However, when the body has too much
cholesterol, the extra begins to build up
in the walls of the blood vessels. This
gradually narrows the blood vessels so
that blood cannot flow freely through
them. This may lead to a heart attack or
stroke.
Minerals serve a variety of purposes.
Calcium, for example, builds bones and
teeth and helps to clot blood. Iron is
needed to build red blood cells and
carry oxygen from the lungs to other
body cells. Other minerals important to
the human body include iodine,
magnesium, sodium, potassium, and
zinc.
The body also needs vitamins. Vitamin
C keeps gums healthy and helps the
body absorb iron. Vitamin D works
with calcium and phosphorus to make
strong bones and teeth. Other vitamins
are A, E, K, and the B-complex vitamins.
Water is important in many chemical
reactions in the body. In fact, every cell
in the body must be bathed in water.
Water also helps the body get rid of
wastes and maintain its temperature.
Water makes up about 60 percent of an
adult’s body weight.
Food Pyramid
Different foods contain different
nutrients. Some foods are better sources
of nutrients than others are. Scientists
have created a chart called the food
pyramid to show how much of different
groups of food a person should eat in
order to be healthy. In general they
recommend that people eat more
grains, vegetables, fruits, and milk and
fewer fats and oils.
#More to explore
Cholesterol
Food Chain
The term food chain describes the order
in which organisms, or living things,
depend on each other for food. Every
People should try to eat several servings of
grains, vegetables, fruits, dairy products,
and meat or beans every day. They should
eat only a small amount of fats and oils.
44 Food Chain BRITANNICA STUDENT ENCYCLOPEDIA
ecosystem, or community of living
things, has a food chain. Some have
more than one food chain.
Most food chains start with organisms
that make their own food, such as
plants. Scientists call them producers.
Organisms that eat other living things
are known as consumers. A squirrel that
feeds on plants is called a primary consumer.
A hawk that eats the squirrel and
other primary consumers is called a secondary
consumer.
Decomposers are often the final link in a
food chain. Decomposers are bacteria
and other organisms that cause decay.
When plants and animals die, decomposers
break down their tissues. This
adds nutrients to the soil so that new
plants may grow. Then the food chain
begins again.
A food web is a group of food chains
within an ecosystem. Most living things
eat more than one type of animal or
plant. So their food chains overlap and
connect. For example, the hawk that ate
the squirrel also may eat fish. This
makes the hawk a part of two food
chains, or a food web.
#More to explore
Ecosystem • Living Thing
Food Poisoning
Food sometimes gets contaminated or
infected with harmful things. A person
who eats such food can get an illness
A food chain in the ocean begins with tiny one-celled organisms called diatoms. They
make their own food from sunlight. Shrimplike creatures eat the diatoms. Small fish then
eat the shrimplike creatures, and bigger fish eat the small fish.
BRITANNICA STUDENT ENCYCLOPEDIA Food Poisoning 45
called food poisoning. Food poisoning is
usually not serious, but some types are
deadly.
Symptoms
The symptoms of food poisoning usually
begin within hours of eating the
contaminated food. The most common
symptoms are fever, stomach cramps,
nausea, vomiting, and diarrhea.
Causes
Certain microorganisms (tiny living
things) cause most types of food poisoning.
Bacteria and other microorganisms
can infect eggs, meat, vegetables, and
many other foods. After entering the
body these microorganisms release poisons
that make people sick.
Some chemicals can also cause food poisoning.
These chemicals are often added
to food while it is being grown, processed,
or prepared. For example, many
farmers spray chemicals on crops to kill
weeds and insects. Some people may
have a bad reaction to those chemicals
when they eat the crops.
Some plants and animals contain natural
poisons that are harmful to people.
These include certain kinds of seafood,
grains, nuts, seeds, beans, and fungi
(mushrooms). Molds called mycotoxins
may also cause food poisoning.
Prevention
When people handle food properly, the
risk of food poisoning is very small.
Microorganisms multiply rapidly in
dirty places and in warm temperatures.
This means that people should never
touch food with dirty hands or put food
on unwashed surfaces. Food should be
refrigerated to stop microorganisms
from growing. Meat needs to be cooked
thoroughly to kill any dangerous microorganisms.
People should also wash food covered
with chemicals before eating it. Finally,
people should not eat wild mushrooms
or other foods that grow in the wild.
Some of these foods may contain natural
substances that are poisonous to
humans. In addition, some types of fish,
such as the moray eel, and certain shellfish
can be poisonous.
Treatment
Most people recover from food poisoning
after a few days of resting and drinking
extra fluids. People who cannot
