicate the median standard deviation of each variable. The REHY zone (top center) represents a 30-min monitored water consumption session. The mean VAS ratings measured during the thirst awareness visit to the laboratory (Day 1) are not depicted in this figure but appear in Table 3, column 3.
Figure 3
Figure 3
Relationships between visual analogue scale (VAS) ratings of thirst and plasma osmolality (POSM; panel A) and between thirst and body mass (panel B). The dual y-axes are scaled to span the approximate minimum and maximum values of each variable. The dissociation of plasma osmolality and thirst appeared only on Table 3. (lower right quadrant of panel A). The re-emergence of euhydration is represented by body mass values on Day 3 (upper right quadrant of panel B) that are statistically similar to baseline (initial value on Day 1). The horizontal dashed lines that intersect each axis represent the group mean values measured during the thirst awareness visit to the laboratory on Day 1.
Figure 4
Figure 4
In the present investigation, the existence and intensity of physiological signals and perceptual inputs vary, depending on hydration status. We propose that, with increasing dehydration in humans, the convergence of physiological signals and perceptual inputs, their intensity, and their increasing aversive mood create increased strength of motivation to seek water and drink. Our current insufficient understanding of the conversion of subconscious homeostatic signals and conscious subjective perceptions into human motivation and behaviors (▼symbol) relies on few recent animal studies.
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