Water is life – every child knows this simple formula. But when it comes to the specific amount we should drink daily, there's a great deal of confusion. The famous "eight-glass rule" persists, even though it's long been scientifically disproven. Current research shows that fluid requirements are highly individual and influenced by numerous factors. A large-scale Japanese long-term study has now provided, for the first time, precise figures on when drinking truly extends life – and why more isn't always better.
Why hydration is so important
Water is the main component of our body – it makes up about 50 to 65 percent of body weight. It is found in cells, muscles, organs, and even bones. Blood and tissues also consist mostly of water. Cells, tissues, and organs need water to function properly. .
The body needs water for numerous vital functions. :
-
Formation of bodily fluids such as tears, saliva, sweat and urine
-
Supporting brain and body in thinking, movement, mood and energy
-
Excretion of waste products via urine and stool
-
Regulation of body temperature through sweating
-
Supplying the body with nutrients and supporting digestion
-
Maintaining fluid balance in and around cells and tissues
-
Prevention of kidney stones and urinary tract infections
Even a slight lack of fluids can lead to feeling less energetic and difficulty thinking clearly. Dehydration is also linked to obesity and diabetes. .
The 8-glass rule: A myth persists.
The most well-known drinking rule states that we should drink eight glasses of water (250 ml each) daily – a total of about two liters. This "8x8 rule" is easy to remember, but not scientifically proven. Its origin is unclear, but it may date back to a 1945 recommendation by the Food and Nutrition Board, which spoke of a total water intake of about 2.5 liters per day – with the crucial, often ignored addition that "the majority of this amount is contained in prepared foods". .
Modern research has moved away from this rigid guideline. Fluid intake must be individually tailored and depends on factors such as... :
-
Body size and composition
-
Activity level and sweat rate
-
Ambient temperature and humidity
-
Altitude (increased fluid loss through respiration)
-
Diet (high-fiber, high-protein or high-salt diets increase the need)
-
Pregnancy and breastfeeding
-
Illnesses with fever, vomiting or diarrhea
The European Food Safety Authority (EFSA) and the Institute of Medicine of the National Academies (NASEM) provide guidelines for total daily water intake from all sources (beverages + food). :
-
For men : approximately 3.7 liters per day
-
For women : approximately 2.7 liters per day
These amounts include not only water, but also other beverages such as tea, coffee and milk, as well as the liquid from solid foods, which contributes about 20 percent to the total intake. Many fruits and vegetables consist of almost 100 percent water, including melons, strawberries, cucumbers, and zucchini. .
New research: The optimal drinking range for a long life
A groundbreaking study from Waseda University in Japan, published in the American Journal of Clinical Nutrition , has for the first time precisely examined the relationship between water balance and mortality. The researchers analyzed data from over 63,000 adults aged 40 to 79 years over an average period of 19 years.
The results show a significant L-shaped relationship between the amount of water in the body (water turnover) and mortality. :
-
Women with the lowest water turnover had a 26 percent increased risk of dying from all causes , while men had an 18 percent increased risk .
-
The risk of dying from cardiovascular disease was increased by 24 percent in women and by 29 percent in men .
-
The correlation was particularly strong in respiratory diseases: Here, the risk in the group with the lowest water turnover increased by a factor of 2.96 for women and by a factor of 3.06 for men .
The researchers used this information to determine the optimal range for daily fluid intake, at which the risk of death is lowest. :
-
Women : 1.28 to 1.4 liters
-
Men : 1.49 to 1.57 liters
These values refer to pure drinking water – the total fluid intake from food and other beverages is additional.
The mechanics of hydration: Why water alone is not always enough
Optimal hydration depends not only on the amount of water consumed, but also on the balance of electrolytes and nutrients. Sports physiologist Dr. Stacey Sims explains that water alone does not contain so-called "transporters"—molecules that support water absorption in the body. .
For optimal hydration, the body relies on molecules that transport fluid through the intestinal cells. These transporters include sodium (salt) and carbohydrates such as glucose. This process is called cotransport: sodium and glucose facilitate water absorption in the intestine by drawing water along with them during transport into the cells via osmosis. .
