We all know that water is essential for human life and dehydration is dangerous. But there are some surprising effects of even mild dehydration that you may not be aware of.

Little-known risks: In addition to impaired cognitive function and lethargy, dehydration is associated with increased risk for falls, gum disease and bladder cancer.

What you need to know about water’s effect on your mental and physical well-being…


It’s difficult to pinpoint exactly how much water we need.

Problem: Depending on activity levels, metabolism and environmental factors, such as heat and humidity, one person may require up to eight times as much water as someone else to stay hydrated.

Solution: Even though we often hear that most people should drink eight glasses of water a day, there isn’t really any scientific evidence to support this approach. The first official recommendation for water consumption was issued in 2004 by the Institute of Medicine, the health arm of the government-sponsored National Academy of Sciences. This so-called adequate intake (AI) for males age 19 and older is 15 eight-ounce cups daily… for females of the same age group, 11 cups daily.

Sound like a lot of liquid? Remember this represents total water intake, including the water that comes from food — and for most people, this amounts to roughly 20% to 25% of the total. When focusing on water alone, the AI includes about 13 cups of beverages, including water, for males age 19 and older… and nine cups for females of the same age group.

Important: Contrary to what many people believe, there is no evidence to show that coffee, tea and other caffeinated beverages contribute to dehydration. So it’s fine to count these beverages as part of your daily fluid intake.

Since it’s so difficult to establish strict guidelines for water consumption, the most convenient indication may be your urine.

Simple self-test: If you urinate at least four times daily and the urine is colorless or pale yellow, you are probably well-hydrated.

Note: Some vitamin supplements, such as B vitamins, can cause urine to be yellow even if the person is hydrated.


The first symptoms of dehydration usually are dry mouth and thirst. If dehydration progresses, headache, dizziness, sleepiness and muscle weakness may occur.

Dehydration that produces a 2% drop in body weight (due to water loss via sweat, vomiting, diarrhea, etc.) is associated with declines in short-term memory, attention and other mental functions. Similar levels of dehydration can lead to fatigue and reduce strength and endurance. Chronic low-water intake can increase risk for illnesses such as…

Urinary tract infection. When female factory workers significantly increased their water intake and urination frequency by three times or more during their shifts for two years, the rate of urinary tract infections dropped from 9.8% to 1.6%.

Bladder cancer. Not all research findings agree, but one large study that followed 47,000 men for 10 years found that those with the highest levels of fluid consumption (10.6 cups daily) had half the risk for bladder cancer as men who consumed the least fluids (5.5 cups).


Some people are at higher risk for acute and chronic dehydration than others. Key risk factors…

Age. The sensation of thirst is blunted as we age, so “drink when you’re thirsty” becomes a less reliable guide. For many people, appetite also lessens with age — so you can end up getting less water from food.

Other indirect age-related factors also may come into play. For example, people troubled by incontinence often limit water intake.

Exercise. During exercise, you lose more water through sweating. So make sure that you drink enough water when you exercise — generally one to two cups before… one to two cups during… and one to two cups after your workout. This is especially important in hot and humid weather or at high altitudes.

Illness. Many chronic illnesses (diabetes and kidney disease among them) raise the risk for dehydration. Diarrhea and vomiting can present an acute dehydration danger — and water alone won’t replace the minerals, such as sodium and potassium, that you lose. If either is severe or prolonged, or if you can’t keep liquids down, consult your doctor.


When dehydration is mild to moderate, the treatment is simple — drink more liquids.

Severe dehydration is a medical emergency that requires immediate medical help. Some symptoms are the same as those for mild dehydration but greatly magnified — extreme thirst, profound sleepiness or lethargy and very dry mouth. Sweating and urination come to a virtual halt.

Older adults especially may experience irritability and lethargy, while severe dehydration also may lead to delirium (marked by disorientation and delusions) or unconsciousness.

If you experience any of these symptoms — or witness them in another person, especially an older adult — contact a doctor.