Nutrition Basics - Chapter 6: Proteins

Defining Protein and Amino Acids

  • Proteins are macromolecules formed from amino acid monomers, constituting roughly 20%20\% of the human body and present in over 100,000100,000 variations.

  • Amino acids consist of a central carbon, an amino group, a carboxyl group, and a unique side chain (R group).

  • Classification groups include nonpolar (hydrophobic), polar (hydrophilic, uncharged), acidic (negatively charged), and basic (positively charged).

  • Nutritional categories:

    • Essential: 99 amino acids (e.g., Histidine, Lysine, Tryptophan) that must be sourced from diet.

    • Nonessential: Amino acids synthesized by the body.

    • Conditionally Essential: Become essential during specific life stages like child growth (e.g., Arginine, Glycine, Proline).

Protein Synthesis and Structure

  • Building Process:

    • Transcription: DNA is copied into mRNA.

    • Translation: mRNA is decoded to synthesize a protein.

    • Folding: Amino acid sequences transform into specific shapes.

  • Chemical Bonding: Amino acids are joined by peptide bonds.

  • Levels of Organization: Proteins are structured into Primary, Secondary, Tertiary, and Quaternary levels, making them more complex than lipids or carbohydrates.

Digestion, Absorption, and Metabolism

  • Digestion: Begins mechanically in the mouth. Chemical digestion occurs in the stomach via hydrochloric acid and Pepsin, then continues in the small intestine with pancreatic enzymes like Chymotrypsin and Trypsin.

  • Absorption: Individual amino acids are transported into the blood from the small intestine.

  • Protein Turnover: The body continuously recycles proteins, with approximately 250 g250\,g dismantled and rebuilt daily.

  • Nitrogen Balance: Maintains the equilibrium between protein intake and excretion.

Biological Functions of Proteins

  • Structure and Motion: Collagen provides strength and flexibility to bones, tendons, and skin (30%30\% of bone tissue).

  • Enzymes: Act as catalysts to lower the energy required for chemical reactions.

  • Hormones: Act as chemical messengers (e.g., turning enzymes on/off).

  • Fluid and Acid-Base Balance: Albumin helps maintain water distribution and proper pH levels in the blood.

  • Protection: Lysozyme and antibodies defend against pathogens like SARS-CoV-2.

  • Energy Production: Disassembled amino acids contribute roughly 10%10\% of daily energy via the citric acid cycle if needed.

Protein-Related Health Conditions

  • Deficiency Syndromes:

    • Kwashiorkor: Severe protein and micronutrient deficiency leading to swelling and liver malfunction.

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

  • High-Protein Diets: Intakes exceeding 30%30\% of calories may correlate with cardiovascular disease, diabetes, and kidney abnormalities.

Dietary Guidelines and Sources

  • RDA (Recommended Dietary Allowance): Calculated for adults as (Weight in lbs.÷2.2 kg/lb)×0.8 g/kg(Weight \text{ in lbs.} \div 2.2 \text{ kg/lb}) \times 0.8 \text{ g/kg}.

  • AMDR (Acceptable Macronutrient Distribution Range): 10−35%10-35\% of daily calories.

  • Quality & Sources:

    • Complete: Food containing all 99 essential amino acids (animal products, soy).

    • Incomplete: Food lacking some essential amino acids (most plants).

    • Complementary Proteins: Combinations of plant foods (e.g., beans and grains) to provide all essential amino acids.

  • PDCAAS (Protein Digestibility Corrected Amino Acid Score): A quality metric where Milk, Egg white, Whey, and Soy score a maximum of 1.01.0.