Where Does The 8 Glasses Rule Come From?
The '8 glasses a day' rule has no solid scientific basis. It likely derives from a 1945 US Food and Nutrition Board recommendation of 2.5 litres per day, but that same document noted that most of this water would come from food. The 'glasses' figure was never research-backed.
The correct answer is: it depends. A 60kg sedentary person has very different hydration needs to a 90kg athlete training twice daily in the heat.
The Evidence-Based Baseline
The most reliable formula is approximately 35ml per kg of bodyweight per day as a baseline, adjusted upward for activity level, heat and other factors. This accounts for all fluid sources including food (which provides roughly 20% of total water intake).
| Factor | Adjustment |
|---|---|
| Sedentary | Baseline (35ml/kg) |
| Lightly active | +12% above baseline |
| Moderately active | +25% above baseline |
| Very active / athlete | +40-60% above baseline |
| Hot climate | +15-30% above baseline |
| Training session | +500-750ml on top |
Dehydration and Performance
Even mild dehydration of 1-2% of bodyweight measurably reduces physical performance, cognitive function and mood. At 2% dehydration, strength and endurance both decline. By the time you feel thirsty, you're already mildly dehydrated.
During moderate exercise, the average person loses 0.5-1.5 litres of sweat per hour. For sessions over 60 minutes in heat, replacing electrolytes alongside fluid is important to prevent hyponatremia.
Does Coffee Count?
Yes, despite its mild diuretic effect, research shows that moderate caffeine intake (up to 4-5 cups per day) does not cause net fluid loss in regular coffee drinkers. Tea, coffee and other caffeinated drinks all contribute to daily fluid totals.
