How much water, actually.

Where eight glasses came from

Vreeman and Carroll went looking for the origin in The BMJ (2007) and traced it to a 1945 Food and Nutrition Board recommendation: a suitable allowance of water for adults is about 2.5 liters daily. The next sentence, which is the one that got lost, reads “Most of this quantity is contained in prepared foods.” A figure for total water became a figure for glasses of it.

Heinz Valtin went looking for evidence rather than origin, in the American Journal of Physiology (2002), and reported the finding in four words: “No scientific studies were found in support of 8 × 8.” He was careful about scope, his conclusion covers healthy adults in a temperate climate living largely sedentary lives, and he says plainly that larger intakes are advisable in illness, in vigorous work and in hot climates.

The official figure, and what it counts

The number people reach for instead comes from the Institute of Medicine’s Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005): an Adequate Intake for total water of 3.7 liters a day for men and 2.7 for women.

Total water means all of it. The report is explicit that total water intake includes drinking water, water in beverages, and water that is part of food. In the national survey the report drew on, beverages supplied about 81% of it and food the remaining 19%. Reading 3.7 liters as a target for plain water misreads the number by roughly a fifth before anything else is considered.

An Adequate Intake is also not a requirement. The same report says there is not a single level of water intake that would ensure adequate hydration and optimal health for half of all apparently healthy people in all environmental conditions, and the Academies, announcing it in 2004, put the practical upshot plainly: the vast majority of healthy people meet their daily needs by letting thirst be their guide. No tolerable upper level was set. The National Library of Medicine says the same thing in Water in diet: there is no specific recommendation for how much water you should drink.

What actually goes wrong, in both directions

Real dehydration is measured as a percentage of body mass lost, and the effects are graded rather than sudden. Wittbrodt and Millard-Stafford pooled 33 studies in Medicine & Science in Sports & Exercise (2018) and found cognitive performance impaired once water deficits exceed about 2% of body mass, with attention affected most; below 2% the effect did not separate from zero. The American College of Sports Medicine draws the line in the same place in its position stand on fluid replacement, recommending athletes prevent losses beyond 2% of body weight, while noting sweat rates vary enough between people that generic fluid plans do not fit anybody in particular.

The other direction is real too, and less discussed. Drinking substantially more than the kidneys can excrete dilutes blood sodium. StatPearls describes exercise-associated hyponatremia as a serum sodium below 135 mmol/L developing during or within a day of activity, and notes that exertion itself can trigger inappropriate antidiuretic hormone secretion, so water is retained rather than passed at exactly the wrong moment. More is not automatically better.

Things that change the answer

The report itself notes that higher total water intakes are required for people who are physically active or exposed to hot environments, and that altitude above about 4,300 meters adds roughly 200 milliliters a day in respiratory losses. Adequate Intakes rise in pregnancy, to 3.0 liters, and in lactation, to 3.8. The National Library of Medicine, on its page on dehydration, lists diarrhea, vomiting, heavy sweating, excess urination and fever as the ordinary causes of losing more than usual, and notes that some people lose their sense of thirst as they age.

One genuine exception to thirst as a guide is worth stating precisely, because it is where the general advice does not apply: OSHA tells workers under occupational heat load not to rely on feeling thirsty to prompt them to drink. That is a specific setting with a specific hazard, not a general correction to the Academies.

Coffee, tea, and whether they count

They count. Killer and colleagues ran a counterbalanced crossover in PLOS ONE (2014) with 50 habitual coffee drinkers taking four cups a day against water, and found no difference in total body water, no difference in 24-hour urine volume, and no difference across the hydration markers they measured. Valtin reached the same conclusion in 2002. Two caveats belong with it: the trial studied men only, all of them already habituated to caffeine.

Water and appetite

The usual citation here is Dennis and colleagues in Obesity (2010), and it is worth reading closely rather than repeating. Forty-eight adults aged 55 to 75, all on a reduced-calorie diet, drank 500 milliliters of water before each meal for twelve weeks and lost about 2 kg more than the group that did not. But the acute effect on how much they ate at a meal (about 43 calories) was significant at the start and no longer significant by week twelve.

So: a modest effect, in older adults, all of whom were already dieting, that faded. It does not support the broader claim that drinking water causes weight loss, and the paper does not make that claim.

What this means for tracking

Tracking water is useful for the same reason tracking anything is useful: it converts an impression into a number you can look at over two weeks. What it is not is a target handed down from a study. If you want a figure to aim at, that is a conversation for a clinician who knows your medications, your kidneys and your climate. Several common medications and several conditions change the answer substantially in both directions.

How Loopa handles this

Water logging is free in Loopa with no cap, on the phone and on the web, and it goes into the same day as your food so it sits beside everything else rather than in its own app. You can count plain water only, or turn on “count all liquids” and have tea, coffee, milk and the liquid part of what you ate count toward the total, which is closer to what the research above is actually measuring. You set your own goal; Loopa does not hand you one.