‘I’m a doctor - here are 6 mistakes leaving you chronically dehydrated'

The UK is currently in the midst of its fourth heatwave of the year, with sweltering temperatures soaring into the low 30s Celsius across large areas of the country. This relentless heat isn’t just uncomfortable—it directly impacts how our bodies handle hydration, and alarmingly, many are mishandling it.

According to Dr Harry Jarrett (PhD, MRSB, MSc, BSc), Head of Science and Research at supplement company Heights, a staggering two-thirds of Brits are chronically dehydrated, even during normal weather - leading to potential changes in mood, energy, exercise performance, concentration, and even cardiovascular function. Here are the most common - and most correctable - hydration mistakes Dr Jarrett sees, and how to fix them:

Coffee, tea, fizzy drinks, and fruit juice don’t hydrate the same as water. Many are diuretic or loaded with sugar. Heights’ data shows that 39% of people rely on fizzy drinks, and 35% on coffee during warm weather. These options may actually increase water loss. Dr Jarrett recommends water or electrolyte-enhanced (hypotonic/isotonic) drinks to retain hydration more effectively.

The NHS recommends around 1.5-2 litres (6-8 glasses) daily in temperate conditions. But with higher temperatures, your body loses fluid through sweat - even before you begin moving. In high heat, your needs can easily double to 3 litres or more just to keep systems humming. Not adjusting accordingly can leave you fatigued, foggy, and faint.

Even a 1-2% drop in body water can impair memory, alertness, and cognitive performance. In a recent survey from Heights, 39% reported fatigue, and 24% reported brain fog multiple times per week. By the time thirst hits, you may already be in decline. The best strategy is drinking regularly throughout the day, not just when you feel thirsty.

When you sweat, you lose more than just water - you lose key electrolytes like sodium, potassium, and magnesium, essential for nerve and muscle function. Yet Heights found 67% of people don’t even know what electrolytes are. Without them, rehydration is incomplete, and performance - both physical and mental - suffers.

Thirst is a late warning sign. By then, blood pressure, temperature control, and memory function may already be compromised. Dr Jarrett advises an evenly spaced drinking habit, especially before and after exposure to heat or exercise, replenishing both fluids and electrolytes.

Even moderate exercise can cause you to lose between 0.5 and 1.5 litres per hour through sweat and respiration. Aim to consume at least 1.5 times the amount lost within two hours of finishing, preferably with electrolytes. This helps restore both hydration and overall bodily function.

“Hydration isn’t merely quenching thirst. It is fundamental to keeping virtually every physiological system working optimally, particularly the brain”, explained Jarrett. “A strategic, evidence-based approach helps sustain mental clarity and physical resilience, especially in the face of unrelenting heat.”