Water - How Much to Drink?

How much water to drink? The short answer is enough that you pee enough.

Our body's water content is strongly regulated to stay very close to normal hydration. There is no such thing as chronic dehydration unless there is also chronic water rationing. Enough water is enough - more is not better - and too much has risks.

Water comes from beverages, water content of food, and the metabolic water created by converting food to energy (example: sugar metabolized to carbon dioxide and water). Water loss from breathing, from sweating, and as water content of feces are not under strong physiological control. Instead, urine volume is regulated. Normal urine production is 1,200 to 2,000 ml/day (think 1 to 2 quarts). Drinking more than needed increases urine production - can be 10 liters a day or more.

Urine production cannot drop to zero because that is how our bodies dispose of metabolic waste products. Our kidneys can concentrate all the waste into an disposal volume of 500 ml/day, but not much smaller. Drinking more does not dispose of more waste - it just dilutes the same amount into a larger volume of urine.

ADEQUATE INTAKE

There is debate on what is considered Adequate Intake (AI). The U.S. government, through the Dietary Reference Intakes (DRIs) for water, published by the Institute of Medicine in 2005, decided that men ages 19 and older should consume 3.7 liters (125 ounces) per day, and women 19 and older should consume 2.7 liters (91 ounces) per day.

These numbers refer to total water intake (TWI), which includes beverages and water content of foods. The typical split is a tad under 80% from beverages and a tad over 20% from food. Hence, men and women should be drinking 100 and 73 ounces, respectively. The reality here is that the IOM wasn't sure how much water is really needed, so it sort of rationalized that the average intake is an adequate intake.

Europe disagrees. The European Food Safety Authority decided in 2010 that an AI for men is 2.5 L and for women 2.0 L. The World Health Organization likes 2.5 L/day for men and 2.2 L/day for women. But keep in mind that the WHO assumes average weights for men and women as being 154 and 128 pounds, whereas the current averages for U.S. adults are 196 and 166 pounds. Adjusting for weight would put the WHO recommendations closer to the U.S. numbers.

A old rule of thumb that tries to take into account a need for more water with an increase in calories being consumed (because physical work or exercise requires more calories and needs more water to compensate for sweating and breathing losses) is to estimate water needs as 1.5 ml per calorie intake from food and drink. By this math a sedentary person taking in 1800 calories would need 2.7 L, whereas a larger and/or more physically active person consuming 3000 calories per day would need 4.5 L.

CONSEQUENCES OF INADEQUATE INTAKE

There is no evidence that drinking more reduces risk of dying. This from a study that tracked 12,650 people for 6.7 years. What it did was look at how many people died in the lowest 25% for water intake (they averaged 1.75 L/day) and compared the three higher quartiles to the lowest. No statistically significant differences among the four groups.

Higher total water intake does reduce the risk of getting kidney stones. Higher total water intake also reduces the risk of exercise-induced asthma attacks.The evidence for weight management is iffy. Drinking water 30 to 60 minutes before a meal reduces calories eaten in that meal. There is evidence that when water is substituted for calorie-containing beverages, total calories per day decreases. In a large epidemiological study, people who drank more water weighed less. But as water consumption might co-occur with other lifestyle choices, it is not clear if the water intake was responsible for the weight difference.

Evidence for health benefits beyond those mentioned above is either weak or contradictory. Dehydration will increase the risk of constipation, but for adequately hydrated people, drinking more water will not alleviate constipation. The idea of more hydration improving skin health has not been proven. There is inconsistent data for more water lowering the risk of bladder cancer, colon cancer or urinary tract infections.

EXCEPTIONS TO THE RULE OF THE BODY MANAGING HYDRATION

Short-term, dehydration and overhydration are easily achieved, with potential for serious consequences. People doing vigorous physical activity can lose a quart an hour via a combination of perspiration and the moisture of exhaled air. A review of 2,135 athletes who had completed endurance events found that 50% had experienced more than a 3% weight loss. Given that we are 60-70% water, that is roughly 5% water loss. It is well documented that body weight loss of more than 2% is known to compromise exercise capacity and mental function.

The same study reported that 11% were overhydrated. i.e., weighed more at the end of the event than at the beginning, and one-tenth of those had symptoms of hyponatremia (low blood sodium). When so much water is consumed that sodium in the blood is diluted, water moves into cells, resulting in swelling. Puffy hands, ankles and face seem harmless, but brain cells also swell, increasing pressure. Symptoms progress from headache and nausea to stupor, seizures and death. Sports drinks contain some sodium, but not enough to prevent hyponatremia if consumed in excess.

For some people, vigorous exercise shuts off urine production even if they are overhydrated. This can persist for hours after exercise ceased. If under the mistaken belief that they are dehydrated because they are not urinating, and then they drink more, this exacerbates the problem. It is essential to do a body weight check before an endurance event and at the end. If weight has gone up, do not drink anything. If weight has gone up a lot, get to medical care quickly.

WATER AND BODY WEIGHT

As easy way to understand the impact of water on body weight is to weigh oneself just before going to bed and second thing in the morning (after a morning pee). Most people find they are 1.5 to 3.0 pounds lighter in the morning. If that were true body weight loss it would represent 5,000 to 10,000 calories expended (using the loose rule of 3,500 calories per pound), whereas the real estimate of calories burned while at complete bed rest for 7-8 hours is more on the order of 500 calories. So most of that weight loss is from water as urine, breathing out moist air, and water loss through skin. That last occurs even it not noticable as perspiration.

The intestinal tract contains water. Food takes several hours to get from the mouth to the start of the large intestine. At this point the water content is quite high, as in addition to the water content of the food and whatever beverages were consumed with the food, the salivary glands and stomach and small intestine digestive secretions contribute at least a quart more. Once into the large intestine more than 90% of the water is recovered/resorbed, but at any time the large intestine contains 2.5-5.0 pounds of feces, with 75%-90% of that being water.

Normal bowel movements are about a half-pound a day of which about 70% is water, but severe, prolonged diarrhea can be dehydrating because the large intestine did not have an opportunity to recover water.