Why is plain water inadequate for rehydration in patients with dehydration?

Why Plain Water is Inadequate for Rehydration in Dehydration

Plain water alone is insufficient for treating dehydration because it lacks the essential electrolytes—particularly sodium—needed to restore fluid balance across body compartments and prevent further fluid losses. 1, 2

The Physiological Basis: Why Electrolytes Matter

The fundamental problem with water-only rehydration lies in the sodium-glucose cotransport mechanism at the intestinal brush border. During dehydration, the absorption of water from the gut requires sodium to be present—one sodium ion is linked with one glucose molecule, which drives water absorption. 1 This cotransport process remains intact during acute diarrhea and is the physiologic foundation for oral rehydration therapy. 1

When you drink plain water without electrolytes:

• Water alone fails to replace the sodium and other electrolytes lost through sweat, vomiting, or diarrhea, leading to incomplete restoration of extracellular fluid volume. 3, 4
• Hypotonic fluids (like plain water) can actually worsen fluid losses in patients with high-output diarrhea or significant gastrointestinal losses. 2
• The osmotic pressure of extracellular fluid decreases with salt loss, and water alone cannot correct this imbalance. 5

Evidence from Exercise and Dehydration Studies

Research demonstrates the inadequacy of water-only rehydration:

• In soldiers exercising in heat, those drinking water alone lost 2.4 liters of total body water compared to only 1.5 liters in those receiving oral rehydration solution. 4
• Plasma volume decreased by 5% with water versus only 3% with electrolyte-containing oral rehydration solution. 4
• Extracellular water contributed most to total body water loss during dehydration, and effective rehydration requires adequate electrolyte replacement in each fluid compartment—something water alone cannot provide. 4

The Sodium Requirement

For effective rehydration, beverages should contain at least 50 mmol/L of sodium, with the WHO standard oral rehydration solution containing 90 mmol/L sodium. 1, 2, 3 This sodium concentration is critical because:

• Sodium determines the osmotic pressure of interstitial fluids and the degree of tissue hydration more than any other ion. 5
• Without adequate sodium replacement, the kidney cannot retain water effectively, leading to continued urine losses despite drinking. 3
• The volume of beverage consumed must exceed sweat or fluid losses to account for ongoing obligatory urine losses, which requires sodium to minimize these losses. 3

Important Clinical Distinctions

Type of Dehydration Matters

The evidence distinguishes between different dehydration scenarios:

• For low-intake dehydration in older adults (raised serum osmolality >300 mOsm/kg), plain water IS appropriate along with other preferred beverages—oral rehydration solutions and sports drinks are NOT indicated. 1, 6 This represents pure water deficit without significant electrolyte loss.

• For volume depletion from diarrhea, vomiting, or excessive sweating, oral rehydration solution with 50-90 mEq/L sodium is essential as first-line treatment. 2, 6 This involves loss of both water and electrolytes.

The Critical Warning

The American Gastroenterological Association explicitly warns against using plain water, tea, or sugar water alone for rehydration in patients with high-output diarrhea or jejunostomy, as hypotonic fluids worsen fluid losses. 2

Practical Algorithm for Fluid Selection

For diarrheal illness or volume depletion:

• Use WHO-ORS (90 mEq/L sodium) or reduced osmolarity ORS (50-90 mEq/L sodium) 2, 6
• Administer 50 mL/kg over 2-4 hours for mild dehydration (3-5% deficit) 2, 6
• Administer 100 mL/kg over 2-4 hours for moderate dehydration (6-9% deficit) 2, 6

For low-intake dehydration in older adults:

• Encourage increased intake of preferred beverages including water, tea, coffee, juice, or milk 1
• Avoid oral rehydration solutions and sports drinks in this population 1

For severe dehydration (≥10% deficit) with shock:

• Immediate IV isotonic fluids (lactated Ringer's or normal saline) at 20 mL/kg boluses 6

Common Pitfalls to Avoid

• Do not assume all dehydration requires electrolyte solutions—older adults with simple low fluid intake should drink their preferred beverages, not ORS. 1
• Do not use plain water for rehydration when solid food intake is not possible, as food normally replaces electrolytes lost in sweat. 3
• Do not forget that excessive plain water intake during prolonged exercise (>4 hours) can cause hyponatremia, particularly when sodium losses through sweating are high. 7