Proteins and Amino Acids

Chapter

6

Photo courtesy of the USDA

Protein

• The word protein was coined by the Dutch chemist Gerardus Mulder in 1838, and comes from the Greek word protos, meaning “of prime importance.”

Protein

• Proteins are a major component of all plant and animal tissues, second only to water.

• When we eat more protein than we need, the excess is either used to make energy or is stored as fat.

Protein Deficiency

• When the diet lacks protein, the body breaks down tissues such as muscle and uses it as a protein source.• This causes loss, or wasting, of muscle,

organs, and other tissues.

• Protein deficiency also increases susceptibility to infections and impairs digestion and absorption of nutrients.

Amino Acids Are the Building Blocks of Protein

• Proteins are sequences of amino acids

• Types of amino acids• Essential: must come

from diet• Nonessential: can be

made in the body

Amino Acids Are the Building Blocks of Protein

• Protein structure• Chain of amino acids

• Sequence of amino acids determines shape• Shape of protein determines function

• Denaturing protein structure• Disrupts function• Caused by heat, acid, oxidation, agitation

Functions of Body Protein

Protein Digestion and Absorption

• Stomach• Proteins are denatured by hydrochloric

acid• Pepsin begins digestion

• Small intestine• Pancreatic and intestinal proteases and

peptidases complete digestion• Amino acids absorbed into the

bloodstream

Proteins in the Body

• Protein synthesis• Directed by

cellular DNA

• Amino acid pool• Protein turnover • Synthesis of non-

protein substances

Proteins in the Body

• Protein excretion• Deamination of amino acids• Amino groups converted to urea for

excretion

• Nitrogen balance• Nitrogen intake vs. nitrogen output

Proteins in the Diet

• Recommended protein intake• Adult RDA = 0.8 grams/kilogram body

weight• Infant RDA = ~ 1.5 grams/kilogram body

weight

• Protein consumption

Proteins in the Diet

• Protein quality• Complete proteins

• Supply all essential amino acids• Animal proteins, soy proteins

• Incomplete proteins• Low in one or more essential amino acids• Most plant proteins

• Complementary proteins• 2 incomplete proteins = complete protein

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Proteins in the Diet

• Evaluating protein quality• Amino acid composition• Digestibility• Protein Digestibility–Corrected Amino

Acid Score (PDCAAS)• Used to determine %DV

• Protein and amino acid supplements• Generally not needed with risks

unknown

Pros and Cons of Vegetarian Eating

• Types of vegetarian diets• Semi-vegetarian• Lacto-ovo vegetarian• Vegan

• Health benefits vs. health risks• Less fat, saturated fat, and cholesterol• Restrictive diets may lack nutrients• Careful planning needed for children, pregnant

women

The Health Effects of Too Little Protein

• Protein-energy malnutrition (PEM)• Kwashiorkor• Marasmus

The Health Effects of Too Much Protein

Denaturation

• Heat, pH, oxidation, and mechanical agitation are some of the forces that can denature a protein, causing it to unfold and lose its functional shape.

Structural Proteins

• Proteins provide structure to all cells, including hair, skin, nails, and bone. As part of muscle, they transform energy into mechanical movement.

Hormones

• Hormones are formed in one part of the body and carried in the blood to a different location where they signal cells to alter activities.

Proteins and the Immune System

• Protein antibodies are a crucial line of defense against invading bacteria and viruses.

Proteins in the Blood

• Blood proteins attract fluid into capillaries.

• This counteracts the force of the heart beating, which pushes fluid out of capillaries.

Proteins Act as Carriers

• Lipoproteins have embedded proteins that help them transport fat and cholesterol in the blood.

Protein and Gout• A recent study sought to

explore whether or not the risk of gout is independently increased by consumption of (1) a protein-rich diet, (2) a diet high in meat, and (3) a diet high in seafood.

• The results showed that a protein-rich diet was not associated with an increased risk of gout. However, the risk of gout increased 21 percent per additional portion of meat per day and 7 percent per addition portion of seafood per week. The study also found that consumption of dairy protein (especially in low-fat dairy products) reduced risk of gout.

ADA Position on Vegetarian Eating

• It is the position of the American Dietetic Association and Dietitians of Canada that appropriately planned vegetarian diets are healthful, nutritionally adequate, and provide health benefits in the prevention and treatment of certain diseases.

Protein (Nitrogen) Balance

• A pregnant woman adds protein so she has a positive nitrogen balance.

• A healthy person who is neither gaining nor losing nitrogen is in nitrogen equilibrium.

• A person who is severely ill and losing protein has a negative nitrogen balance.

Proteins in the Diet

• Meat, eggs, milk, legumes, grains, and vegetables are all sources of protein.

• Fruits contain minimal amounts and, along with fats, are not considered protein sources.

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