shape and size, some consisting only of20–30 amino
acids and others of several thousands. They are
present in every living cell. In the skin, hair, callus,
cartilage, muscles, tendons and ligaments, proteins
hold together, protect, and provide structure to the
body. As enzymes, hormones, antibodies, and globu-
lins, they catalyze, regulate, and protect the body
chemistry. Important biomolecules like hemoglobin,
myoglobin and various lipoproteins, that carry oxygen
and other substances within the body are also
proteins.
Besides providing energy to the body, dietary pro-
tein is also required for growth—especially by chil-
dren, teenagers, and pregnant women, tissue repair,
immune system function, hormone and enzyme pro-
duction, and for lean muscle mass and tone mainte-
nance. When eaten, the proteins contained in foods are
broken down into amino acids, an important dietary
source of nitrogen. To make the proteins that it needs
(protein biosynthesis), the body also needs them.
There are 20 amino acids and the body can make
some of them from components within the body, but
it cannot synthesize nine of them, accordingly called
the ‘‘essential amino acids’’ since they must be pro-
vided in the diet. They include: histidine, isoleucine,
leucine, methionine, phenylalanine, threonine, trypto-
phan, and valine. Protein that comes from animal
sources are called ‘‘complete proteins’’ because they
contains all of the essential amino acids while protein
from plants, legumes, grains, nuts, seeds and vegeta-
bles are called ‘‘incomplete proteins’’ because they are
lacking one or more essential amino acid(s).
Proteins are complex molecules and the body
needs time to break them down. This is why they are
a slower and longer-lasting source of energy than car-
bohydrates. According to the Dietary Reference
Intakes(RDI) published by the Unites States Depart-
ment of Agriculture (USDA), adults need to eat about
60 grams of protein per day (0.8 g per kg of weight).
Adults who are physically very active or trying to build
muscle need slightly more. Children also need more. If
more protein is consumed than is needed, the body
stores its components as fat, which can be broken
down and used for energy as need arises. Proteins are
broken down during digestion, which exposes them to
acid in the stomach and to degradation by the action
of enzymes called proteases. Some ingested amino
acids are converted to carbohydrates (gluconeogene-
sis), which is also used under starvation conditions to
generate glucose from the body’s own proteins, partic-
ularly those found in muscle.Carbohydrates
There are two basic types of carbohydrates,
depending on their size. Simple carbohydrates (mono-
sachharides) are those that cannot be broken down
into simpler sugars. They include various forms of
sugar, such as glucose and fructose. Complex carbohy-
drates are larger and consist of long strings of simple
carbohydrates (disachharides, oligosachharides, poly-
sachharides). They include sucrose, lactose, maltose,
maltodextrins, fructo-oligo-saccharides, starch, amy-
lose, and amylopectin. The human body uses carbohy-
drates in the form of glucose and it can convert both
simple and complex carbohydrates into energy very
quickly. The brain needs to use glucose as an energy
source, since it cannot use fat for this purpose. This is
why the level of glucose in the blood must be constantly
maintained above the minimum level. The body also
stores very small amounts of excess carbohydrate as
energy reserve. The liver stores some as glycogen, a
complex carbohydrate that the body can easily and
rapidly convert to energy. Muscles also store glycogen,
which they use during periods of intense physical activ-
ity. The amount of carbohydrates stored as glycogen is
equivalent to about a day’s worth of calories. A few
other body tissues store carbohydrates as complex car-
bohydrates that cannot be used to provide energy.
Carbohydrates have two major roles: they are the
primary energy source for the brain and they are a
source of calories to maintain body weight. A diet
containing an optimum level of carbohydrates may
help prevent body fat accumulation. They are also
involved in the construction of the body organs and
nerve cells, and in the definition of a person’s biolog-
ical identity such as their blood group. Dietaryfiber,
which is a carbohydrate, also helps keep the bowel
functioning properly. Because they are smaller, simple
carbohydrates can be broken down by the body more
quickly and they are the fastest source of energy.
Fruits, dairy products, honey, and maple syrup contain
large amounts of simple carbohydrates, which provide
the sweet taste in most candies and cakes. ComplexThe three functions of macronutrientsPromote growth and Regulate body
Provide energy development functions
Carbohydrates Proteins Proteins
Poteins Lipids Lipids
Lipids (fats and oils) Vitamins Vitamins
Minerals Minerals
Water Water(Illustration by GGS Information Services/Thomson Gale.)
Macronutrients