How Metabolism Actually Works (Not What You Think)

What Is Metabolism?

Metabolism refers to all chemical reactions in your body that sustain life — converting food into energy and using that energy for cellular functions. In the fitness context, it refers to your Total Daily Energy Expenditure (TDEE): how many calories your body burns each day.

The Four Components of Metabolism

1. BMR (Basal Metabolic Rate) — 60–70% of total: Calories burned at complete rest. Driven primarily by lean muscle mass, organ size, age, and genetics. This is largely fixed in the short term.

2. TEF (Thermic Effect of Food) — ~10%: Calories burned digesting food. Protein has the highest TEF at 20–30% of its calories. Carbs: 5–10%. Fat: 0–3%. This is why high-protein diets modestly increase metabolic rate.

3. EAT (Exercise Activity Thermogenesis) — 5–30%: Calories burned through intentional exercise — gym sessions, sport, running.

4. NEAT (Non-Exercise Activity Thermogenesis) — 15–50%: Every calorie burned through non-exercise movement: walking to your car, fidgeting, standing, cleaning, taking the stairs. Research by Levine et al. found NEAT can vary by 2,000 kcal/day between people of similar size.

Why Some People "Eat Anything and Don't Gain Weight"

The answer is almost entirely NEAT. People with naturally high NEAT unconsciously move more throughout the day — they fidget, pace, stand rather than sit, and take more incidental steps. Studies show lean individuals sit on average 2.5 hours less per day than obese individuals, accounting for a 350 kcal/day difference purely through NEAT.

What Actually Affects Your Metabolic Rate

Muscle mass: Each kilogram of lean muscle burns approximately 13 kcal/day at rest. Building muscle permanently raises BMR — the most sustainable way to increase resting metabolic rate.

Age: BMR declines approximately 1–2% per decade after age 30, primarily due to age-related muscle loss (sarcopenia). Resistance training counteracts this.

Diet history: Prolonged severe caloric restriction causes metabolic adaptation — the body reduces metabolic rate by 15–25% to conserve energy. This is why crash diets fail long-term.

Thyroid function: The thyroid regulates metabolic rate. Hypothyroidism can reduce BMR by 40%. If you suspect thyroid issues, consult a doctor — no amount of diet optimisation substitutes for medical evaluation.