Water: The Forgotten Nutrient
The human body is approximately 60% water by weight. Every biochemical reaction occurs in an aqueous environment — from ATP synthesis in mitochondria to neurotransmitter signaling in the brain. A mere 2% reduction in body water impairs cognitive function, reaction time, and mood. Yet most people chronically underhydrate, mistaking mild dehydration for fatigue or hunger.
The Sweat Equation
At rest in comfortable temperatures, the body loses about 1.5 liters of water daily through urine, breathing, and skin evaporation. Exercise dramatically increases this through sweating — the body's primary cooling mechanism. Sweat rates range from 0.5 to 2.0 liters per hour depending on intensity, fitness level, and environmental conditions. Sweat also carries electrolytes: approximately 900 mg of sodium per liter, plus potassium, magnesium, and chloride.
Electrolyte Balance
Hydration is not just about water volume — it requires maintaining the electrochemical gradient across cell membranes. Sodium is the primary extracellular electrolyte, while potassium dominates intracellularly. When you sweat heavily and replace only with plain water, blood sodium drops. Below 130 mmol/L, this causes exercise-associated hyponatremia — a condition that has killed marathon runners who drank too much water without electrolytes.
Personalized Hydration Strategy
This simulator calculates your individual water needs based on weight, activity, and temperature. It also tracks sodium balance — crucial for anyone exercising more than 60 minutes or in temperatures above 30°C. The goal is not maximum water intake but optimal hydration: enough to maintain performance without the risks of overhydration. Monitor urine color (pale yellow = ideal) as a practical daily check.