The Foundation
Your metabolism is not fixed — and it responds to what you do
Metabolism is the collective term for all the chemical reactions in your body that convert food into energy and use that energy to sustain life. It is not simply "how fast you burn calories" — it is the sum of thousands of interconnected processes governing energy production, storage, and utilisation at every moment of every day.
The widespread belief that metabolism is largely predetermined — that some people are stuck with a "slow metabolism" they cannot change — is mostly wrong. While genetics play a role, the primary drivers of metabolic rate are body composition, physical activity, and hormonal environment — all of which are significantly modifiable through lifestyle. Understanding metabolism moves you from fatalism to agency.
Metabolic Adaptation
The body's metabolism adapts to its environment. Sustained caloric restriction causes metabolic adaptation — the body reduces its energy expenditure to match reduced intake. This is why crash diets produce diminishing returns over time and why long-term body composition management requires a different strategy than short-term weight loss.
60–75%
Of total daily energy expenditure is resting metabolic rate — not exercise
~13 kcal/kg
Daily energy expenditure per kilogram of muscle mass (metabolically active tissue)
10%
Of total energy expenditure is the thermic effect of food (digestion)
The Components
Where your energy actually goes
| Component | % of Total | What it is | Modifiability |
|---|
| Basal Metabolic Rate (BMR) | 60–75% | Energy to maintain life at rest — organ function, cell maintenance, temperature | Moderate — driven by muscle mass, organ size, hormones |
| Non-Exercise Activity (NEAT) | 15–30% | All movement that isn't deliberate exercise — walking, fidgeting, standing | High — most variable component |
| Exercise Activity (EAT) | 5–15% | Deliberate training and sport | High — but smaller fraction than most people assume |
| Thermic Effect of Food (TEF) | ~10% | Energy cost of digesting food — protein highest (25–30%), fat lowest (0–3%) | Low — affected by diet composition |
Energy Systems
How your body produces energy during exercise
1
ATP-PCr system (0–10 seconds)
Explosive efforts — a sprint, a maximum lift — rely on pre-stored ATP (adenosine triphosphate) and phosphocreatine. No oxygen required. Produces maximum power but depletes in seconds. This is why maximum efforts can only be sustained briefly.
2
Glycolytic system (10 seconds–2 minutes)
Carbohydrate (glucose/glycogen) is broken down to produce ATP. Can operate without oxygen (anaerobic glycolysis). Produces lactic acid (the burn). Powers high-intensity intervals, 400m runs, hard sets in the gym.
3
Oxidative system (2+ minutes)
The primary energy system for sustained activity. Uses oxygen to metabolise carbohydrates AND fats. Lower power output than anaerobic systems but virtually unlimited duration. Walking, jogging, cycling, daily activity — all primarily oxidative.
Fat Burning Myth
The "fat burning zone" — low-intensity exercise where fat is the primary fuel — is real but overemphasised. While low-intensity exercise burns a higher proportion of fat, high-intensity exercise burns more total calories and creates greater post-exercise oxygen consumption (EPOC). For body composition change, total energy deficit matters more than fuel source during the session.
Insulin and Blood Sugar
The central metabolic hormone
Insulin is released by the pancreas in response to rising blood glucose. It signals cells to take up glucose — for immediate energy, or storage as glycogen (liver, muscle) or fat. Insulin sensitivity — how well cells respond to insulin's signal — is the central marker of metabolic health.
High insulin sensitivity means low insulin is needed to handle blood glucose. Insulin resistance — reduced cellular response requiring more insulin — is the foundation of type 2 diabetes, metabolic syndrome, and a major driver of visceral fat accumulation. Exercise, particularly resistance training, dramatically improves insulin sensitivity by increasing the number and activity of glucose transporters in muscle cells.
Optimising Your Metabolism
Practical levers you can pull today
1
Build muscle — the metabolic engine
Muscle tissue is the primary driver of resting metabolic rate variation between individuals. Each kilogram of muscle burns approximately 13 kcal/day at rest — and far more during activity. Resistance training 3× per week, progressively, is the most powerful intervention for long-term metabolic health.
2
Maximise NEAT — the underrated multiplier
NEAT is the most variable and underestimated component of energy expenditure. Stand instead of sit. Take stairs. Walk to meetings. 10,000 steps vs 3,000 steps per day represents 200–400 kcal difference — equivalent to a 30-minute run, but accumulated without dedicated exercise time.
3
Prioritise protein — highest thermic effect
Protein has a thermic effect of 25–30% — meaning roughly a quarter of the calories in protein are used in the process of digesting it. Higher protein intakes also preserve muscle during caloric restriction, preventing the metabolic adaptation that derails most diets.
4
Avoid drastic caloric restriction
Cutting calories aggressively reduces metabolic rate through two mechanisms: direct metabolic adaptation (the body becomes more efficient) and muscle loss (less metabolically active tissue). Modest deficit (250–500 kcal/day) with high protein and resistance training preserves metabolic rate while losing fat.
5
Sleep and manage stress cortisol
Sleep deprivation reduces insulin sensitivity, increases hunger hormones (ghrelin), and promotes fat storage. Chronic stress elevates cortisol, which drives glucose release and fat accumulation — particularly visceral fat. These are not minor factors — they are central metabolic regulators.
Metabolism follows the signals you give it
Train consistently, eat enough protein, sleep well, stay active throughout the day. The metabolism responds to lifestyle — not willpower or restrictive interventions.
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