"Drink 8 glasses of water a day" — this is probably the most frequently cited and simultaneously least well-supported health advice. The truth is more nuanced: fluid needs are individual, depending on body weight, activity, climate, and diet. And both too little and too much water can be problematic.
Why does hydration matter?
Water makes up 50–70% of adult body weight. Popkin et al. (2010, Nutrition Reviews) documented in a comprehensive review that water is involved in virtually every physiological process: thermoregulation, nutrient transport, waste removal, protein synthesis, and blood volume maintenance.
Perrier et al. (2021, European Journal of Nutrition) presented the "hydration for health" hypothesis — long-term suboptimal hydration is associated with increased risk of chronic kidney disease, urolithiasis (kidney stones), cognitive dysfunction, and metabolic disorders. Chang et al. (2016, Annals of Family Medicine) found in NHANES data analysis that inadequate hydration is associated with higher BMI — likely partly because people confuse thirst with hunger.
What happens with dehydration?
Nuccio et al. (2017, Sports Medicine) confirmed in a systematic review that even a 2% loss of body weight as fluid measurably reduces cognitive function, reaction time, and endurance performance in team sport athletes.
Goulet et al. (2024, Sports Medicine) confirmed in a meta-analysis of RCTs that dehydration significantly impairs endurance performance — the effect was consistent regardless of thermal conditions, though greater in heat.
Sawka et al. (2007, Medicine & Science in Sports & Exercise) concluded in the American College of Sports Medicine (ACSM) position stand that the goal of hydration strategy during exercise is to prevent body weight loss greater than 2% while avoiding overdrinking — both are harmful.
How much water do you actually need?
Armstrong & Johnson (2018, Nutrients) cautioned in a review that the universal "8 glasses" recommendation (1.9 L) lacks a solid scientific basis. Actual needs depend on:
Body weight: Approximately 30–35 ml/kg body weight per day is a reasonable starting point for sedentary adults (Popkin et al., 2010). For an 80 kg person, that's 2.4–2.8 L of total fluid (including water from food — food covers 20–30% of daily intake).
Activity: ACSM recommends 400–800 ml of fluid per hour of exercise, depending on intensity and temperature (Sawka et al., 2007). During intense exercise in heat, needs can double.
Climate: In hot, humid environments, fluid needs increase by 50–100%. At altitudes above 2,500 m, losses also increase due to faster breathing.
When is too much water dangerous?
Hew-Butler et al. (2015, British Journal of Sports Medicine) warned in a consensus statement about exercise-associated hyponatremia (EAH) — a condition where excessive drinking dilutes blood sodium below safe levels. This is a serious condition that can be fatal in extreme cases.
EAH is more common than many think — it occurs in marathon runners, ultra-endurance athletes, and even recreational exercisers who "force" themselves to drink water despite not being thirsty. The consensus recommends: drink to thirst, not to a schedule — the thirst mechanism in healthy people is a sufficiently reliable signal.
What to drink?
Water is the foundation — but it's not the only fluid that counts. Tea, coffee (yes, coffee hydrates too — the diuretic effect is smaller than the fluid intake with moderate consumption), milk, soups, and water from fruit and vegetables all contribute to daily intake (Armstrong & Johnson, 2018).
During exercise lasting more than 60 minutes or in heat, electrolytes (especially sodium) are crucial — not just for hydration, but for maintaining muscle function and preventing cramps. A simple homemade recipe: a pinch of salt and juice of half a lemon in 500 ml of water.
References
- Armstrong, L.E. & Johnson, E.C. (2018). Water intake, water balance, and the elusive daily water requirement. Nutrients, 10(12), 1928.
- Chang, T. et al. (2016). Inadequate hydration, BMI, and obesity among US adults: NHANES 2009–2012. Annals of Family Medicine, 14(4), 320–324.
- Goulet, E.D.B. et al. (2024). Dehydration and endurance performance in competitive athletes: systematic review and meta-analysis. Sports Medicine, 54(1), 175–189.
- Hew-Butler, T. et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. British Journal of Sports Medicine, 49(22), 1432–1446.
- Nuccio, R.P. et al. (2017). Fluid balance in team sport athletes and the effect of hypohydration on cognitive, technical, and physical performance. Sports Medicine, 47(10), 1951–1982.
- Perrier, E.T. et al. (2021). Hydration for health hypothesis: a narrative review of supporting evidence. European Journal of Nutrition, 60(3), 1167–1180.
- Popkin, B.M. et al. (2010). Water, hydration, and health. Nutrition Reviews, 68(8), 439–458.
- Sawka, M.N. et al. (2007). Exercise and fluid replacement: ACSM position stand. Medicine & Science in Sports & Exercise, 39(2), 377–390.
Important notice
This article is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment, and is not a substitute for professional medical consultation.
All decisions regarding health, nutrition, exercise, or lifestyle changes should always be discussed with your physician, who understands your complete medical history.
The author is not a medical doctor and assumes no liability for any consequences arising from the use of this information without medical supervision.