You have probably heard you should drink eight 8-ounce glasses of water a day. When a Dartmouth kidney specialist went looking for the research behind that number, he could not find any.
Heinz Valtin, professor emeritus of physiology at Dartmouth Medical School, published a review in the American Journal of Physiology in August 2002 examining exactly this question. He reported finding no scientific proof that every person must drink at least eight glasses of water a day.
Valtin, a kidney specialist and the author of two standard textbooks on kidney and water balance, described the popular counsel, known as "8 x 8," as more urban myth than medical dogma.
Where the rule actually came from
The review traced the number back to a Food and Nutrition Board recommendation of roughly 1 milliliter of water per calorie of food. The same recommendation added that most of that quantity is already contained in prepared foods.
Also read: 8 warning signs of eczema most people mistake for ordinary dry skin
That qualifier tended to drop out as the guidance was passed along. What was left, over the decades, was the tidy but unsupported instruction to pour yourself eight glasses.
What health authorities actually say
Instead of a universal number, the National Academies of Sciences, Engineering, and Medicine set adequate total water intake at 2.7 liters (91 ounces) for women and 3.7 liters (125 ounces) for men. Those figures cover water from all sources, including plain water, other beverages, and food.
The report noted that most healthy people adequately meet their daily hydration needs by letting thirst be their guide. Roughly 20 percent of total water intake comes from food, with the rest from beverages of all kinds.
The National Heart, Lung, and Blood Institute notes similarly that fluid needs can be met with water, other drinks, and fruits and vegetables with high water content.
Also read: 7 warning signs of cervical cancer most women write off as normal
Individual needs shift with body size, exercise, hot weather, altitude, illness, and medications such as diuretics. A construction worker in summer heat and a sedentary office worker in air conditioning do not need the same amount of fluid.
Practical signals worth watching
Rather than counting glasses, the National Academies panel concluded that thirst reliably guides most healthy adults to adequate intake.
Mainstream US health authorities also point to urine color as a rough check, with pale yellow generally taken as a sign of adequate hydration. Valtin himself cautioned that dark urine is often over-interpreted and does not necessarily indicate meaningful dehydration.
Some groups need extra vigilance. Older adults have a blunted thirst response, young children can become dehydrated quickly during illness, and anyone exercising heavily, working outdoors in heat, or running a fever loses more fluid than usual and should replace it deliberately.
Also read: How long should you be able to balance on one leg? The answer depends on your age and gender
Also read: Not all magnesium supplements are the same: which form is right for you
This article is made and published by Emil Martesen, who may have used AI in the preparation.