Your Meals Count Toward Your Daily Water, a Fact the Eight-Glasses Rule Left Out

About 19 percent of the water you take in on a typical American day arrives in food.

That share comes from the survey behind a 2004 water report from the Institute of Medicine, part of the National Academies. The report set adequate intake at about 3.7 liters of total water a day for men and 2.7 liters for women.

Total water means all of it, so the glass by the sink counts, and so do the coffee, the milk in your cereal, the soup at lunch and the watermelon after dinner. Count your day that way and six popular beliefs about drinking water stop holding up.

1. Eight glasses was never a rule

Eight 8-ounce glasses add up to 64 ounces, or about 1.9 liters.

In that survey, men aged 19 to 30 got about 3.0 liters a day from drinks and women got about 2.2 liters, which puts both groups past eight glasses before you count a bite of food. Food added about 0.7 liter for men and 0.5 liter for women.

Daily intake for U.S. adults aged 19 to 30 in the survey behind the 2004 report. The panel adopted the median totals as its adequate intake and set no upper limit for water.

Where the eight came from

Heinz Valtin, a physiologist at Dartmouth Medical School, went hunting for the source in a 2002 review for the American Journal of Physiology. He concluded, “No scientific studies were found in support of 8 x 8.”

One lead in that review goes back to 1945, when the Food and Nutrition Board of the National Research Council called 2.5 liters a day a suitable allowance for adults, or about 1 milliliter for each calorie of food. The board’s next sentence read, “Most of this quantity is contained in prepared foods.” Valtin passes along a suggestion that readers skipped that line and made 2.5 liters into eight glasses to drink.

That second sentence deserves more fame than the rule it was cut from, since it puts your plate in the count.

2. Milk, tea and soup count too

Juice and coffee count as well, and the 2004 panel’s chair said so when the report came out.

“We don’t offer any rule of thumb based on how many glasses of water people should drink each day because our hydration needs can be met through a variety of sources in addition to drinking water.”

Lawrence Appel, chair of the Institute of Medicine panel, in the National Academies’ February 2004 news release

Water you eat

Look in USDA’s FoodData Central database and you’ll find that iceberg lettuce holds 96 grams of water in every 100 grams, raw cucumber with the peel holds 95 and watermelon holds 91. Plain whole-milk yogurt comes in at 88, and even food you’d call dry has some, with cooked white rice at 68 and whole-wheat bread at 39.

Food	Grams of water per 100 grams
Iceberg lettuce, raw	96
Cucumber with peel, raw	95
Watermelon, raw	91
Plain whole-milk yogurt	88
White rice, cooked	68
Whole-wheat bread	39

A wedge of watermelon weighing 280 grams gives you about 256 grams of water, a little more than an 8-ounce cup.

Plain water is still a good everyday choice, and it’s free. Harvard Health Publishing gives about 4 to 6 cups of plain water a day for generally healthy people, and it says that amount shifts with how much water you get from other drinks and food.

3. Moderate coffee counts like water

Caffeine does make your kidneys pass a little more water for a while. The 2004 panel judged that effect to be short-lived and found no convincing evidence that it adds up to a water shortfall over the day, so caffeinated drinks count toward your total like any other.

A 2014 study in PLOS ONE tested this on ordinary coffee drinkers. Fifty men who drank coffee regularly had 4 cups a day of 200 milliliters each for three days, and in a separate three-day stretch they drank the same amount of water. Blood and urine markers of hydration showed no significant differences between the two stretches. The authors concluded that moderate coffee “provides similar hydrating qualities to water” for men who already drink it.

The study tested only men, at a moderate dose of about 4 milligrams of caffeine per kilogram of body weight, which is a fair description of a normal coffee habit and nothing like a pot before noon. If your mornings look like that study, your coffee goes on the drinks side of your tally.

A Hmong-American man in his early 50s with short black hair greying at the temples, in a sage work shirt with rolled sleeves, pours coffee from a steel stovetop pot into a deep teal mug at a honey-wood table on a sunlit porch, beside a glass of water, a plate of sliced peaches and a folded cloth. — *Each of the study’s 4 daily cups held 200 milliliters, a little under a standard 8-ounce cup.*

4. Thirst is an early signal

A popular line says that by the time you feel thirsty, you’re already dehydrated. Valtin’s review found the timing runs the other way.

Thirst can switch on when your blood becomes less than 2 percent more concentrated. Most experts, he writes, define dehydration as starting later, once you’ve lost 3 percent or more of your body weight. That matches a rise of at least 5 percent.

Rise in blood concentration at each point, from Valtin’s 2002 review. A 3 percent weight loss for a 170-pound adult is about 5 pounds of water.

That leaves thirst as an early warning, with room to spare.

If you’re healthy, you can mostly relax. The 2004 news release said the vast majority of healthy people meet their daily needs “by letting thirst be their guide.”

The signal does fade with age. Harvard Health notes that older people don’t sense thirst as much as they did when they were younger. The panel also said people who are very active or live in hot climates may need more water. If either describes you, drink with meals and at breaks, without waiting for your mouth to feel dry.

5. Moderately yellow urine is normal

Urine darkens when you pass less of it and lightens when you pass more, and color charts built on that idea are a common way to teach athletes about hydration. The 2004 report is more cautious. It says “no precise relationship between urine color and hydration level exists,” and that diet, medications and vitamins can all change the color.

Valtin describes urine at the normal daily volume he measured in 69 students as moderately yellow. Next to the pale yellow or clear that popular advice holds up, that shade can look dark, even though it’s normal.

Water-clear urine all day mostly means your body is passing water it didn’t need.

6. Overdrinking is rare but real

The 2004 panel set no upper limit for water, since healthy kidneys clear the extra. It did warn that in rare cases, drinking far more than you lose can dilute your blood sodium, a condition called hyponatremia.

What happened in Boston

At the 2002 Boston Marathon, researchers tested blood from 488 runners at the finish. Of those, 13 percent had hyponatremia, meaning sodium of 135 millimoles per liter or less, and the other 87 percent did not. Their 2005 paper in the New England Journal of Medicine linked it to gaining weight during the race, a sign of drinking more than you sweat, and to finishing times over 4 hours.

Marathon runners per 100 tested at the 2002 Boston Marathon finish line. Of 766 runners who enrolled before the race, 488 gave a usable blood sample.

A marathon is an extreme day, and the 2004 panel called dangerous overdrinking highly unusual.

The 2004 advice fits a routine you likely have already. You drink with meals and when you’re thirsty, add some on hot days, and let soup, coffee and cucumber do their part. For a cue, leave one clear glass beside the kitchen tap.

Thirst that won’t quit no matter how much you drink, or a big change in how often you urinate, is worth a visit to a clinician.