 |  Perspectives in Nutrition, 5/e Gordon M. Wardlaw,
Ohio State University
Margaret W. Kessel,
Ohio State University
Proteins
Chapter 7 Summary- Amino acids, the building blocks of proteins, contain a very usable form of
nitrogen for humans. Of the 20 types of amino acids found in food, 9 must be
consumed as food and the rest can be synthesized by the body.
- High-quality, also called complete, protein foods contain ample amounts of
all nine essential amino acids. Furthermore, foods derived from an animal source
provide high biological value protein. Lower-quality, also called incomplete,
protein foods lack sufficient amounts of one or more essential amino acids.
This is typical of plant foods, especially cereal grains. Different types of
plant foods eaten together often complement each other's amino acid deficits,
thereby providing high-quality protein in the diet.
- Individual amino acids are linked together to form proteins. The sequential
order of amino acids determines the protein's ultimate shape and function. This
order is directed by DNA in the cell nucleus. Diseases such as sickle cell
anemia can occur if the amino acids are incorrect on a polypeptide chain.
When the three-dimensional shape of the protein is unfolded--denatured—by treatment
with heat, acid or alkaline solutions, or other processes, the protein also
loses its biological activity.
- Protein digestion begins in the stomach, dividing the proteins into breakdown
products containing shorter chains of amino acids. In the small intestine, these
polypeptide chains eventually separate into amino acids. These free amino acids
are absorbed by the enterocytes and travel via the portal vein to the liver.
- Important body components—such as muscles, connective tissue, transport proteins,
visual pigments, enzymes, some hormones, and immune bodies—are made of proteins.
These proteins are in a state of constant turnover. Proteins also provide carbons,
which can be used to synthesize glucose when necessary.
- The protein RDA for adults is 0.8 g per kg of healthy body weight. For a typical
70-kg (156-lb) person, this corresponds to 56 g of protein daily; for a 55-kg
(120-lb) person, this corresponds to 44 g/day. The North American diet generally
supplies plenty of protein: Men typically consume about 95 g of protein daily,
and women consume closer to 65 g. The combined protein intake is also of sufficient
quality to support body functions.
- Almost all animal products are nutrient-dense sources of protein. The high
quality of these proteins means that they can be easily converted into body
proteins. Plant foods generally contain less than 20% of their energy content
as protein; however, legumes are an excellent source of high-quality protein
if eaten with grains or animal products.
- Protein quality can be measured by determining the extent to which the body
can retain the nitrogen contained in the protein absorbed; this is called biological
value. In addition, the balance of essential amino acids in a food can be compared
with an ideal pattern. The comparison with the ideal pattern is referred to
as the chemical score. When multiplied by the degree of digestibility, the chemical
score yields the Protein Digestibility Corrected Amino Acid Score (PDCAAS).
- Undernutrition can lead to protein-energy malnutrition in the form of kwashiorkor
or marasmus. Kwashiorkor results primarily from an inadequate energy and protein
intake in comparison with body needs, which often increase with concurrent disease
and infection. Kwashiorkor often occurs when a child is weaned from human milk
and fed mostly starchy gruels. Marasmus results primarily from extreme starvation—a
negligible intake of both protein and energy. Marasmus commonly occurs during
famine, especially in infants. Variations of these diseases appear in some hospitalized
Americans.
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2002 McGraw-Hill Higher Education