Experts now say that
eight-glassed-a-day rule is wrong. Here,
new guidelines to determine how much you really should be drinking.
Dehydration was K.C. Guevara’s
main concern when she started the 2003 Boston Marathon.
The weather was in the low 70’s and sunny- easily 20 degrees hotter
than usual for April. To keep
hydrated, the then 26-year-old drank about 3 liters of water before the race
and, from miles 5-22, started tossing back 3-ounce cups of water at hydration
stations set up along the course. But Guevara was worrying about the wrong
Through most of the race, she felt
unusually tired. By the time she
crossed the finish line, “I knew something was wrong,” she says. “I
felt dizzy and light-headed, and it was difficult to think straight.”
Guevara made a beeline for the medical tent, where she found her mouth
wouldn’t cooperate with her brain. “I
babbled like a 5-year-old,” she says. Though
she was still on her feet, a blood test revealed she was entering a state known
as hyponatremia, the result of way too much water and way too little sodium.
Unchecked, hyponatremia can cause
brain cells to become swollen with water, potentially leading to confusion,
seizures, coma- and even death. (In fact, a participant in the previous year’s
marathon did die of the condition.) Fortunately
for Guevara, doctors were ready with doses of intensely salty bouillon, and
after two hours she was fine. “It changed my mind about my fluid needs,”
Guevara says. “During the summer I used to drink 2 gallons a day.
I just thought that’s what my body needed.”
The new water recommendations
Obviously, Guevara overdid it.
She drank the equivalent of about 19 cups of water over a six-hour
Drinking yourself to death with
water used to be considered virtually impossible; hyponatremia is certainly
still rare in healthy people, and far less common than dehydration.
But stories like Guevara’s have caused race directors, researchers,
coaches, everyday exercisers and the Institute of Medicine (IOM) of the National
Academies of Sciences to look more seriously at how much fluid the body really
needs. In February the IOM
published the first-ever Dietary Reference Intakes for water and for
electrolytes such as sodium and potassium chloride (which control the movement
of water in and out of cells of your muscles and organs.)
“[The guidelines are] part of an ongoing effort to emphasize nutrition,
not just for preventing disease, but for making people optimally healthy,”
says Stella L. Volpe, Ph.D., R.D., a member of the IOM panel and the Miriam
Stirl Term Endowed Chair in Nutrition at the University of Pennsylvania School
of Nursing in Philadelphia.
To establish the new water-intake
recommendations, the IOM panel spent two years examining hundreds of studies
from peer-reviewed scientific journals on everything from normal fluid balance
and kidney function to fluid needs for those who are ill.
The panel concluded that healthy, sedentary women ages 19-50 who live in
a temperate climate are adequately hydrated when they get:
- 2.7 liters, or 91 ounces, of water a day (for men
its 3.7 liters). Divided into
8-ounce portions, that’s the equivalent of 11.4 glasses a day.
- 81 percent of hydration from drinking water and
other beverages, including coffee and tea (that’s about nine 8-ounce
glasses a day) and the remaining 19 percent from foods, especially fruits
and vegetables. A Diet that
includes plenty of produce also helps maintain a healthy electrolyte balance
by adding potassium.
You may be thinking, “So what
else is new?” After all, these
guidelines come pretty close to matching the 8-ounce glasses of water a day
“rule” that we’ve accepted- and tried to comply with- for the past 30
years. The so called “8x8”
guideline directed us to drink eight glasses of water on top of
everything else we drank. So it was
actually more difficult to follow than these new recommendations.
For example: Let’s say you drink two cups of coffee and one cup of
juice every morning. That’s
already 24-ounces; one-third of your 74-ounce liquid needs under the new
guidelines- before you’ve even left the house!
The best thing about these new Dietary Reference Intakes is that you can
meet your hydration needs through a variety of sources.
And, for the first time, there is a
scientific basis for the recommendation that we require about nine glasses of
liquid a day.
Perhaps just as important is the
IOM’s finding that all beverages count toward the nine-glass goal (coffee and
tea included) and that we can get part of our hydration needs met from food.
“Every food you eat contains water- including dry foods like bread,”
explains Leslie Bonci, R.D., director of the sports nutrition program at the
University of Pittsburgh Medical Center. In
fact, most fruits and vegetables are 80-99 percent water, but even cake, bread
and cheese are more than 20 percent water each.
