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    44-46 Water metabolism
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    44-46 Water metabolism

    Water metabolism: total, intracellular and extracellular body water, measurement, dehydration and fluid excess.

    Water is the most abundant molecule in the human body and is continuously renewed. It acts as a solvent and transport medium, removes metabolic heat, and regulates cell volume and function. This application reports three related values: extracellular water, cellular (intracellular) water and total body water, as percentages. Total body water makes up roughly 50–60% of body weight in adults: more in lean people and men, less in women, older people and people with obesity, because fat tissue contains little water. About two thirds is inside cells and one third is extracellular (interstitial fluid, blood plasma, lymph and cerebrospinal fluid). Body water is controlled within about 1% by thirst and by the kidneys under the influence of antidiuretic hormone.

    Water balanceBody compositionHydration
    Total body water≈ 50–60% of body weight
    Intracellular≈ 2/3 of body water
    Extracellular≈ 1/3 of body water
    Reference methodisotope (deuterium) dilution
    Distribution of body water in a lean adult: total body water about 60 percent of body weight, of which about two thirds is intracellular and one third extracellular, divided into interstitial fluid and plasma Body water compartments (lean adult, 70 kg) Intracellular water ≈ 2/3 · ≈ 28 L Interstitial ≈ 10–11 L Plasma ≈ 3 L extracellular water ≈ 1/3 · ≈ 14 L total body water ≈ 60% of body weight ≈ 42 L
    Figure 1. Approximate distribution of body water in a lean adult man weighing 70 kg. The proportion of water is lower in women, older people and people with more body fat.

    Measurement

    The reference method is isotope dilution: after drinking a known amount of deuterium- or oxygen-18-labelled water, its concentration in saliva, urine or blood gives total body water; bromide or sodium dilution gives extracellular water. Bioelectrical impedance is a convenient estimate used in clinics and body composition devices, but it is affected by hydration, recent meals and exercise.

    Clinical significance

    Reduced body water (dehydration)

    Insufficient intake, sweating, fever, vomiting, diarrhoea, diuretics, uncontrolled diabetes, diabetes insipidus. Even a loss of a few percent of body weight reduces plasma volume, raises heart rate and impairs temperature regulation and physical and mental performance. Older adults and infants are most at risk.

    Excess body water

    Heart, kidney or liver failure, nephrotic syndrome, inappropriate antidiuretic hormone secretion, excessive fluid intake or infusions. An increased extracellular share appears as oedema; excess free water lowers blood sodium (hyponatraemia).

    Thirst is triggered mainly by a rise in plasma osmolality sensed in the hypothalamus, and also by low blood volume through baroreceptors and the renin–angiotensin system; the kidneys adjust water excretion under the control of vasopressin.

    When to see a doctor

    Persistent thirst, very dark or very little urine, dizziness on standing, confusion, or swelling and rapid weight gain need medical assessment.

    References

    1. Jéquier E, Constant F. Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr. 2010;64(2):115–123. doi:10.1038/ejcn.2009.111
    2. Popkin BM, D’Anci KE, Rosenberg IH. Water, hydration, and health. Nutr Rev. 2010;68(8):439–458. doi:10.1111/j.1753-4887.2010.00304.x

    This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

    Published on 26 April 2024