Dehydration occurs when fluid losses exceed intake, reducing total body water below the level needed for normal physiological function. Mild dehydration — as little as 1–2% of body weight — measurably impairs cognitive performance and physical endurance. Thirst is a late signal; by the time you feel thirsty, mild dehydration may already be present.
Early warning signs (1–2% fluid loss)
- Dry or sticky mouth
- Reduced frequency of urination — urine dark yellow or amber (well-hydrated urine is pale straw coloured)
- Mild headache
- Fatigue or low energy that appears disproportionate to activity
- Difficulty concentrating or mild irritability
- Slightly reduced skin elasticity (skin tent persists briefly when pinched)
Moderate to severe symptoms (3–8% fluid loss)
As dehydration worsens, the body compensates by reducing non-essential fluid losses — hence decreased urination — and prioritising blood flow to vital organs. Symptoms escalate from discomfort to medical urgency.
- Marked reduction in urination; very dark urine
- Rapid heart rate — the heart works harder with lower blood volume
- Dizziness and lightheadedness, particularly on standing (orthostatic hypotension)
- Muscle cramps — from electrolyte losses
- Confusion and disorientation — particularly in older adults and children
- Sunken eyes and dry skin with poor turgor
- Rapid breathing
How much water do you actually need?
The '8 glasses a day' rule has no specific scientific basis. Fluid needs vary significantly with body size, ambient temperature, activity level and diet. The UK NHS and European Food Safety Authority recommend around 2.0 litres/day for women and 2.5 litres/day for men — including all beverages and moisture from food (roughly 20% of total intake).
A more reliable guide than a fixed target: monitor urine colour. Pale yellow (like straw) indicates good hydration. Colourless urine suggests overhydration; dark yellow to amber indicates under-hydration; brown urine warrants medical evaluation.
"Thirst is not the early warning sign it used to be in evolutionary terms — it arrives late. Urine colour is a far more reliable indicator."
Omar Khalil, Health Educator, Looms
Groups at highest risk
- Older adults — thirst sensation diminishes with age; kidney concentration ability declines
- Infants and young children — higher surface-area-to-volume ratio; unable to communicate thirst
- People with diabetes — osmotic diuresis from high blood sugar drives fluid loss
- People in hot climates or exercising vigorously — sweat losses can exceed 2 litres per hour
- Those taking diuretic medications
When dehydration is an emergency
Seek urgent medical attention for: no urine output for 8 hours, confusion or altered consciousness, very rapid heart rate with dizziness, inability to keep fluids down, sunken eyes in an infant, or in the context of severe vomiting or diarrhoea that persists beyond 24 hours. Intravenous fluid replacement may be required.