Follow your thirst
Under normal circumstances,
however, hydration isn’t something we need to think about.
“The minute you drink too much fluid, the kidney’s want to pee it
out,” says kidney expert Heinz Valtin, M.D., a retired professor of physiology
at Dartmouth Medical School in Hanover, N.H. (Valtim tried to find scientific
studies to support the widely cited 8x8 water guideline, but his 2002 published
report came up empty.) “When you
don’t have enough fluid, the kidneys retain it.
The system that regulates the body’s fluid balance is accurate,
sensitive and fast. On it’s own,
it’s remarkably efficient,” he says.
The new water-intake report backs
up Valtin’s observation that our bodies know what they’re doing: Most people
meet their daily hydration needs simply by drinking when they’re thirsty- and
that includes beverages other than water, since they’ve almost entirely H20.
That means your day’s tally should factor in soft drinks, coffee and
tea, which will come as a great relief to anyone who remembers the old hydration
adage to drink an extra cup of water for every cup of caffeinated beverage
consumed. That too is passé, it
turns out: A study at the
University of Nebraska published in 2000 found no significant differences in
hydration when subjects drank caffeinated beverages and when they drank the same
beverages without caffeine.
One Size Doesn’t Fit All
Helpful though they are, however,
the new guidelines are just averages and wouldn’t have helped marathoner K.C.
Guevara, since they don’t account for activity or weather. In its report, the
I0M panel grants that “higher intakes of total water will be required for
those who are physically active or are exposed to a hot environment.” Yet it
doesn’t say what those amounts are.
So if you’re running in the heat,
for example, it’s tricky to figure your needs down to the ounce because each
person’s body handles fluid and electrolytes differently. “You can have two
identical runners next to each other-same height, weight and conditioning- and
one will lose more fluid than the other,” says Douglas Casa, Ph.D., A.T.C.,
director of athletic training education at the University of Connecticut in
Storrs. “You can’t apply one-size-fits-all standards to hydration and
The conventional wisdom that
you’re already dehydrated by the time you’re thirsty is an exaggeration that
borders on being a myth in itself. When a lack of fluids makes minerals and
other components in your blood more concentrated by about 2 percent, you
naturally become thirsty. But you’re not actually considered dehydrated until
your blood concentrates by 5 percent. These numbers sound small, but ‘that’s
a large amount of leeway,” says Valtin. “If you’re healthy, you could do
nothing but follow your thirst and probably be OK.”
Still, Casa says that when doing an
aerobic activity for extended periods of time (as with long bike rides, hikes
and runs) be careful not to consume too much fluid. Extended exercise, he says
makes the kidneys less efficient at eliminating excess fluid because blood is
shunted away from organs to working muscles - a problem that may have affected
Guevara during her marathon.
On top of that, some people are
more prone to sodium loss when they exercise heavily. “I’m a significant
sodium loser,” says University of Pittsburgh’s Bonci. “I get a gritty
feeling on my skin and I notice white caking around my waistband, sports bra -
anyplace I sweat more.” If you notice such signs after an extended workout,
it’s an indication your body is excreting a lot of sodium, so go ahead and
down a sports drink to be safe.
Such drinks can help replace
electrolytes and sodium, but Casa recommends that vigorous athletes ensure
proper fluid balance by taking another step: Weigh yourself before and after
your workouts and drink what you think is best while you exercise. “If
you’re lighter when you’re done, you’ve lost fluid and you need to drink
more. If you’re heavier, you need to drink less,” he explains. “With
practice and training, you’ll see how your fluid needs change with intensity
What to drink for a hot-weather
Summer is the time to move your
exercise outside. But when the temperature is hotter and/or more humid than
you’re accustomed to, you’ll need to pay special attention to your fluid
intake to replace what you sweat out (and to prevent heat cramps, heat
exhaustion or heatstroke). The American College of Sports Medicine offers these
general recommendations for working out on a hot day:
- Down 20 ounces of water (about 2 1/2 cups) or a
sports drink two or three hours before exercising.
- Consume another 10 ounces of water or a sports
drink about 15 minutes before starting.
- During your workout, swig 10 ounces about every
- After exercising, drink 20 ounces for every pound
you lose working out. (to calculate your needs weigh yourself before and