Carlos Solis
Water is essential to every cell in your body — and the consequences of not drinking enough go beyond feeling tired. A 2023 study from the U.S. National Institutes of Health linked insufficient hydration to faster aging, greater vulnerability to chronic disease, and a higher risk of premature death.
Even modest fluid loss changes mood and energy levels, and some studies show it can affect cognitive function. A 2% drop in body weight from dehydration equals about 1 to 1.5 liters of fluid loss and often happens before you feel thirsty. Drinking water also supports weight loss: swapping calorie-containing drinks for water increases fat oxidation.
1. How much water to drink each day
The old “eight glasses a day” rule is outdated. The European Food Safety Authority (EFSA) recommended in 2010 that adult women aim for about 2 L of total fluid a day and men about 2.5 L. In hot weather or during intense exercise, increase that toward roughly 3 L.
Remember that fluids come from more than just what you drink. Professor Stuart Galloway points out that people typically get roughly 80% of their daily fluid from beverages and about 20% from food. For an average man, that’s around 500 ml from food, but people who eat a lot of fruits and vegetables can get up to 40% of their fluid intake from food.
2. Should you add electrolytes?
During intense exercise you lose not only water but also salt in sweat. Athletes can shed 1 to 2 L of sweat per hour, and sometimes as much as 4 L. Around 2.5 g of salt leaves the body with each liter of sweat, and prolonged heavy sweating can create an electrolyte imbalance.
Electrolytes do more than help with hydration: they regulate acid–base balance and support muscle and nerve function. A shortage of electrolytes can cause cramps, weakness, nausea, and headache. In those situations, replacing plain water with an electrolyte solution can be necessary.
The common tip to add a spoonful of salt to a glass of water has a physiological basis: salt slows the passage of fluid through the intestine and eases absorption, which helps keep fluid in the bloodstream longer. Typical sports drinks contain about 20 mmol/L of electrolytes, while oral rehydration solutions used to treat diarrhea contain 50–75 mmol/L — and those higher concentrations are the most effective for restoring balance.
3. Milk is sometimes better than water
A 2023 study compared roughly a dozen drinks and found that milk retained fluid better than water: urine volume measured two and four hours after drinking showed that milk held on to fluid more effectively.
The reason is simple: milk contains not just water but proteins (notably casein), fats, sugars, and electrolytes. Casein slows gastric emptying, and the sodium and potassium in milk help keep fluid in the body. That doesn’t mean fizzy sugary drinks are a good substitute — their high sugar content can force the body to use its own water stores to process sugars in the small intestine.
As for coffee, caffeine has a mild diuretic effect, but even a 40 mL espresso delivers a net gain of fluid. Alcohol doesn’t dehydrate as much as people often assume: drinking one to 1.5 L of beer probably won’t produce more urine than the same volume of water. Stronger, less-diluted drinks such as wine or whiskey have a greater diuretic effect.
4. Spread your fluids throughout the day
It’s easier to maintain optimal hydration when you sip steadily instead of waiting long stretches between drinks. Water temperature makes little difference for overall hydration, unless you’re trying to cool your body in hot weather.
A simple strategy: start the day with a 500 ml glass of water, since you don’t drink overnight and are partially dehydrated in the morning. Then aim to drink roughly the same amount with each meal, and let other beverages like coffee or tea add to your daily total.
5. How to tell you’re dehydrated
If you only drink when you feel thirsty, you’re probably partially dehydrated much of the time. Thirst often lags behind actual hydration levels.
Urine color is another informal guide, but remember the urine you see may have sat in the bladder for a few hours and reflects your hydration state three or four hours earlier. Urinating five times a day or fewer is a reliable sign you’re not drinking enough.
6. Don’t drink too much either
Drinking excessive amounts of fluid is uncommon but can be harmful. The body usually eliminates extra fluid, but a very high intake can disrupt electrolyte balance. One danger is diluting blood sodium, which causes hyponatremia and can lead to serious complications.
The situation is complicated because drinking large amounts can raise secretion of vasopressin, the hormone that retains water. Some medications increase thirst, and people on those drugs may drink more than the kidneys can safely remove.
Kidneys have a limit on how much urine they can produce — roughly 1 L per hour. If intake exceeds the elimination rate, water can move into cells and cause water intoxication. Swelling in skeletal muscles is rarely fatal, but if water shifts into the brain, it can cause cerebral edema and, in extreme cases, death.
Based on material from BBC Science Focus
