Encyclopedia of Health and Medicine

184 Nutrition and Diet
Hunger sends HORMONE and NERVE signals to the
APPETITE and hunger centers in the BRAIN, each of
which responds with other neurohormonal messages
that intensify the physical and psychological
urges to eat. Hunger subsides only when the body
receives food (in contrast to appetite, which abates
after time even when the person does not eat).
See also DIGESTIVE ENZYMES; DIGESTIVE HORMONES;
METABOLISM; STARVATION.
hydration Maintenance of the body’s fluid level.
About 60 percent of the body’s weight is water.
The typical adult requires three quarts (two liters)
of water daily to remain adequately hydrated.
Health experts recommend drinking six to eight
cups of water each day to meet this need, though
most people acquire much of the water they need
through the foods they eat. Many foods, notably
fruits and vegetables, have high water content
that helps supply the body with water. Soups,
sauces, fruit and vegetable juices, and pastas and
rice cooked in water also supply fluid to the diet.
Health experts consider water a vital nutrient
because the body cannot live without it. Though
water contains no calories, it does contain trace
minerals that are necessary for metabolic functions.
A person can survive only about five to
seven days without water.
Thirst is not a good indication of proper
hydration. By the time a person feels
thirsty, the body is experiencing significant
fluid depletion. In DEHYDRATION,
many people do not feel thirsty.
DEHYDRATION is a serious condition that results
from inadequate water consumption, and can
occur much more rapidly than expected during
intense physical exercise and in hot temperatures
as the body loses significant water through sweat.
Distance athletes and WEEKEND WARRIORS are at
particular risk for dehydration during competition,
the former because their efforts are so intense that
it is difficult to drink enough water often enough
to keep up with water loss and the latter because
they often do not realize the intensity of their
efforts and fail to properly hydrate before and during
competitive activities including swimming and
other water sports. Dehydration leads to electrolyte
imbalances as the salts in the body become
more concentrated, resulting in numerous physiologic
consequences including mental confusion
and impaired cognitive function, irregular HEART
RATE, fluctuations in BLOOD PRESSURE, and MUSCLE
cramps.
WATER CONSUMPTION FOR
HYDRATION DURING PHYSICAL ACTIVITY
90 minutes before activity: 12 ounces of cold water
15 minutes before activity: 12 ounces of cold water
During activity: 4 ounces of cold water every 15 minutes
15 minutes after activity: 16 ounces of cold water
Fluids that contain sugar or CAFFEINE actually
draw water from the body. Excess sugar pulls
water into the gastrointestinal tract as it makes its
way through the digestive process. Caffeine is a
mild diuretic, acting on the KIDNEYS to cause them
to extract more water from the BLOOD. Beverages
such as sodas (soft drinks) also contain high quantities
of electrolytes, which are minerals in the
form of salts. These, too, may act on the kidneys
to increase the water the kidneys pull from the
blood to pass with the URINE.
In a clinical context hydration may refer to the
long-term infusion of fluids via PARENTERAL NUTRI-
TION or ENTERAL NUTRITION (feeding tube) into a person
who is in a PERSISTENT VEGETATIVE STATE as a
means of preserving life.
See also COGNITIVE FUNCTION AND DYSFUNCTION;
CONDITIONING; END OF LIFE CONCERNS; HEAT EXHAUS-
TION; HEAT STROKE.