L7 Macronutrients: Protein and Structure

Macronutrients: Protein

• Protein is derived from the Greek word "proteos", meaning "holding first place" or "primary".
• Proteins are large molecules composed of amino acids.
• Amino acids are the building blocks of proteins.
• Approximately 20 different amino acids are commonly found in plants and animals.
• Proteins differ from carbohydrates (CHO) and fats due to nitrogen content, their amino acid structure differs from other molecules, unlike glucose molecules.
• The sequence and number of amino acids determine protein's nature.

Protein Structure

• Primary structure: Peptide bonds form a linear chain of amino acids.
• Secondary structure: Attractions between R groups of amino acids create helices and pleated sheet structures.
• Tertiary structure: The helices and pleated sheets fold into a compact domain.
• Quaternary structure: Individual polypeptides form larger protein structures.

Amino Acids

• All amino acids share a basic structure: an amino group (-NH2), an acid group (-COOH), a hydrogen group (-H), and a unique side chain (-R).
• The -R group differentiates amino acids.
• Some amino acids contain a sulfur group ("S").
• Peptide bonds connect amino acids.

Types of Amino Acids

• Essential amino acids: Nine amino acids that the body cannot produce enough of, and must be obtained from the diet.
• Nonessential amino acids: Eleven amino acids that the body can synthesize from other compounds.
• Conditionally essential amino acids: Six amino acids that the body typically produces, but may need to be consumed in the diet under specific circumstances or conditions.
• Semi-essential amino acids: Two amino acids that are considered essential for children, but not for adults.
  • Histidine
  • Arginine

Protein Digestion and Absorption

• Whole proteins are not readily absorbed.
• Proteins are digested into amino acids, dipeptides, tripeptides, and polypeptides in the small intestine.
• Most (99%) of ingested protein is absorbed as amino acids.
• Amino acids are water-soluble and are absorbed directly into the bloodstream via active transport.
• Cells combine amino acids to form the needed protein.
  • Cells will produce nonessential amino acids if the essential amino acids are present.
• The protein digestion process occurs in phases involving the mouth, stomach, pancreas, and small intestine.

Protein Functions

• Growth and maintenance: Proteins are part of every cell and needed for building tissues (e.g., children's tissues, blood, hair, skin).
• Energy source: When carbohydrate and fat stores are adequate, amino acids can be broken down for energy and are used to make proteins. When both are not adequate, proteins are broken down for energy resulting in protein deficiencies.
• Regulatory role: Proteins play a vital role in numerous bodily functions, including chemical reactions (enzymes), chemical messengers (hormones), immunity (antibodies), fluid balance, and transporting nutrients.

Protein Food Sources

• Animal sources: meat, fish, dairy products, eggs are complete proteins (contain all essential amino acids).
• Plant sources: cereals, legumes, nuts, and seeds are incomplete proteins (lack some essential amino acids).
• Complementary proteins are a diet that combines proteins that complement each other to provide a full amino acid profile.

Protein Recommendations and Factors Influencing Protein Needs

• The recommended daily allowance (RDA) varies based on gender, pregnancy, and lactation.
• Several factors, such as growth phases, illness, and dietary quality, influence protein requirements.

Clinical Implications of Low/High Protein Diets

• Low protein intake can lead to severe protein-energy malnutrition (PEM).
• Low protein intake during pregnancy may lead to low birth weight.
• High protein intake from animal sources may increase saturated fat and reduce fiber, and could result in kidney problems.
• Vegan diets necessitate paying particular attention to protein complementation for avoiding deficiencies.

Acceptable Macronutrient Distribution Range (AMDR)

• The AMDR for protein is 10% to 35% of total calories.
• Protein needs are typically met when 10% of your energy intake comes from protein, assuming adequate calorie intake.