Therefore, under certain conditions, electrolyte-rich or carbohydrate-containing drinks can contribute more effectively to hydration than pure water – especially during longer and more intense physical activities or in elderly people taking medications that can lead to dehydration. .
How can I tell if I'm drinking enough?
The body has sophisticated mechanisms to regulate its fluid balance. The simplest and most reliable indicators are: :
Thirst : Contrary to a widespread myth, thirst is not a sign of existing dehydration, but a finely tuned early warning system of the body. It signals in good time that it's time to drink. Only for older people, whose sense of thirst may diminish, is a proactive drinking strategy advisable.
Urine color : An excellent indicator of hydration status :
-
Pale yellow (like light lemon juice) : optimal hydration
-
Dark yellow to amber : Warning sign – you should drink more
-
Dark orange to brownish : severe dehydration – act immediately
Important: Certain foods (beetroot), vitamins, and medications can alter urine color. .
Special situations: When demand increases
Sports and heat
During physical activity, especially in hot environments, fluid loss through sweating can range from 0.3 to 2.0 liters per hour. It's important to drink before, during, and after exercise. For intense workouts longer than 60 minutes, isotonic drinks with electrolytes can be helpful to replenish lost fluids. .
High temperatures and humidity
You sweat more in hot and humid environments. You should hydrate before going outdoors and then drink small amounts regularly. .
High altitude
At high altitudes, the body loses more fluid through respiration. Increased fluid intake is therefore necessary. .
Older people
The risk of dehydration is particularly high in older people. The updated S3 guideline "Clinical Nutrition and Hydration in Old Age" (2025) recommends specific target values for the first time. :
-
The total fluid intake should generally be 30 ml per kg of body weight per day .
-
If dehydration is suspected, the diagnosis should be checked based on serum osmolality in conjunction with the overall clinical picture.
-
Approximately one-third of frail elderly people are affected by hypertonic dehydration (pure water deficiency without electrolyte loss). .
Is it possible to drink too much?
Yes, excessive drinking can be dangerous. Excessive water intake in a short period can dilute the sodium level in the blood so drastically that it leads to life-threatening hyponatremia. Symptoms include headache, nausea, confusion, and in severe cases, seizures and coma. Endurance athletes who drink large amounts of water without replacing the electrolytes lost through sweat are particularly at risk. .
What counts towards fluid intake?
The following contribute to the daily fluid balance :
-
Drinks : Water (tap or mineral water), unsweetened herbal and fruit teas, coffee and black tea (in moderation – their diuretic effect is negligible once you get used to it), diluted fruit juices, milk
-
Foods with high water content : fruits (melon, strawberries, oranges), vegetables (cucumber, tomatoes, zucchini), soups and broths, yogurt and other dairy products
Alcohol has a strong diuretic effect and should not be included in fluid intake calculations. .
Conclusion
The question of the right amount of fluid to drink cannot be answered with a single number that applies to everyone. The days of the rigid 2-liter rule are scientifically outdated. Instead, the rule is: the body reliably signals what it needs.
The new long-term Japanese study provides us with the first concrete indications of an optimal drinking range that minimizes the risk of death: approximately 1.3 to 1.5 liters of water daily for men and women, in addition to fluids from food. .
The best indicators are your own thirst and the color of your urine. Those who pay attention to these signals, consciously drink more in hot weather and during exercise, and maintain a balanced fluid intake are on the right track. Hydration isn't rocket science – it's the most natural thing in the world when you listen to your body.
Official sources & studies:
-
Mayo Clinic: Water: How much should you drink every day? (January 2026)
-
S3 guideline "Clinical nutrition and hydration in old age" (DGEM, 2025) – esanum.de
-
Watanabe D et al.: Association between predicted level of water turnover deficit and mortality (American Journal of Clinical Nutrition, 2024) – quoted from 163.com
-
ICMR-National Institute of Nutrition: Dietary Guidelines for Indians (2024) – cited from Telangana Today
