Metabolism

What Is Metabolism?

Metabolism refers to the complete set of chemical reactions that occur in living organisms to sustain life. In nutrition, metabolism most often describes how the body converts food into energy and building materials. It encompasses two complementary processes: catabolism — the breakdown of molecules to release energy — and anabolism — the synthesis of complex molecules from simpler ones for growth and repair.

The primary currency of cellular energy is adenosine triphosphate (ATP). Metabolic pathways such as glycolysis, the citric acid cycle, and oxidative phosphorylation convert carbohydrates, fats, and proteins into ATP.

Components of Total Energy Expenditure

Total daily energy expenditure (TDEE) has three main components:

• Basal Metabolic Rate (BMR): Energy used at complete rest to maintain vital functions — breathing, circulation, and cell repair. Accounts for 60–75% of TDEE in most sedentary adults.
• Thermic Effect of Food (TEF): Energy cost of digesting, absorbing, and processing nutrients. Approximately 10% of TDEE. Protein has the highest TEF (20–30%), followed by carbohydrates (5–10%) and fat (0–3%).
• Physical Activity Energy Expenditure (PAEE): Energy from all movement, including formal exercise and non-exercise activity thermogenesis (NEAT). The most variable component, ranging from 15% in sedentary individuals to over 50% in very active people.

Factors That Influence Metabolic Rate

Lean muscle mass is the largest determinant of resting metabolic rate — each kilogram of muscle burns approximately 13 kcal per day at rest, compared to roughly 4.5 kcal per kilogram of fat. Age-related muscle loss (sarcopenia) reduces BMR by approximately 1–2% per decade after age 30. Thyroid hormones are major regulators of basal metabolism; hypothyroidism can reduce BMR by 30–40%. Sleep deprivation, severe calorie restriction, and certain medications also affect metabolic rate.