Protein Digestion & Utilisation Basics

Proteins are broken down into amino acids, which serve as building blocks for muscle, enzymes, hormones, and immune structures. Understanding protein digestion and utilisation explains how dietary protein supports body functions.

Protein Structure and Amino Acids

Proteins are polymers composed of amino acids linked by peptide bonds. The human body requires 20 amino acids. Nine are essential, meaning they cannot be synthesised by the body and must be obtained through diet. Eleven are non-essential, as the body can manufacture them from other amino acids or precursors. The combination of amino acids in a protein determines its structure and function.

Protein Digestion Process

Protein digestion begins in the stomach, where hydrochloric acid denatures proteins and the enzyme pepsin initiates peptide bond cleavage. In the small intestine, additional enzymes (trypsin, chymotrypsin, carboxypeptidases) continue breaking down proteins into smaller peptide chains and individual amino acids. These are then absorbed across the intestinal wall into the bloodstream.

Amino Acid Utilisation

Amino acids are transported throughout the body and incorporated into new proteins through protein synthesis. This process is particularly active in muscle tissue, where muscle protein synthesis is the primary mechanism by which muscle mass is maintained or increased. Amino acids also serve as precursors for neurotransmitters, hormones, and immune molecules.

The rate of muscle protein synthesis is influenced by several factors, including amino acid availability (particularly leucine), insulin levels, and mechanical stimulus from exercise. Protein distributed across multiple meals produces steadier amino acid availability compared to infrequent large intakes.

Thermic Effect of Protein

Protein requires more energy for digestion, absorption, and processing compared to carbohydrates or fats. This is termed the thermic effect of food. Approximately 20-30% of protein calories are expended in digestion, compared to 5-10% for carbohydrates and 0-3% for fats. This property influences total energy balance when macronutrient composition varies.

Protein Requirements

Total daily protein requirements vary based on body weight, activity level, and age. Current recommendations range from 0.8 grams per kilogram of body weight for sedentary individuals to 1.6-2.2 grams per kilogram for those engaged in regular resistance training. Individual responses to protein intake vary, and optimal amounts depend on numerous factors.

Important Context