What does dehydration do to your body?
Most people drink water when they are thirsty and assume that covers hydration. At a cellular level, hydration depends on electrolytes as much as water volume. Without sodium, potassium and magnesium, the water you drink stays in the wrong places.
Water moves into your cells through osmosis, driven by the concentration of electrolytes on each side of the cell membrane. Sodium is the dominant electrolyte in the fluid surrounding your cells, and its concentration creates the osmotic gradient that pulls water from outside cells to inside. When you sweat, you lose both water and electrolytes simultaneously. If you replace only the water, you dilute the sodium concentration in the extracellular fluid, reducing the osmotic gradient and reducing the efficiency with which water enters cells. You can drink significant quantities of water in this state and still feel dehydrated, because the cellular dehydration is driven by electrolyte imbalance rather than absolute fluid volume.
What is happening
The three electrolytes most relevant to daily hydration are sodium, potassium and magnesium. Sodium drives the primary osmotic gradient and regulates extracellular fluid volume. Potassium is the primary intracellular electrolyte and is essential for nerve transmission and muscle contraction. Magnesium is a cofactor for the sodium-potassium ATPase pump, the enzyme that actively maintains the electrochemical gradient across cell membranes by pumping sodium out and potassium in. If magnesium is insufficient, the pump operates inefficiently and the cellular ion balance degrades regardless of how much sodium and potassium you are consuming. All three work as a system.
The situations where electrolyte losses become significant enough to matter are broader than most people assume. Sustained physical work in any environment, even a cooled one, produces meaningful sweat electrolyte loss over a long shift. Alcohol consumption drives diuresis and electrolyte excretion overnight. Stress hormones, particularly a hormone that regulates salt balance, increase sodium excretion during sustained psychological stress. Long-haul air travel in dry, pressurised cabin air dehydrates through respiratory and skin losses. In any of these contexts, plain water is an incomplete hydration solution, and the persistent fatigue, headache, muscle cramping and cognitive dullness that characterise them are partly electrolyte-driven rather than purely fluid-driven.
What to do next
Electrolyte supplements, from dissolved tablets or sachets, are the most practical way to ensure adequate electrolyte replacement without the sugar content of commercial sports drinks. The effective formats contain meaningful quantities of sodium (at least 400 to 600 milligrams per serving), potassium (at least 200 milligrams) and magnesium (at least 50 milligrams). Adding them to water during or after a physically demanding shift, in the morning after significant alcohol consumption, or during or after travel closes the gap that plain water leaves and produces a noticeably faster recovery in how you feel. The difference between hydrating with electrolytes and hydrating with water alone, after conditions that have depleted both, is physical enough to be noticeable within 20 to 30 minutes.
Related reading: Heat and sun hydration, Hydration basics, Hydration at work.
Sources: EFSA water guidance, EUFIC hydration guidance, WHO physical activity guidance, WHO healthy diet guidance.
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