Nutritional Science: Protein Quality, Metabolism, and Essential Factors

Posted on Oct 15, 2025 in Human Nutrition and Dietetics

Protein Quality and Biological Value

Protein quality is defined by the number, nature, and type of amino acids present. A protein has higher quality if its range of amino acids is more complete and closer to physiological needs.

Defining Protein Quality

  • High Quality Protein: Contains all essential amino acids in proper proportion relative to the body’s needs.
  • Limiting Amino Acid: An essential amino acid found in the smallest amount relative to the body’s requirement.

Protein Quality Indicators

Indicators are categorized as Chemical or Biological:

Chemical Indicators

  1. Aminogram (Amino Acid Profile)
  2. Chemical Index of Essential Amino Acids
  3. Cómputo (Calculation)
  4. Available Lysine

Biological Indicators

  1. Protein Growth Performance Coefficient
  2. Net Protein Retention
  3. Biological Coefficient of Digestibility
  4. Biological Value (BV)
  5. Utilization
  6. Net Protein Production Value

Protein Complementation

The consumption of several proteins with different, but complementary, amino acid patterns—even if they individually have low biological values—produces a protein mixture with a Biological Value (BV) higher than expected.

For example: milk + cereals, or legumes + cereals.

Dietary Protein Needs

Adults: 0.57 g of high-quality protein per kilogram of body weight (typically recommended as 50% vegetable, 50% animal sources).

Protein Denaturation

Denaturation is the structural modification of the native protein molecule without breaking the primary covalent bonds.

Causative Agents of Denaturation

The causative agents include:

  • Physical Agents: Heat, pressure, radiation, pH changes.
  • Chemical Agents: Urea, detergents, etc.

Results of Denaturation

As a result of denaturation, the protein exhibits:

  • Decreased solubility.
  • Loss of crystallization power.
  • Decreased enzyme activity.
  • Increased viscosity and susceptibility to proteolysis.

Effects of Heat on Proteins

Heat exposure can have both beneficial and harmful effects:

  • Beneficial: Inactivation of trypsin inhibitors, improved digestion, and improved availability of sulfur amino acids (Methionine, Tryptophan, Threonine).
  • Harmful: Amino acid oxidation (e.g., lysine losses), non-enzymatic browning, and formation of new, non-absorbable amino acid links.

Energy Metabolism and Atwater Factors

The Atwater System (Metabolizable Energy)

The Atwater Number represents the calories derived from the combustion of 1 gram of physiologically active substance. This value determines the metabolizable energy (E) supplied by food.

Physiological Energy Values (Kcal/g)

  • Carbohydrates (CH): 4
  • Fat: 9
  • Protein (P): 4
  • Alcohol: 7

Heat of Combustion

Heat of Combustion refers to the calories released during the complete combustion of 1 gram of an active ingredient.

Basal Metabolism (BMR)

Basal Metabolism (MB/BMR) is the amount of energy needed to maintain the activity of internal organs when the individual is at complete rest. This energy supports organ maintenance, protein synthesis, active transport, and basic functions (respiration, circulation).

Simplified Calculation: MB (Kcal/day) = 1 Kcal × weight (kg) × 24 hours.

Factors Affecting Basal Metabolism

Basal Metabolism is influenced by several factors:

  • Body Size: Larger surface area means more heat is dissipated, generally increasing BMR.
  • Body Composition: More muscle increases BMR; more fat decreases BMR.
  • Sex: Men generally have a higher BMR than women.
  • Pregnancy and Lactation: Increases BMR.
  • Age: BMR decreases starting around age 20.
  • Febrile Process: If body temperature increases by one degree Celsius, the metabolic rate increases.
  • Ambient Temperature: If ambient temperature increases, BMR decreases. If ambient temperature decreases, BMR increases.
  • Sleep: Decreases BMR.
  • Pharmacodynamic Agents: Increases BMR.
  • Fasting: Decreases BMR.
  • Altitude: Increases BMR.
  • Hormone State: Hyperthyroidism increases BMR, and vice versa.

Characteristics of Essential Nutrients

An essential nutrient must meet the following criteria:

  • It is necessary for life and is present in reasonably constant concentrations in healthy tissues.
  • It is not synthesized in the body (or not in sufficient amounts).
  • It must be supplied in the diet: Deficiency leads to structural or physiological abnormalities, and its addition to a deficient diet reverses these abnormalities.

Examples of Essential Nutrients

  • Carbohydrates (CH): None are strictly essential.
  • Lipids: Linoleic acid and Linolenic acid.
  • Protein (Amino Acids): Methionine (Met), Valine (Val), and Tyrosine (Tyr).
  • Minerals: Calcium (Ca), Phosphorus (P), Sodium (Na), Iron (Fe), and others.
  • Vitamins: A, D, K, and B Complex vitamins.