Overview

• The two-liter-a-day guideline originated in a 1945 US nutrition directive linking one milliliter of water to each kilocalorie consumed.
• Researchers found no universal evidence supporting a mandatory two-liter intake, noting hydration needs vary with physical activity, diet, temperature and gender.
• The German Nutrition Society now advises healthy adults in moderate climates to drink 1.5 liters of water or unsweetened tea daily and to exclude sugary or alcoholic beverages.
• The German Sport University Cologne recommends 400–600 milliliters before exercise and about 200 milliliters every 20 minutes during activity to maintain performance.
• Experts caution that consuming only one liter a day can cause dehydration symptoms while rapidly drinking five or more liters risks dangerous electrolyte imbalances.