Why is sodium chloride (NaCl) preferred over water for hydration in patients with dehydration or significant electrolyte imbalances?

Why Sodium Chloride Solutions Are Superior to Plain Water for Hydration

Sodium chloride solutions are better than plain water because water alone causes rapid dilution of plasma sodium and osmolality, which triggers increased urine production and suppresses the thirst mechanism, ultimately delaying effective rehydration and potentially worsening dehydration. 1

Physiological Basis for Sodium Chloride Superiority

The Problem with Plain Water

• Plain water ingestion after fluid loss causes a fall in plasma osmolality and sodium concentration, which paradoxically stimulates urine production rather than fluid retention 1
• This reduced plasma sodium concentration also suppresses the natural thirst drive, preventing adequate voluntary fluid intake 1
• The net result is that plain water delays the rehydration process because most of it is excreted rather than retained 1

How Sodium Chloride Solutions Work Better

• Addition of sodium chloride to water increases actual fluid intake while simultaneously reducing urine output, creating a dual benefit for rehydration 1
• Sodium chloride solutions maintain plasma osmolality, preserving the stimulus to drink and reducing renal water excretion 1
• Maximal sodium absorption occurs with solutions containing 120 mmol/L sodium chloride combined with 30 mmol/L glucose, which optimizes both sodium and water absorption 1

Clinical Evidence Across Different Populations

In Diabetic Ketoacidosis (DKA)

• Isotonic saline (0.9% NaCl) is the first-line fluid for initial resuscitation at 15-20 ml/kg/hour during the first hour 1
• This addresses the typical total body water deficit of 6-9 liters and sodium deficit of 7-10 mEq/kg that occurs in DKA 1
• Plain water would be catastrophic in this setting, as it would worsen the existing electrolyte imbalances and fail to restore intravascular volume 1

In Short Bowel Syndrome and Intestinal Failure

• Patients with high-output jejunostomy or short bowel syndrome should limit hypotonic fluids (water, tea, coffee) because these actually stimulate fluid secretion and worsen stomal losses 1
• The leaky jejunal epithelium allows hypotonic fluids to draw sodium and water into the intestinal lumen, paradoxically increasing dehydration 1
• These patients require isotonic high-sodium oral rehydration solutions (120 mmol/L sodium) to replace stoma sodium losses and achieve net absorption 1

In Exercise-Related Dehydration

• Solutions containing 5-12% carbohydrate with electrolytes demonstrate superior fluid retention compared to plain water at 2-4 hours post-exercise 1
• Studies show 16.1% greater fluid retention with 12% carbohydrate-electrolyte solutions versus water (95% CI: 7.45-24.75) 1
• During vigorous exercise, fluids should contain 20-30 mEq/L of sodium to prevent exercise-induced hyponatremia from excessive sodium-free fluid intake 2

The Rehydration Index Concept

• A lower urine output in the hours following fluid ingestion indicates better intravascular volume restoration 1
• Plain water produces high urine volumes because the kidneys rapidly excrete the excess free water to maintain osmolality 1
• Sodium-containing solutions result in lower urine volumes, meaning more of the ingested fluid is actually retained and used for body weight restoration 1

Critical Clinical Scenarios Where This Matters Most

Severe Dehydration with Electrolyte Depletion

• Sodium chloride deficiency produces symptoms nearly identical to Addison's disease: nausea, vomiting, muscle cramps, and potentially convulsions 3
• In conditions like pyloric obstruction, extensive burns, or postoperative intestinal paralysis, sodium chloride has a lifesaving effect that plain water cannot provide 3
• The toxic symptoms following intestinal obstruction are accompanied by marked reduction of blood chloride, requiring sodium chloride replacement 3

Prevention of Hyponatremia

• Excessive intake of sodium-free fluids can cause dangerous hyponatremia, particularly during prolonged exercise or in patients with impaired water excretion 2
• Sodium maintains over 90% of the osmotic pressure of interstitial fluids and determines tissue hydration 3
• Plain water administration in volume-depleted states risks dilutional hyponatremia with potentially severe neurological consequences 4

Practical Implementation

For Acute Dehydration

• Start with isotonic saline (0.9% NaCl) at 15-20 ml/kg/hour for initial resuscitation 1
• Once hemodynamically stable, transition to 0.45% NaCl at 4-14 ml/kg/hour if corrected sodium is normal or elevated 1
• Add 20-30 mEq/L potassium (2/3 KCl and 1/3 KPO4) once renal function is confirmed 1

For Oral Rehydration

• Use glucose-containing reduced osmolarity WHO oral rehydration solutions rather than plain water, sports drinks, or juice 2
• The optimal formulation contains 120 mmol/L sodium chloride with 30 mmol/L glucose 1
• Avoid fruit juices, colas, and other hypertonic solutions in patients with high intestinal output, as these worsen fluid losses 1

Common Pitfalls to Avoid

• Never use plain water as the primary rehydration fluid in patients with significant dehydration or ongoing losses 1
• Do not assume that any fluid intake is beneficial—hypotonic fluids can actually worsen net fluid balance in short bowel syndrome 1
• Avoid excessive free water intake during exercise, which can cause exercise-associated hyponatremia 2
• Remember that approximately 25% of total body sodium is in bone, so severe depletion requires substantial replacement that water alone cannot provide 3