Hydration: How Much Water Do You Actually Need

Eight glasses of water a day is one of the most repeated pieces of health advice in the English language. Nobody has found where it came from. The nearest candidate is a 1945 US nutrition board recommendation of about 2.5 liters of water a day, which continued, in the sentence most people never quote, that most of this is already contained in food.

The body's needs are real and variable. The rule is neither.

What the body needs

Adults lose about 2 to 3 liters of water a day through urine, breath, sweat, and stool, more in heat or with exercise. That must be replaced. The US National Academies set adequate intakes of about 3.7 liters a day for men and 2.7 for women from all sources, and then noted that most healthy people meet their needs by drinking when thirsty.

"All sources" is the important phrase. Roughly 20 percent of water intake comes from food; fruit, vegetables, soup, and yogurt are largely water. Coffee, tea, milk, and juice count fully. The old claim that caffeinated drinks dehydrate you was tested and found wrong; the mild diuretic effect is smaller than the volume of the drink.

Thirst works

For healthy adults, thirst is a well calibrated signal. It switches on when the blood becomes about 1 to 2 percent more concentrated, well before performance or health is affected. The idea that by the time you are thirsty you are already dehydrated is not supported for ordinary daily life. Thirst does become less reliable in older adults, during hard exercise, in heat, and in illness, which are the situations where a deliberate habit helps.

The sign that beats any target

Urine color. Pale straw to light yellow means adequate hydration. Dark yellow or amber means concentrated urine and a need to drink. Clear urine means you are drinking more than you need, which is harmless in most people but not a goal. First morning urine is normally darker.

Some things change the color without changing hydration: B vitamins turn it bright yellow, beets and some medications turn it orange or red. Ribbon Checkup's piece on what urine color tells you covers those. For a more precise read, a urine test strip's specific gravity pad measures concentration directly; see specific gravity and hydration.

When you need more

Exercise. Sweat rates run from half a liter to two liters an hour depending on the person, the effort, and the heat. For sessions under an hour, drinking to thirst is enough. For longer efforts, a rough target is 400 to 800 milliliters an hour, more in heat. Weighing yourself before and after a long session gives a personal figure: each kilogram lost is about a liter to replace.

Heat. Needs rise and thirst lags. Older adults in heat waves are the group most often hospitalized for dehydration, and the advice for them is to drink on a schedule rather than by thirst.

Illness. Fever, vomiting, and diarrhea all increase losses. Oral rehydration solution, which contains salt and glucose in the right ratio, is absorbed better than water alone in significant diarrhea.

Kidney stones. People who have had one are advised to drink enough to produce at least 2 to 2.5 liters of urine a day. This is one of the few situations where a high fixed volume has clear evidence behind it. See kidney stones prevention by the numbers.

Pregnancy and breastfeeding. Needs rise by roughly 300 and 700 milliliters a day respectively.

Drinking too much

It is possible to drink more water than the kidneys can excrete, which is about a liter an hour. Beyond that, sodium in the blood falls, a condition called hyponatremia. It has killed marathon runners who drank at every aid station and people who forced fluids during illness or for a cleanse. Symptoms are headache, nausea, confusion, and in severe cases seizures. The people at risk are endurance athletes on long events, people taking drugs that affect the kidneys' handling of water, and anyone drinking large volumes over a short period on the theory that more is better. It is not.

Water versus everything else

Water is the best default because it carries no calories or sugar. Beyond that, the choice of drink matters less than the total. The exception is sugar sweetened drinks, which contribute more added sugar to the average diet than any other source; replacing those with water is one of the highest value dietary changes there is, for reasons that have nothing to do with hydration. The nutrition label guide explains what to look for.

The practical version

Drink when thirsty. Keep water available so that is easy. Check urine color once a day. Drink more deliberately in heat, during long exercise, when ill, and if you are over 70 or have had a kidney stone. Ignore the number of glasses.