“How much water should I drink every day?” This commonly asked, and seemingly simple question, is deceptively complex.
Traditionally, the answer has been the “8 x 8” rule of thumb; that is, drink eight 8-ounce glasses of water (64 ounces) per day. This “one-size-fits-all” approach is not based on any scientific evidence, nor does not take into consideration a person’s size, activities, and environment.
Let’s discuss some frequently asked questions about water to get to the bottom of this question.
Why do we need to drink water?
Water is required for all forms of life and for the many biochemical reactions that sustain life. By weight, water makes up 60 percent of a man’s body and 50 percent of a woman’s body. The amount of water in the body determines the volume of our cells – we don’t want our cells to shrink because that leads to cellular dysfunction, especially in the brain. Water is integral in this respect.
We need to drink it because our bodies are always losing water. If a person starts with a normal amount of water in their body, the minimum daily water intake must match the water lost every day in urine, stool, sweat, and via exhalation. Failure to take in enough water to match water losses will result in dehydration, which will cause cells to shrink.
How does the body maintain a normal amount of water?
A normal thirst mechanism – and a source of water – are required to protect against dehydration. Loss of body water leads to shrinkage of cells in the “thirst center” of the brain. This produces the feeling of thirst and motivates us to seek out water to quench it. Before thirst is stimulated in the brain, cells close to the thirst center sense the increased concentration of body fluids. The shrinkage of these cells causes them to release a hormone called “anti-diuretic hormone”(ADH). Water balance is maintained by the combination of water intake and the action of ADH, which signals the kidney to conserve (i.e., not excrete) water.
What are the potential benefits of drinking a lot of water?
Beyond preventing dehydration, there are a number of benefits to maintaining an adequate to high daily intake of water. Drinking a lot of water:
- Helps prevent kidney stones
- Helps prevent urinary tract infections
- Helps keep mucous membranes (mouth, eyes, and nose) moist
- Helps prevent constipation
- Helps maintain a normal body temperature
- Helps lubricate and cushion joints
- Helps protect the spinal cord and other sensitive body tissues
- May help prevent headaches and improve mental alertness
- May help protect cells in the kidney by diluting toxins
What are the potential risks of drinking a lot of water?
Other than the fact that “the more you drink, the more you will pee,” drinking a lot of water poses minimal risks for healthy individuals who have normal function of the heart, liver, and kidneys. Such individuals are able to excrete more than 12 liters of water per day if necessary. On the other hand, drinking a lot of water may be problematic for patients with urinary incontinence.
That said, there are particular groups of patients who are at risk of having too much water in their body. This condition—clinically known as hyponatremia—is the most common metabolic disturbance in hospitalized patients. Almost all cases of hyponatremia are due to a problem in the kidney’s ability to excrete water. Hyponatremia is most commonly due to abnormal release of ADH and it can happen with:
- Organ dysfunction (e.g., congestive heart failure, liver cirrhosis, kidney disease)
- Certain tumors (e.g., lung cancer, breast cancer, primary CNS disease, or metastatic cancer in the brain)
- Certain medications (e.g., thiazides, chlorpropamide, carbamazepine, and SSRI anti-depressants such as fluoxetine and sertraline).
Unless the hyponatremia is due to a medication that can be stopped, the patient must strictly limit his or her water/fluid intake.
So, how much water should one drink every day?
There still is no consensus on the optimal amount of water a human should ingest every day.
According to the Institute of Medicine (2005), under normal circumstances an “adequate intake of total water” for adults is about a gallon (128 ounces; 3.7 liters) per day for men, and about ¾ gallon (96 ounces; 2.7 liters) per day for women. The latter increases to 3.0 liters/day for pregnant women and 3.8 liters/day for women who are breastfeeding.
Total water includes the water content of beverages (e.g., water, coffee, tea, milk, soda) and the water content of food (e.g., fruits, vegetables, meat, soup). Depending on the diet, the water content of food can contribute up to 25 percent of total daily water intake.
But again, one size may not fit all. The first general rule is to “obey your thirst.” If you are not thirsty and the color of your urine is clear or light yellow, your fluid intake is likely adequate. Patients who benefit from increased water intake include those with a history of kidney stones and those with polycystic kidney disease. Endurance athletes and those who pursue activities requiring high levels of exertion—especially in warm climates—will need more water and should consult with trained professionals.
Some authorities have recommended that one observe the relative color of their urine as a way to detect if they are becoming dehydrated. There are now “hydration calculators” available online (e.g., http://www.camelbak.com/ko/HydratED/HydrationCalculator.aspx), which may be a very helpful tool for people to estimate their fluid intake needs. In addition to age, gender, height, and weight, this calculator takes into account the level and duration of activity, climate, rate of sweat loss, and color of the urine.
Alan Segal, MD, is a nephrologist at The University of Vermont Medical Center where he is also Director of the Nephrology Fellowship Program. He is also an associate professor at the Larner College of Medicine at UVM.