"Drink plenty of fluids": A systematic review of evidence for this recommendation in acute respiratory infections

[Extract] Doctors often recommend drinking extra fluids to patients with respiratory infections. Theoretical benefits for this advice are replacing insensible fluid losses from fever and respiratory tract evaporation, correcting dehydration from reduced intake, and reducing the viscosity of mucus.1 2 To many this advice is self evident and justified on the basis that even if the benefit is uncertain, or at best small, at least it is harmless.

However, there are theoretical reasons for increased fluid intake to cause harm. Antidiuretic hormone conserves fluid by stimulating water reabsorption from the renal collecting ducts. Increased antidiuretic hormone secretion has been reported in adults and children with lower respiratory tract infections of bronchitis, bronchiolitis, and pneumonia of viral and bacterial aetiology.3 4 It is uncertain if this also occurs in upper respiratory tract infections.

Several mechanisms have been proposed for this increased hormone secretion, acting through fever, hypoxia, hypercarbia, pain, emotion, or nausea. Secretion may be stimulated by a resetting of osmostat receptors to lower levels.3 Also, lung hyperinflation and pulmonary infiltrates may stimulate hormone secretion by causing a false perception of hypovolaemia by intrathoracic receptors.4 This would be in keeping with findings that antidiuretic hormone secretion in pneumonia increases proportionally with the extent of lung parenchymal involvement.3

Giving extra fluids while antidiuretic hormone secretion is increased may theoretically lead to hyponatraemia and fluid overload. Clinical symptoms of hyponatraemia are irritability, confusion, lethargy, coma, and convulsions. Fluid restriction may be appropriate management to prevent this.