Notes on Protein in the Human Diet

Introduction to Protein

• Protein is essential for the human diet.
• Found in various foods: eggs, dairy, seafood, legumes, meats, nuts, and seeds.
• Functions of proteins: fight infections, help cells divide, and more.

Structure of Proteins

• Proteins are chains of amino acids linked by peptide bonds.
• Amino acids consist of:
  • Central carbon atom
  • Amino group (–NH2)
  • Carboxylic acid group (–COOH)
  • Hydrogen atom
  • Unique side chain (R group) for each amino acid
• Exception: Proline has a ring structure.

Amino Acids

• Humans use about 20 amino acids to create proteins, including:
  • Non-essential: can be made by the body
    • Alanine, Asparagine, Aspartic acid, Glutamic acid, Serine
  • Conditionally essential: can be synthesized under normal conditions but not during stress or illness
    • Arginine, Cysteine, Glutamine, Glycine, Proline, Tyrosine
  • Essential: must be obtained from food
    • Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine

Protein Digestion and Absorption

1. Proteolysis process: Protein breakdown begins in the stomach.
   • Hydrochloric acid denatures protein, making it accessible.
   • Pepsin breaks down proteins into smaller oligopeptides.
2. Small intestine:
   • Oligopeptides are further digested by pancreatic enzymes into tripeptides, dipeptides, and amino acids.
   • Amino acids are absorbed into intestinal cells.
   • Some amino acids are used to synthesize intestinal enzymes; the rest enter the bloodstream.

Protein Sources

• Animal-based foods: Eggs, dairy, seafood, and meat provide all nine essential amino acids.
• Soy foods: Unique plant-based source providing all essential amino acids.
• Plant-based sources:
  • Whole grains, legumes, nuts, seeds may lack some amino acids but can be combined (e.g., rice and beans) to provide a complete profile.
  • Volume of food may need to be higher for similar protein content.

Protein Requirements

• Daily protein needs vary by age and activity level:
  • General recommendation: 0.8g per kg of body weight.
  • Children:
    • Ages 1-3: 13g
    • Ages 4-8: 19g
    • Ages 9-13: 34g
  • Adults:
    • Males 14-18: 52g
    • Males 19+: 56g
    • Females 14+: 46g
• Special populations (pregnant women, athletes) may have increased needs.

Achieving Protein Intake

• Examples of meals to meet protein needs:
  • Greek yogurt + chicken salad + fish = 64g protein.
  • Tofu scramble + edamame + lentils + brown rice = 64g protein.

Protein Deficiency Risk

• Higher risk in individuals with:
  • Malnutrition
  • Trauma and burn injuries
  • Conditions affecting nutrient absorption (e.g., inflammatory bowel disease)
• Generally safe to consume high-protein diets unless specific health conditions exist (e.g., kidney disease).

Conclusion

• Protein is vital for the body; have a mix of non-essential, conditionally essential, and essential amino acids.
• Adequate protein can be achieved through varied diets, regardless of being omnivorous, vegetarian, or vegan.