Nutrition Basics: Chapter 6 - Proteins

Composition and Classification of Proteins

• Proteins: Large macromolecules composed of amino acid monomers; they comprise approximately $20\,\%$ of the human body.

• Amino Acid Structure: Consists of a central carbon atom, an amino group, a carboxyl group, and a unique side chain (R group).

• Classification by Side Chain:

  • Nonpolar: Hydrophobic with long/bulky side groups.

  • Polar: Hydrophilic and uncharged.

  • Acidic: Hydrophilic and negatively charged.

  • Basic: Hydrophilic and positively charged.

• Nutritional Classification:

  • Nonessential: Synthesized by the body in sufficient amounts.

  • Essential: Must be obtained from the diet (total of nine).

  • Conditionally Essential: Become essential during specific life stages, such as child growth (e.g., Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine).

Protein Synthesis and Structural Organization

• Building Steps:

  • Transcription: Copying DNA into messenger RNA (mRNA).

  • Translation: Decoding mRNA to synthesize a protein.

  • Protein Folding: Amino acid sequences transform into a dictated functional shape.

• Structural Levels: Primary structure, Secondary structure (Alpha helix, Beta sheet), Tertiary structure, and Quaternary structure.

• Denaturation: Physical unraveling of a protein's folded structure into a long strand of amino acids due to external stresses like heat, acid, high salt concentrations, alcohol, or mechanical agitation.

Digestion, Absorption, and Turnover

• Stomach: Gastric juices containing hydrochloric acid and the enzyme pepsin initiate protein breakdown.

• Small Intestine: Pancreatic enzymes chymotrypsin and trypsin dismantle fragments into individual amino acids for transport to the blood.

• Protein Turnover: Continuous process of breaking down and building new proteins; approximately $250\,g$ are dismantled and rebuilt daily.

• Nitrogen Balance: A state where nitrogen input equals use and excretion. Higher intake is needed during pregnancy or growth.

Essential Functions in the Human Body

• Structure and Motion: Collagen (primarily glycine and proline) makes up $30\,\%$ of bone tissue and is found in tendons and skin.

• Enzymes and Hormones: Proteins catalyze chemical reactions and act as chemical messengers (e.g., to turn enzymes on or off).

• Fluid and pH Balance: Albumin is the most abundant blood protein and is critical for maintaining water distribution and proper pH.

• Transport and Protection: Includes channel/carrier proteins and the immune system (e.g., lysozyme and antibodies).

• Energy Production: Approximatley $10\,\%$ of dietary protein is catabolized daily for energy through the citric acid cycle.

Clinical Health and Protein Deficiency

• Kwashiorkor: Severe protein and micronutrient deficiency characterized by swelling (edema) of feet and abdomen and liver malfunction.

• Marasmus: Severe protein and energy deficiency resulting in emaciation and growth retardation.

• Excessive Intake: Diets where protein exceeds $30\,\%$ of calories may increase risks for cardiovascular disease, diabetes, and kidney/liver abnormalities.

Dietary Sources and Recommendations

• Recommended Dietary Allowance (RDA): For adults, it is $0.8\,g/kg$ of body weight.

  • Calculation: $(Weight\,in\,lbs. \div 2.2\,kg/lb) \times 0.8\,g/kg$

• Acceptable Macronutrient Distribution Range (AMDR): $10\text{--}35\,\%$ of total calories.

• Protein Quality:

  • Complete Sources: Contain all nine essential amino acids (mostly animal-based, plus soy).

  • Incomplete Sources: Lack certain essential amino acids (mostly plant-based, plus gelatin).

  • PDCAAS: Measurement of protein quality; milk protein, egg whites, whey, and soy rank highest at $1.00$.

• Supplements: Scientifically, protein or amino acid supplements are not proven to improve performance or muscle mass more effectively than whole food sources.