35 millilitres per kilogram: your personal water requirement

What the science says

The German Nutrition Society (DGE) recommends that adults drink around 1.5 litres of fluid per day – in addition to the water you consume through food. This brings the body’s total daily intake to around 2 to 3 litres. As a guide to individual requirements, 35 ml of water per kilogram of body weight per day is recommended.

The reason for this recommendation lies in the body’s water balance: every day, an adult loses around 2.5 litres of water – through urine, breath, the skin and stools. The body must replenish this amount to maintain its functions. We consume about one litre through food, and the rest through drinks.

What happens if the balance is off?

If you consume less water than your body loses, a deficit arises. This may sound harmless, but it has far-reaching consequences. Your blood becomes thicker, which puts a strain on the cardiovascular system. The kidneys are less able to excrete waste products efficiently. Digestion slows down. And your brain – which consists of 73% water – is particularly sensitive: even a fluid loss of just 1.5% of body weight leads to a measurable drop in concentration and alertness.

Conversely, it is also possible to have ‘too much’, albeit rarely. So-called water intoxication (hyponatraemia) can occur if very large quantities are drunk in a short space of time and the sodium level in the blood becomes too diluted. This mainly affects extreme athletes and is not a realistic risk in everyday life.

Why requirements are so individual

These guidelines are exactly that: guidelines. Your actual requirements depend on several factors that can change from day to day.

Body weight and body composition: A lighter body needs less, a heavier one more. Muscle tissue stores significantly more water than fat tissue. This explains why men (on average 60% water content) have a slightly higher requirement than women (on average 55%).

Exercise and sport: Those who engage in physical activity lose more water through sweat. For every hour of intense exercise, this can amount to up to an extra litre. Anyone doing physical work – in the garden, on a building site, or around the house – should also factor in this additional requirement.

Temperature and climate: On hot days, in heated rooms or in dry air, fluid requirements increase significantly. The body loses more moisture through the skin and breathing, often without us even realising it.

Diet: Fruit, vegetables, soups and smoothies provide plenty of water. Those who eat a predominantly dry diet – bread, muesli, meat – need to drink more accordingly. A salty or protein-rich diet also increases water requirements, as the body needs more fluid to process it.

Age: As we get older, the sensation of thirst diminishes – a phenomenon that is well documented in medicine. The DGE recommends that older people aged 65 and over consume at least 1.3 litres of fluid a day through drinks. In practice, many older people drink significantly less. In care homes in particular, dehydration is one of the most common preventable health risks.

Specific life stages: Breastfeeding women have increased fluid requirements – the DGE recommends 1.7 litres a day from drinks. Fluid requirements also rise sharply in cases of fever, vomiting or diarrhoea.

How can you tell if you’re not drinking enough?

Thirst is the most obvious sign – but unfortunately a late one. If you feel thirsty, your body is already dehydrated. Earlier signs are often more subtle: tiredness, headaches, difficulty concentrating and dry skin are among the most common. Dark urine is also a reliable indication that your body needs more water.

Research clearly shows how quickly dehydration takes effect. Even a water loss of just 2% of body weight leads to a measurable decline in cognitive performance: attention, coordination and decision-making ability all deteriorate. For an 80 kg person, this amounts to just 1.6 litres – a quantity that can be lost in as little as two hours on a warm day with moderate physical activity.

Dehydration can reduce concentration by around a third. When dehydrated, the brain has to work much harder to achieve the same performance – meaning it uses more energy for less result.

The role of habit

Drinking – like so much in our daily lives – is a habit. Those who have got into the habit of drinking regularly do so almost automatically. On the other hand, anyone who ‘forgets’ to drink all day and realises in the evening that the bottle is still almost full knows the problem. It can help to establish fixed drinking routines: a glass of water straight after getting up, one with every meal, one before and after exercise. Some people place a water bottle in plain sight on their desk – the visual reminder alone helps.

Regular hydration is particularly important for children. Children’s bodies have a proportionally higher water content than those of adults, and they become dehydrated more quickly. Studies show that even mild dehydration can impair children’s academic performance.

Not all water is the same

As well as the quantity, the type of water also plays a role. The DGE recommends water and unsweetened tea as the best thirst-quenchers. Coffee, long considered a ‘water thief’, is now viewed in a more nuanced way – moderate amounts certainly count towards fluid intake. Sweetened drinks and juices, on the other hand, should be the exception; juice spritzers should be made with a ratio of one part juice to three parts water.

Our tip

Drink regularly throughout the day – don’t wait until you feel thirsty. A glass of fresh, filtered water in the morning gently gets your body going. And if you’re unsure in the evening whether you’ve drunk enough: a glance at the colour of your urine will tell you. A pale straw-yellow colour means everything is fine.