Protein: Sources and Physiological Functions

Understanding Protein Structure

Proteins are complex molecules composed of amino acids—organic compounds linked in specific sequences. The human body contains approximately 20 amino acids, nine of which are classified as essential because the body cannot synthesise them and must obtain them from food sources. The remaining eleven amino acids are non-essential, as the body can produce them internally from other compounds.

Essential amino acids include leucine, isoleucine, valine, methionine, phenylalanine, threonine, tryptophan, histidine, and lysine. The pattern and balance of these amino acids varies across protein sources, influencing their nutritional quality and utilisation by the body.

Animal-Based Protein Sources

Meat and poultry: Beef, pork, chicken, and turkey provide complete proteins containing all nine essential amino acids. They also provide B vitamins (particularly B12), iron, and zinc. Fat content varies based on cut and preparation method.

Fish and seafood: Salmon, cod, tuna, and shellfish offer complete proteins alongside omega-3 fatty acids (in fatty fish), iodine, and selenium. Different varieties provide varying fat and micronutrient profiles.

Eggs: Complete protein with an exceptional amino acid profile. Eggs also contain choline, lutein, and other bioactive compounds. The yolk contains nutrients including vitamin D, choline, and antioxidants.

Dairy products: Milk, yogurt, and cheese provide complete proteins and are primary sources of calcium and phosphorus. Fermented varieties (yogurt, cheese, kefir) contain beneficial bacteria. Lactose content varies across dairy products.

Plant-Based Protein Sources

Legumes: Beans, lentils, and peas provide substantial protein along with fibre and complex carbohydrates. Most legumes are incomplete proteins—lacking one or more essential amino acids—but combining different legumes or pairing legumes with grains creates complete protein profiles. Preparation method (soaking, cooking) influences nutrient bioavailability.

Whole grains: Quinoa, oats, barley, and other whole grains contribute protein alongside fibre and micronutrients. Quinoa is a complete plant protein, while most other grains require complementary foods.

Nuts and seeds: Almonds, peanuts, sunflower seeds, and flax seeds offer protein alongside healthy fats, fibre, vitamin E, and minerals. Nutrient density is high, but portion size affects caloric intake.

Soy products: Tofu, tempeh, and edamame are complete proteins containing all essential amino acids. Soy also provides iron, calcium (when processed with calcium sulphate), and isoflavones.

Protein Digestion and Absorption

Protein digestion begins in the stomach, where hydrochloric acid and pepsin break protein bonds. This process continues in the small intestine with pancreatic enzymes. The resulting amino acids are absorbed in the small intestine and transported to the liver, where they are distributed for utilisation throughout the body.

Digestion rate varies among protein sources. Animal proteins are typically digested more rapidly than plant proteins due to differences in structure and fibre content. Cooking method and food combination also influence digestive rate—for example, pairing protein with carbohydrates and fats slows digestion compared to consuming protein in isolation.

Digestibility is higher for animal proteins and processed plant proteins compared to whole plant foods, where fibre may limit amino acid absorption. Individual digestive capacity varies based on age, gut health, and enzyme production.

Protein Requirements and Individual Variation

Protein needs vary substantially based on age, activity level, muscle mass, and metabolic status. General dietary guidelines suggest approximately 0.8 grams per kilogram of body weight daily for sedentary adults. However, requirements increase for athletes (1.2–2.2 g/kg), older adults (1.0–1.2 g/kg), and individuals recovering from injury or illness.

Factors influencing protein needs:

Resistance training and muscle-building activity increase protein synthesis demands. Endurance training also increases protein turnover. Older adults experience reduced muscle synthesis efficiency, requiring higher protein intake to maintain muscle mass. Recovery from surgery or illness increases amino acid demand. Certain health conditions alter protein metabolism.

Individual responses to protein intake vary. Some people feel more satiated with higher protein meals; others experience different satiety patterns. Finding personalised protein intake that supports energy, muscle health, and satiety is more useful than applying universal recommendations.

Protein-Energy Interactions

Protein has the highest thermic effect of all macronutrients—approximately 20–30% of the energy from protein is expended during digestion, compared to 5–10% for carbohydrates and 0–3% for fats. This means a higher proportion of protein calories are used in the digestion process itself.

Protein also influences satiety signalling through multiple mechanisms: increased cholecystokinin (CCK) secretion, delayed gastric emptying, and effects on appetite-regulating hormones. This satiety effect contributes to the observation that higher-protein diets are often associated with stable energy intake in population studies.

Combining adequate protein with fibre, whole grains, and vegetables is associated with sustained energy and stable hunger and fullness patterns in many individuals.