tissue because of their large size. Blood fluid is pulled
into the capillary beds from the tissue through the
mechanics of oncotic pressure, in which the pressure
exerted by the protein molecules counteracts the blood
pressure. Therefore, blood proteins are essential in
maintaining and regulating fluid balance between the
blood and tissue. The lack of blood proteins results
in clinical edema, or tissue swelling, because there is
insufficient pressure to pull fluid back into the blood
from the tissues. The condition of edema is serious and
can lead to many medical problems.
Energy provision. Protein is not a significant source
of energy for the body when there are sufficient amounts
of carbohydrate andfatsavailable, nor is protein a
storable energy, as in the case of fats andcarbohydrates.
However, if insufficient amounts of carbohydrates and
fats are ingested, protein is used for energy needs of the
body. The use of protein for energy is not necessarily
economical for the body, because tissue maintenance,
growth, and repair are compromised to meet energy
needs. If taken in excess, protein can be converted into
body fat. Protein yields as much usable energy as carbo-
hydrates, which is 4 kcal/gm (kilocalories per gram).
Although not the main source of usable energy, protein
provides the essential amino acids that are needed for
adenine, the nitrogenous base of ATP, as well as other
nitrogenous substances, such as creatine phosphate
(nitrogen is an essential element for important com-
pounds in the body).
Protein Requirement and Nutrition
The recommended protein intake for an average
adult is generally based on body size: 0.8 grams per
kilogram of body weight is the generally recom-
mended daily intake. The recommended daily allow-
ances of protein do not vary in times of strenuous
activities or exercise, or with progressing age. How-
ever, there is a wide range of protein intake which
people can consume according to their period of devel-
opment. For example, the recommended allowance
for an infant up to six months of age, who is under-
going a period of rapid tissue growth, is 2.2 grams per
kilogram. For children ages seven through ten, the
recommended daily allowance is around 36 total
grams, depending on body weight. Pregnant women
need to consume an additional 30 grams of protein
above the average adult intake for the nourishment of
the developing fetus.
Sources of protein. Good sources of protein include
high-quality protein foods, such as meat, poultry, fish,
milk, egg, and cheese, as well as prevalent low-quality
protein foods, such as legumes (e.g., navy beans, pinto
beans, chick peas, soybeans, split peas), which are high
in protein.Protein–Calorie MalnutritionThe nitrogen balance index (NBI) is used to eval-
uate the amount of protein used by the body in com-
parison with the amount of protein supplied from
daily food intake. The body is in the state of nitrogen
(or protein) equilibrium when the intake and usage of
protein is equal. The body has a positive nitrogen
balancewhen the intake of protein is greater than
that expended by the body. In this case, the body can
build and develop new tissue. Since the body does not
store protein, the overconsumption of protein can
result in the excess amount to be converted into fat
and stored as adipose tissue. The body has anegative
nitrogen balancewhen the intake of protein is less than
that expended by the body. In this case, protein intake
is less than required, and the body cannot maintain or
build new tissues.
Anegative nitrogen balancerepresents a state of
protein deficiency, in which the body is breaking
down tissues faster than they are being replaced. The
ingestion of insufficient amounts of protein, or food
with poor protein quality, can result in serious medical
conditions in which an individual’s overall health is
compromised. The immune system is severely affected;
the amount of blood plasma decreases, leading to med-
ical conditions such as anemia or edema; and the body
becomes vulnerable to infectious diseases and other
serious conditions. Protein malnutrition in infants is
called kwashiorkor, and it poses a major health prob-
lem in developing countries, such as Africa, Central and
South America, and certain parts of Asia. An infant
with kwashiorkor suffers from poor muscle and tissue
development, loss of appetite, mottled skin, patchy
hair, diarrhea, edema, and, eventually, death (similar
symptoms are present in adults with protein deficiency).
Treatment or prevention of this condition lies in
adequate consumption of protein-rich foods.Resources
BOOKS
Berdanier, Carolyn D. (1998).CRC Desk Reference for
Nutrition. Boca Raton, FL: CRC Press.
Briggs, George M., and Calloway, Doris Howes (1979).
Bogert’s Nutrition and Physical Fitness, 10th edition.
Philadelphia, PA: W. B. Saunders.
Johnston, T. K. (1999). ‘‘Nutritional Implications of Vege-
tarian Diets.’’ InModern Nutrition in Health andProtein