Table 3.
Dietary, physiological, metabolic, and behavioral components of human 24-h water balance.
| Total water intake a (L/24h) | Intracellular metabolic water production b | Total solute load c (mOsm/24h) | Urine osmolality d (mOsm/kg) | Maximal renal concentrating ability (mOsm/kg) | Urine volume (L/24h) | Non-renal water loss (L/24h) e | Free water reserve f (L/24h) | |
|---|---|---|---|---|---|---|---|---|
| Functions and characteristics | Contributes to TBW | Product of human metabolism | Metabolized and digested products excreted in urine | Regulates TBW and ECV-ICV osmolality | Inherent quality of the kidneys | Regulates TBW and ECV-ICV osmolality | Excretory and secretory processes | Calculated index of euhydration, based on population statistics |
| Influential factors | Meal timing and contents, idiosyncratic thirst, physical activity, body size, cultural and learned preferences | Metabolic rate and substrates, physical activity, diet macronutrient and energy content, NES responses | Metabolic products, dietary contents, body size, idiosyncratic hunger, learned food preferences | TWI, MRCA, solute load, NRWL, physical activity, NES responses | Life-stage group, male or female sex | TWI, total solute load, NRWL, physical activity, NES responses | Diet, ventilatory rate, physical activity, body size | TWI, total solute load, NRWL, physical activity |
| Organs involved | GI, CNS, NES, mouth and throat | CNS, NES | GI, CNS, NES | Kidneys, CNS, NES | Kidneys, CNS, NES | GI, kidneys, CNS, NES | Skin, GI, respiratory organs | GI, kidneys, CNS, NES |
| Conscious or behavioral influence? | Yes, habitual 24-h water intake | No | Yes, solid food consumption | Yes, secondary to TWI and food contents | No | Yes, secondary to TWI and food contents | Yes, eccrine sweat loss during labor or exercise | Yes, secondary to water and food intake |
| Representative mean, median, or range of values for sedentary adults | ♀, 1.8–2.0 and ♂, 1.9–2.4 (FR, UK); ♀&♂, 1.5–2.5 (13countries); ♀, 2.3 (range: 0.8–4.5) (USA); ♂, 3.0 (range: 1.4–7.7) and ♀, 2.5 (range: 1.2–4.6) (USA); ♀, 1.9 and ♂, 2.3 (GE); ♀&♂, 0.2–3.9 (FR) L/24h | ♀, 0.2–0.3; ♂, 0.3–0.4; ♂, 0.4 L/24h | ♀, 669–781 and ♂, 915–992 (GE); ♂, 951 (USA); ♀&♂, 362–1365 (4 countries); ♂, 750 (USA); ♀, 752 and ♂, 941 (GE) mOsm/24h | ♀&♂, 120–1250 (FR); ♀&♂, 555 (UK) mOsm/kg | ♀&♂, 1430 (UK) mOsm/kg; ♀&♂ range, 1100–1300 (GE); ♀&♂, 1010–1330 (USA) | ♀&♂, 0.2–3.9 (FR); ♀&♂, 1.9 (n = 8, UK) L/24h | ♂, 0.3–0.4 (UK); ♀, 0.5–0.7 and ♂, 0.7–1.3 (GE) L/24h | ♀, 0.4–0.5 and ♂, 0.2–0.3 (GE); ♀, 0.5 and ♂, 0.3 (GE) L/24h |
| Reference citations | [11,14,15,25,38,51] | [25,52,53] | [1,11,12,41] | [38,54] | [1,54,55] | [38,54] | [1,24] | [11] |
a, TWI, total water intake = (plain water + water in beverages + food moisture); b, water generated during substrate oxidation; c, greatly influenced by diet composition; d, in a 24-h sample; e, NRWL includes eccrine sweat, transdermal, respiratory and stool water losses; f, FWR = (24-h urine volume, L/day) − (obligatory urine volume, L/day). The latter term is the water volume necessary to excrete the 24-h solute load, hypothetically calculated as (830 mOsm/kg) − (3–4 mOsm/kg per year > 20 years of age) [1]. Hydration status is inadequate if FWR is negative. Abbreviations: TBW, total body water; NES, neuroendocrine system (central nervous system + hormones); ECV, extracellular volume; ICV, intracellular volume; MRCA, maximal renal concentrating ability; CNS, central nervous system (brain + spinal cord); GI, gastrointestinal organs; GE, Germany; FR, France; USA, United States of America.