Hydration is not simply a matter of water volume. The water in the human body is not pure. It is a solution containing dissolved minerals known as electrolytes, including sodium, potassium, magnesium, chloride, calcium, and phosphate. These electrolytes regulate fluid distribution between cells, control nerve signal transmission, govern muscle contraction, and maintain the pH of blood. When you drink plain water, you replace fluid volume but not the electrolytes that give that fluid its functional properties.
Sodium: The Central Electrolyte
Sodium is the primary electrolyte in extracellular fluid and the main determinant of plasma osmolality, the concentration of the blood. When sodium is low, the kidneys excrete excess water to restore the correct concentration. This means that drinking large quantities of plain water in the absence of adequate sodium can paradoxically worsen cellular hydration by diluting the extracellular fluid and causing water to shift into cells. In athletes who drink heavily during endurance events without replacing sodium, this produces hyponatraemia, a serious and potentially fatal condition.
Potassium and Intracellular Hydration
Potassium is the dominant electrolyte inside cells. The sodium-potassium pump, an enzyme present in every cell membrane, actively maintains the concentration gradient between intracellular potassium and extracellular sodium. This gradient drives electrical signalling in neurons and muscle cells, and it also regulates the movement of water across cell membranes. Low potassium intake, which is extremely common in adults eating a low-vegetable diet, impairs this pump and contributes to poor cellular hydration even when total fluid intake appears adequate.
Magnesium and the Often-Missed Deficiency
Magnesium is involved in over 300 enzymatic reactions in the body and is critical for ATP synthesis, muscle relaxation, and nervous system regulation. It is also a common dietary deficiency, with surveys suggesting that up to 50 percent of adults in developed countries consume below the recommended daily intake. Symptoms of magnesium deficiency, including muscle cramps, poor sleep, fatigue, and anxiety, overlap considerably with symptoms of dehydration, and the two conditions frequently co-exist.
When You Need More Than Water
Plain water is adequate for most people in normal conditions. But electrolyte replacement becomes important during prolonged exercise of more than 60 to 90 minutes, in hot weather with significant sweating, after illness involving vomiting or diarrhoea, and for adults following very low-carbohydrate diets, which increase urinary sodium and potassium excretion. In these circumstances, electrolyte replacement through food or a low-sugar electrolyte supplement is clinically appropriate and clearly beneficial.
- Eat potassium-rich foods daily: bananas, avocado, sweet potato, white beans, and spinach
- Include sodium in meals around exercise rather than avoiding salt entirely
- Consider a magnesium glycinate supplement of 200 to 400mg in the evening if you experience cramps, poor sleep, or anxiety
- During exercise lasting more than 90 minutes, replace electrolytes as well as fluid
- After illness with vomiting or diarrhoea, use an oral rehydration solution rather than plain water alone
Sources
- Maughan, R.J. (2003). Impact of mild dehydration on wellness and on exercise performance. European Journal of Clinical Nutrition, 57(S2), S19-S23.
- DiNicolantonio, J.J., et al. (2018). Subclinical magnesium deficiency: a principal driver of cardiovascular disease and a public health crisis. Open Heart, 5(1), e000668.
- Shirreffs, S.M., & Sawka, M.N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of Sports Sciences, 29(S1), S39-S46.