Heatwaves are no longer isolated acute events but prolonged periods of cumulative physiological stress, a shift that is altering the public-health understanding of hydration. Fabrizio Pregliasco, director of the specialisation school in hygiene and preventive medicine at the University of Milan, notes that each additional day of high temperatures compounds cardiovascular, respiratory and metabolic strain, particularly in the elderly, chronic patients and the frail. Italian emergency departments are recording a rise in admissions for dehydration, heat stroke, cardiac decompensation and falls linked to hypotension and disorientation. The duration of exposure, not just the peak temperature, is what now drives the health impact.

The mechanism hinges on the body’s regulation of water and sodium. Gabriela González, a nephrologist at the Hospital de Clínicas in Buenos Aires, explains that ageing diminishes the kidney’s ability to dilute urine and move water, while the hormone vasopressin tends to increase, causing water retention that dilutes blood sodium. This can lead to hyponatremia—a condition often mistaken for simple dehydration—exacerbated by medications such as diuretics and antidepressants. Mare Sundström, a researcher in environmental medicine at Sweden’s KTH Royal Institute of Technology, underscores that fluid requirements are highly variable: a sedentary office worker may need only one litre a day, while someone exercising in extreme heat might require ten. The common two-litre rule, she says, ignores the water content of food and the blunted thirst sensation in older adults, who must drink even when not thirsty.

What people choose to drink is under new scrutiny. Dietitians in Beirut point to green beverages such as matcha, moringa and green smoothies as alternatives to coffee. Matcha, the only one containing caffeine, also delivers L-theanine; a systematic review and meta-analysis of randomised controlled trials found that the combination can improve alertness, reaction time and mood compared with a placebo. Clinicians caution, however, that matcha can increase heart rate and reduce iron absorption, and is not recommended for cardiac patients or on an empty stomach. For those managing blood sugar, health reports from the Arab world highlight sugar-free sparkling water, fruit-infused water and unsweetened iced tea as safe options that do not raise blood glucose. Indonesian health platforms add that water-rich foods—cucumber, celery, radish, tomato and asparagus—can contribute substantially to daily fluid balance.

Sodium intake is the other side of the equation. Indonesian sources list often-ignored signs of excess salt: intense thirst, swelling in the hands and face, frequent urination and cognitive dulling. Yet González warns that a blanket low-sodium diet can be dangerous for older adults whose regulatory mechanisms are fragile, potentially triggering hyponatremia. The balance between water and sodium, she argues, must be individually assessed through physical examination and blood analysis, not reduced to population-wide slogans.

Pregliasco calls for a structural response that goes beyond colour-coded heat alerts. He urges the deployment of timely epidemiological surveillance systems to track hospitalisations, emergency-department visits and heat-related mortality, enabling targeted protection of the most vulnerable. The next milestone to watch is whether Mediterranean health authorities integrate such real-time monitoring into their summer preparedness plans, moving from reactive warnings to adaptive, data-driven intervention.