Water is the largest single constituent of the human body, accounting for 50–70% of body weight. It is essential for virtually every physiological function — it serves as a solvent, transport medium, temperature regulator, lubricant, and structural component. Unlike macronutrients, water cannot be stored in significant amounts, and water balance must be maintained through regular intake.
Body Water Compartments
Total body water is distributed across three compartments. Intracellular fluid (approximately two-thirds of total body water) is the fluid within cells. Extracellular fluid (approximately one-third) includes interstitial fluid (between cells) and plasma (in blood vessels). Water moves freely between compartments, driven by osmotic and hydrostatic forces. Electrolytes (sodium in ECF, potassium in ICF) maintain the osmotic balance that determines water distribution. Albumin, the major plasma protein, helps retain water in the vascular compartment through oncotic pressure.
Fluid Balance Regulation
The body maintains water balance through a complex system of sensors, hormones, and behavioral responses. Osmoreceptors in the hypothalamus detect changes in plasma osmolality. A 1–2% increase in osmolality triggers thirst and stimulates the release of antidiuretic hormone (ADH, vasopressin) from the posterior pituitary. ADH increases water reabsorption in the kidney collecting ducts, concentrating urine and conserving water. The renin-angiotensin-aldosterone system (RAAS) responds to decreased blood volume or sodium levels. Renin from the kidneys converts angiotensinogen to angiotensin I, which is then cleaved to angiotensin II — a potent vasoconstrictor that also stimulates thirst and aldosterone release. Aldosterone increases sodium reabsorption (and thus water retention) in the distal renal tubule. Atrial natriuretic peptide (ANP) is released by cardiac atria in response to volume expansion, promoting sodium and water excretion.
Daily Water Requirements
Water intake comes from three sources: beverages (70–80%), food moisture (20–30%), and metabolic water (oxidation of carbohydrates, fats, and proteins — approximately 200–300 mL/day). Water is lost through urine (60%), insensible losses through skin and lungs (35%), and feces (5%). Additional losses occur through sweat during exercise or heat exposure.
The Institute of Medicine sets Adequate Intake levels: 3.7 L/day (15.5 cups) for men and 2.7 L/day (11.5 cups) for women, including all water from beverages and food. Thirst is generally an adequate guide for healthy, sedentary individuals in temperate conditions. However, older adults have diminished thirst sensation and require conscious attention to hydration.
Dehydration
Dehydration occurs when water loss exceeds intake. Mild dehydration (1–3% of body weight) causes headache, fatigue, reduced cognitive performance (impaired short-term memory, attention, and psychomotor skills), and decreased physical performance. Moderate dehydration (3–6%) causes dry mouth, decreased urine output, dark urine, dizziness, and tachycardia. Severe dehydration (>6%) causes confusion, weakness, sunken eyes, oliguria or anuria, and can progress to hypovolemic shock, organ failure, and death.
Causes. Inadequate intake (especially in elderly, infants, disabled individuals, and during illness), excessive losses (vomiting, diarrhea, fever, exercise, heat exposure, diuretics, polyuria from diabetes or diabetes insipidus), and burns.
Assessment. Clinical signs (skin turgor, mucous membrane dryness, capillary refill), urine color and specific gravity (darker urine indicates greater concentration), serum osmolality (>295 mOsm/kg indicates dehydration), and serum sodium (>145 mEq/L indicates hypernatremic dehydration).
Treatment. Oral rehydration solution (water, electrolytes, glucose) is effective for mild to moderate dehydration. Severe dehydration requires intravenous fluids. In children with diarrheal illness, WHO oral rehydration solution (ORS) reduces mortality.
Overhydration
Overhydration (water excess) occurs when water intake exceeds the kidneys’ ability to excrete it (up to 15–20 L/day in healthy kidneys). Hyponatremia (serum sodium <135 mEq/L) results from dilution of ECF sodium. Symptoms: nausea, headache, confusion, seizures, and coma (from cerebral edema). Acute hyponatremia is a medical emergency. Causes: excessive water intake (especially during endurance exercise, psychogenic polydipsia, and forced water ingestion), SIADH (increased ADH), kidney failure, and medications (SSRIs, carbamazepine, oxcarbazepine). Prevention: drink to thirst during exercise — do not drink beyond thirst. Sports drinks with electrolytes reduce risk compared to water alone.
Types of Drinking Water
Tap water. Regulated by the EPA (Safe Drinking Water Act). Typically fluoridated for dental health. Considered safe in most developed countries.
Bottled water. Regulated by the FDA as a food product. May be spring, mineral, purified, or artesian water. No evidence of superior health benefits compared to tap water.
Hard vs soft water. Hard water has higher mineral content (calcium, magnesium). Soft water (treated) has sodium. No health concerns from drinking hard water.
Alkaline water. Marketed with claims of neutralizing acidity and improving health. No credible scientific evidence supports these claims beyond standard hydration. The body tightly regulates blood pH; dietary water has negligible effect.
Summary
Water is essential for life, serving as the medium for all biochemical reactions. The body maintains water balance through thirst, ADH, RAAS, and ANP. Adequate intake is approximately 2.7–3.7 L/day from all sources. Mild dehydration impairs cognitive and physical performance. Severe dehydration is life-threatening. Overhydration and hyponatremia are rare but serious. Plain water is sufficient for most activities; sports drinks benefit prolonged, intense exercise. Thirst is an adequate guide for most healthy individuals.