Since 1997
Hydration FAQ
by Neal W. Pollock, Ph.D. on April 10th, 2017

​If 1.5 gallons of water is considered too much to drink (From the Medical Line, Fall 2014), what is your recommendation? Would it not depend on the environment and activity? When I worked as a river guide rowing the Grand Canyon, we would easily drink more than a gallon of water during the day.
​Optimal hydration is a vexing topic — easy to talk about in theory but very difficult to pin down with numbers. Dehydration, often prompted by inadequate fluid intake (hypohydration), will result in depressed fluid volume and increased blood concentration, which, in turn, lead to reduced sweat rate, increased heat storage and decreased central venous pressure. Ultimately, cardiovascular capability, aerobic exercise capacity and the ability to tolerate heat strain would all be impaired, broadly compromising both physical and cognitive performance.

Excessive fluid intake (hyperhydration), conversely, can dilute the normal body water balance. Extreme cases can result in hyponatremia, a condition marked by depressed sodium concentration in the blood. Symptoms can include headache, nausea, vomiting, lethargy, impaired consciousness and, in extreme cases, death.

Body water is gained by fluid intake, some food intake and metabolic production. While the ingredients of some drinks act as diuretics, notably caffeine and alcohol, these are often delivered with large enough fluid volumes that the net impact will be an effective increase in body water volume. While alcohol ingestion is discouraged for other reasons, beer — particularly low-alcohol beer — has been shown to be an effective rehydrating agent. Electrolyte drinks replenish salts and other elements important for physiological function along with the water, reducing the likelihood of hyponatremia (abnormally low sodium levels in the blood) even with substantial fluid intake.

Evaluating state of hydration based on urine color seems logical but is not always valid. Urine production throughout the day is heavily influenced by short-term events. For example, being acutely chilled will cause vasoconstriction and concentration of blood in the core. More core blood means more blood returning to the heart. The heart responds to this increased volume by beating harder and altering hormone release to stimulate fluid elimination in the kidney. This, in turn, will result in a clear urine sample regardless of the underlying state of hydration. Similarly, very dark urine does not necessarily mean that a person is dehydrated, since the urine volume is freely sacrificed to protect plasma volume.

The challenges of short-term variability can largely be avoided by focusing on the first sample of urine produced after waking from a comfortable full-length sleep period. The normal adult production rate of around two or three fluid ounces per hour (one milliliter per kilogram of body mass per hour). Low production rates indicate inadequate kidney function, possibly due to insufficient fluid intake. The color should be a pale yellow, but be aware that some foods and medications (over the counter as well as prescription) can affect normal color. Assuming no interactions with food or medication, consistently rich or darker yellows suggest that increased fluid intake may be warranted. Consistently colorless urine could indicate a pattern of excessive hydration.

Thirst is an unreliable indicator of one's state of hydration. Consciously drinking regularly and checking the first waking urine sample probably provides a better foundation. Given differences in body size, fluid and food intake patterns and environmental pressures among individuals, it is often inappropriate to recommend arbitrary volumes of fluid to be ingested. In the real world for real people, the guidance could range from far too little to far too much.

As an interesting side note, the often-quoted advice to drink eight 8-ounce glasses of water per day was not based on any science. The recommendation was reportedly adopted as a simple promotional campaign for the community when inadequate fluid intake was felt to be a critical issue.

While there is no simple answer to your question, one can apply the preceding information practically. If a person does not regularly develop headaches, feels generally good and has fairly pale, clear yellow urine on the first waking sample, then current fluid intake patterns may be reasonable. If the first waking sample is transparent and the person feels lethargic or has other symptoms that could be associated with hyponatremia, reducing the fluid intake may be a useful test to see if the sense of wellness improves. Eating a well-balanced diet will help ensure adequate electrolyte foundations. Periods of high fluid stress, such as exercise in hot and humid conditions, can be met with a combination of plain water and electrolyte drinks, but realize that the calories in most electrolyte drinks may be more than necessary. Most important, any frank concerns should be discussed with a health-care professional.


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