unit five - proteins

1. Introduction

• Derived from the Greek word “proteos,” meaning “the first.” Proteins form a fundamental part of the body’s structure.

• Essential components of the body:

• Active, versatile, and functional molecules responsible for complex cellular tasks.

• Constitute 15–20% of an adult’s body weight:

• 40% in muscle tissue.

• 30% in skin and blood.

• The rest in other tissues and fluids (not found in bile or urine under normal conditions).

2. Proteins

Chemical Composition:

• Polymers of simpler substances: amino acids (AA).

• Composed of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).

• Energy yield: 4 kcal/g when oxidized.

Amino Acids (AA):

• Over 200 types of amino acids exist in nature.

• Only 20 amino acids form part of human body proteins.

• 9 Essential Amino Acids:

• Leucine, Isoleucine, Valine, Lysine, Methionine, Threonine, Phenylalanine, Tryptophan, Histidine.

• Branched-chain amino acids (leucine, isoleucine, valine):

• Vital for protein synthesis.

• Comprise one-third of skeletal muscle.

2.1 Classification by Nutritional Value

• Protein Quality:

• Foods contain proteins of varying quality, depending on their essential amino acid content and digestibility.

• The limiting amino acid is the one present in the lowest proportion compared to cellular demand.

• Protein Complementation:

• Combining different foods can balance the deficiencies of amino acids, enhancing overall protein quality.

Protein Requirements:

• General recommendations: 0.8 g/kg body weight.

• Increased needs: 1.0–1.2 g/kg, up to 2.0 g/kg in specific conditions.

• Protein efficiency decreases with:

• Sedentary lifestyle.

• Obesity.

• Aging.

• Restrictive diets.

• Poor sleep quality.

2.2 Functions of Proteins

1. Structural (Plastic) Function:

• Provides essential amino acids for tissue synthesis.

• Protein turnover:

• Approximately 200–300 g of proteins are degraded and synthesized daily, maintaining a dynamic balance.

• Daily protein intake is essential to repair wear and tear.

2. Energy Function:

• When energy sources (carbohydrates and fats) are insufficient, or protein intake exceeds requirements, proteins are oxidized for energy (4 kcal/g).

• This process is less efficient than lipid or carbohydrate oxidation.

• Results in nitrogen waste, requiring energy for urea formation.

3. Regulatory Function:

• Involves the regulation of cellular activities through:

• Hormones (e.g., insulin, growth hormone).

• Enzymes that catalyze metabolic reactions.

• Neurotransmitters that facilitate nerve impulses.

4. Transport Function:

• Maintains osmotic balance by transporting substances:

• Gases (e.g., hemoglobin transports oxygen).

• Lipids (e.g., serum albumin).

5. Defensive Function:

• Protects against foreign organisms and particles:

• Antibodies (immunoglobulins).

• Blood clotting factors (e.g., fibrinogen, thrombin) prevent blood loss when vessels are damaged.

2.3 Protein Quality

Definition:

• Protein quality refers to the ability of a protein source to meet an individual’s nitrogen and amino acid requirements.

Factors Influencing Protein Quality:

1. Digestibility:

• Refers to how easily a protein is broken down and absorbed.

2. Chemical Integrity:

• The balance and completeness of amino acids in the protein.

3. Antinutritional Factors:

• Can reduce protein availability by up to 50%.

• Examples:

• Natural Antinutrients: Tannins, phytates, etc.

• Storage or Processing Effects: Formation of compounds during food storage or cooking.

Comparison of Protein Sources:

• Animal Proteins:

• Higher efficiency and digestibility.

• Examples: Meat, fish, eggs, dairy.

• Plant Proteins:

• Generally less efficient due to lower digestibility and being encased in carbohydrates.

Measuring Protein Quality

1. Protein Score:

• Compares the amino acid content of a test protein to that of a reference protein.

• Formula:

Protein Score = mg of amino acid in test protein/mg of amino acid in reference protein

2. PDCAAS (Protein Digestibility Corrected Amino Acid Score):

• Adjusts protein score based on digestibility.

• Calculated by multiplying protein score by digestibility percentage.

3. Applications:

• PDCAAS values are useful for selecting protein sources in dietary planning based on their quality.

2.4 Food Sources of Protein

Animal-Based Protein Sources

Food Protein per 100g Carbohydrates

Meat 20–25 g <0.5 g

Fatty Fish 20–25 g <0.5 g

Lean Fish 15–20 g <0.5 g

Egg (1 unit) ~6 g <0.5 g

Plant-Based Protein Sources

Food Protein per 100g Carbohydrates

Firm Tofu 15 g <2 g

Soy Tempeh 18 g <2 g

Textured Soy Protein 50 g 25 g

Legumes (dry) 20–25 g 50 g

Legumes (cooked) 6–8 g 15 g

Canned Legumes 5–6 g 10 g

Legume Pasta (dry) 20–25 g 50 g

Summary

1. Proteins are essential nutrients:

• They serve critical roles in energy production, structural integrity, regulation, and defense.

2. Quality and source matter:

• Animal proteins tend to have higher quality, but plant proteins can be complemented to improve their nutritional value.

3. Daily protein intake is necessary:

• To maintain the body’s dynamic balance between protein synthesis and degradation.

4. Choose diverse sources:

• A mix of animal and plant proteins ensures adequate amino acid intake while promoting overall health.

Summary

• Proteins are essential macronutrients that serve critical roles in the body.

• Protein sources vary in quality and digestibility, with animal proteins generally having higher efficiency.

• Combining plant-based proteins can ensure adequate amino acid intake in vegetarian and vegan diets.

• A balanced intake of diverse protein sources is key to maintaining health and supporting physiological functions.

Protein Functions: Recap

1. Structural Function:

• Provides the building blocks for tissue growth and repair.

2. Energy Function:

• Secondary energy source, yielding 4 kcal/g when oxidized.

3. Regulatory Function:

• Hormones (e.g., insulin, growth hormone) and enzymes regulate metabolic processes.

4. Transport Function:

• Proteins like hemoglobin and albumin transport oxygen, lipids, and other substances.

5. Defensive Function:

• Antibodies protect against infections, and clotting factors prevent blood loss.

Protein Quality and Evaluation

1. PDCAAS (Protein Digestibility Corrected Amino Acid Score):

• Assesses protein quality by measuring amino acid content and digestibility.

2. Limiting Amino Acids:

• Plant proteins often lack one or more essential amino acids but can be complemented by combining food sources.

Dietary Protein Sources

Animal-Based Sources:

• High in quality, providing all essential amino acids.

• Examples: Meat, fish, eggs, and dairy.

Plant-Based Sources:

• Often incomplete in amino acids but can be combined to achieve complete protein intake.

• Examples: Legumes, tofu, tempeh, and soy products.

Protein Intake Guidelines

1. General Recommendations:

• 0.8 g/kg body weight for adults.

2. Increased Needs:

• 1.2–2.0 g/kg for athletes, elderly individuals, or during recovery from illness.

Summary of Key Points

• Proteins are indispensable for maintaining and repairing body tissues, energy production, and metabolic regulation.

• A balanced intake of high-quality proteins supports optimal health.

• Plant-based proteins can achieve equivalent nutritional value to animal proteins when combined appropriately.