Everyone knows humans need water and we can’t survive without it. We’ve all heard we should be aiming for eight glasses, or two liters, of water per day.
This target seems pretty steep when you think about how much water that actually is, and don’t we also get some water from the food we eat?
We asked five medical and sports science experts if we really need to drink eight glasses of water per day.
All five experts said no.
Here are their detailed responses.
You only need to drink to thirst. The best gauge of your hydration level is the color of your urine. You should aim for light yellow in color; if very dark, then you’re dehydrated and need more water; if clear (like water), then you don’t need so much water. Excessive water intake can be dangerous, particularly in those with heart conditions. The kidney has a remarkable ability to concentrate water, so if you are “getting dry,” the kidney will concentrate the urine and send a message to the brain to drink more.
No, it’s not necessary to drink eight glasses of water a day. It appears the origin of the recommendation to drink eight glasses of water a day may have come from a publication by the National Academy of Sciences Food and Nutrition Board in 1945 stating “A suitable allowance of water for adults is 2.5 liters daily in most instances.” The recommendation also stated that “most of this quantity is contained in prepared foods,” a fact which is often overlooked. We do get a lot of our water intake from the foods we consume. Cauliflower and eggplant, for example, are 92% water. A one size fits all approach is unlikely to be helpful. Healthy adults may not need to drink an additional eight glasses of water a day. On the other hand, persons with certain diseases or living in very hot climates may require larger intakes of fluid.
Eight glasses, which is just less than two liters of water, is very roughly the basal water required by a fasting, well adult per day, who is doing nothing at all (for example, staying in the hospital), with no special losses (such as vomiting or diarrhea). In day-to-day life, we usually have additional losses (exercise or sweating during a hot day), and we receive water from other sources. There are the obvious ones from our diet such as beverages, and juicy and moist foods, such as fruits and vegetables. Less obvious is water from the metabolism of food. The conversion of fats, carbohydrates, and proteins to energy in our bodies all produce water. Rather than focusing on the number of glasses, simply drink fluids when thirsty. Aiming for more water (especially in place of sweetened drinks) is often a good idea to improve health.
A person’s daily water requirements are highly individual and dependent upon a number of internal and external factors. While eight glasses of water per day is recommended as a base requirement to meet daily physiological needs, the actual volume of water required in a day is dependent on one’s day-to-day activities, health, and the climate in which they reside. Research shows even just a mild level of dehydration can negatively affect both mental and physical performance. This is further accentuated for individuals who are highly active and who live in hot environments. A simple and easy reminder to ensure you are drinking enough is to drink to thirst, and for days when activity levels are higher than normal or in hotter environments to increase the regularity of drinking and the total volume.
Many factors will determine how much water (via all foods and fluid, not just water!) your body needs. These include body size and composition (weight, muscle, and fat), how much you sweat (physically active, hot or humid environment, too much clothing) or urinate (taking certain medication, being at high altitude), your health (having fever, vomiting, or disease) or status (pregnant, breastfeeding), and diet (high-water content foods, carbohydrates). For most healthy adults, rarely feeling thirsty and having light yellow (or colorless) urine usually confirms adequate water intake. Other helpful tips include drinking a glass of low-calorie fluid before and with every meal (to distinguish hunger from thirst) and drinking low-calorie fluid before, during, and after physical activity (especially if you sweat). Although rare, drinking too much fluid can also have negative health consequences, so more is not necessarily better.
Disclosure statements: Toby Mündel has received research funding from the Gatorade Sport Science Institute and Neurological Foundation of New Zealand, which has included research on hydration.