Why is it recommended to drink lots of water when having influenza (flu), especially for vulnerable populations such as the elderly, young children, and individuals with underlying health conditions?

Why Drink Plenty of Water for the Flu

Drinking plenty of fluids during influenza is recommended as standard symptomatic care to prevent dehydration from fever and reduced oral intake, though the evidence supporting this practice is based primarily on clinical experience rather than rigorous trials. 1

The Guideline Recommendation

Multiple clinical guidelines consistently recommend increased fluid intake as part of influenza management:

• The British Thoracic Society and British Infection Society explicitly recommend "drinking plenty of fluids" as symptomatic treatment for adults with influenza. 1
• All children with influenza should receive advice on fluids alongside antipyretic therapy. 1
• The guidelines acknowledge there is "little scientific evidence for most symptomatic and self-help treatment, but experience suggests that some of the following may help, and are unlikely to cause harm," with fluid intake listed among these measures. 1

Why Fluids Matter in Influenza

Preventing Dehydration-Related Complications

Children who cannot maintain fluid intake due to breathlessness, fatigue, or gastroenteritis require fluid therapy to prevent clinical deterioration. 1

The physiological rationale includes:

• Fever increases insensible fluid losses and metabolic demands. 2
• Reduced oral intake is common due to malaise, sore throat, and fatigue. 1, 2
• Gastroenteritis frequently accompanies influenza in children, compounding fluid losses. 1

Evidence from Pneumonia Research

While direct trials in uncomplicated influenza are lacking, research in pneumonia (a common influenza complication) provides compelling indirect evidence:

• A 2022 meta-analysis found that dehydration doubled the odds of medium-term mortality in pneumonia patients (OR 2.3,95% CI 1.8-2.8), with moderate-quality evidence across 128,319 participants. 3
• This effect was consistent across all pneumonia types, including community-acquired and healthcare-associated pneumonia. 3
• Greater degrees of dehydration increased mortality risk further, demonstrating a dose-response relationship. 3

Vulnerable Populations at Highest Risk

Elderly patients, young children, and those with chronic conditions face the greatest dehydration risks during influenza:

• Elderly patients may not mount adequate febrile responses but still experience fluid losses, and acute hypohydration can precipitate adverse medical events in this population. 4, 5
• Infants aged 0-1 years have hospitalization rates comparable to elderly patients (200-1,000 per 100,000) and are particularly vulnerable to dehydration. 2
• Mild dehydration has been associated with multiple adverse health outcomes, especially in young children and frail elderly persons. 5

Critical Caveats and Pitfalls

The Risk of Overhydration

While dehydration is harmful, excessive fluid intake can cause serious complications, particularly hyponatremia:

• A case report documented symptomatic acute hyponatremia requiring hospitalization in a patient who followed advice to "drink more fluids" during a simple infection. 6
• During acute illness, inappropriately elevated antidiuretic hormone (ADH) levels reduce renal water excretion, making patients vulnerable to water intoxication even with normal kidney function. 6
• Severely ill children requiring intravenous fluids and oxygen therapy should receive fluids at only 80% of basal levels to avoid complications of inappropriate ADH secretion, with electrolyte monitoring. 1

Practical Guidance

The recommendation is to drink "plenty" of fluids, not excessive amounts:

• Patients should aim to maintain normal hydration status, not force large volumes beyond thirst. 1
• Oral fluids are preferred over intravenous administration whenever possible. 1
• In critical care settings, conservative fluid strategies have been associated with better outcomes, emphasizing that "more is not always better." 7

When to Escalate Care

Patients should seek immediate medical attention if they develop:

• Inability to maintain oral fluid intake 4
• Signs of severe dehydration (decreased urination, dizziness, confusion) 4
• Worsening symptoms after initial improvement, which may indicate bacterial superinfection 4
• Shortness of breath at rest or persistent high fever for 4-5 days 1

The Bottom Line

Adequate fluid intake during influenza is a low-cost, low-risk intervention supported by clinical guidelines and indirect evidence from pneumonia research, but should be balanced and not excessive. The advice to "drink plenty of fluids" means maintaining good hydration to replace losses from fever and reduced intake, not forcing large volumes that could lead to hyponatremia. 1, 6, 3 Vulnerable populations—particularly the elderly, infants, and those with chronic conditions—require closer monitoring for both dehydration and fluid overload. 2, 4, 5