Download Summary for Chapter 6 – Protein: Amino Acids

Summary for Chapter 6 – Protein: Amino Acids
Chemically speaking, proteins are more complex than carbohydrates or lipids, being made of some 20 different
amino acids, 9 of which the body cannot make; they are essential. Each amino acid contains an amino group, an acid
group, a hydrogen atom, and a distinctive side group, all attached to a central carbon atom. Cells link amino acids
together in a series of condensation reactions to create proteins. The distinctive sequence of amino acids in each
protein determines its unique shape and function.
Digestion is facilitated mostly by the stomach’s acid and enzymes, which first denature dietary proteins, then cleave
them into smaller polypeptides and some amino acids. Pancreatic and intestinal enzymes split these polypeptides
further, to oligo-, tri-, and dipeptides, and then split most of these to single amino acids. Then carriers in the
membranes of intestinal cells transport the amino acids into the cells, where they are released into the bloodstream.
Cells synthesize proteins according to the genetic information provided by the DNA in the nucleus of each cell. This
information dictates the order in which amino acids must be linked together to form a given protein. Sequencing
errors occasionally occur, sometimes with significant consequences.
The protein functions discussed here are summarized in the accompanying table. They are only a few of the many
roles proteins play, but they convey some sense of the immense variety of proteins and their importance in the body.
Growth and maintenance
Enzymes
Hormones
Fluid balance
Acid-base balance
Transportation
Antibodies
Energy and glucose
Proteins form integral parts of most body structures such as skin, tendons,
membranes, muscles, organs, and bones. As such, they support the growth and
repair of body tissues.
Proteins facilitate chemical reactions.
Proteins regulate body processes. (Some, but not all, hormones are proteins.)
Proteins help to maintain the volume and composition of body fluids.
Proteins help maintain the acid-base balance of body fluids by acting as buffers.
Proteins transport substances, such as lipids, vitamins, minerals, and oxygen,
around the body.
Proteins inactivate foreign invaders, thus protecting the body against diseases.
Proteins provide some fuel, and glucose if needed, for the body’s energy needs.
Proteins are constantly being synthesized and broken down as needed. The body’s assimilation of amino acids into
proteins and its release of amino acids via protein degradation and excretion can be tracked by measuring nitrogen
balance, which should be positive during growth and steady in adulthood. An energy deficit or an inadequate protein
intake may force the body to use amino acids as fuel, creating a negative nitrogen balance. Protein eaten in excess
of need is degraded and stored as body fat.
A diet that supplies all of the essential amino acids in adequate amounts ensures protein synthesis. The best
guarantee of amino acid adequacy is to eat foods containing high-quality proteins or mixtures of foods containing
complementary proteins that can each supply the amino acids missing in the other. In addition to its amino acid
content, the quality of protein is measured by its digestibility and its ability to support growth. Such measures are of
great importance in dealing with malnutrition worldwide, but in the United States and Canada, where protein
deficiency is not common, protein quality scores of individual foods deserve little emphasis.
Protein deficiencies arise from both energy-poor and protein-poor diets and lead to the devastating diseases of
marasmus and kwashiorkor. Together, these diseases are known as PEM (protein-energy malnutrition), a major form
of malnutrition causing death in children worldwide. Excesses of protein offer no advantage; in fact, overconsumption
of protein-rich foods may incur health problems as well.
The optimal diet is adequate in energy from carbohydrate and fat and delivers 0.8 grams of protein per kilogram of
healthy body weight each day. U.S. and Canadian diets are typically more than adequate in this respect.
Normal, healthy people never need protein or amino acid supplements. It is safest to obtain lysine, tryptophan, and all
other amino acids from protein-rich foods, eaten with abundant carbohydrate and some fat to facilitate their use in the
body. With all that we know about science, it is hard to improve on nature.