Metabolism is the sum of all chemical reactions occurring within your body's cells to sustain life. These biochemical processes convert nutrients from food into energy and essential building blocks for growth, repair, and normal function. Whether you are sleeping, exercising, or simply breathing, your body continuously requires energy—and metabolism is the mechanism that provides it. Regulated by enzymes and hormones including insulin, thyroid hormones, and cortisol, metabolism ensures energy production matches your body's demands. Understanding metabolism is fundamental to appreciating how your body maintains balance and responds to diet, activity, illness, and environmental changes.

What Is Metabolism and How Does It Work?

Metabolism refers to the sum of all chemical reactions that occur within the cells of your body to maintain life. These biochemical processes convert the nutrients from food and drink into energy and the building blocks needed for growth, repair, and normal physiological function. Every moment—whether you are sleeping, exercising, or simply breathing—your body requires energy, and metabolism is the mechanism that provides it.

The process begins when you consume food containing macronutrients: carbohydrates, proteins, and fats. Through digestion, these are broken down into simpler molecules—glucose from carbohydrates, amino acids from proteins, and fatty acids from fats. Once absorbed into the bloodstream, these molecules are transported to cells throughout the body. Within cells, these substrates undergo chemical transformations to produce adenosine triphosphate (ATP), the primary energy currency of the cell. This occurs both in the cytosol (through glycolysis) and in specialised structures called mitochondria (through more complex pathways).

Metabolism is tightly regulated by enzymes and hormones, including insulin, thyroid hormones (T3 and T4), glucagon, growth hormone, cortisol, and adrenaline. These regulatory molecules ensure that energy production matches the body's demands at any given time. For instance, during physical activity, metabolic rate increases to supply muscles with additional ATP. Conversely, during rest or sleep, metabolic processes slow down but never cease entirely, as vital functions such as breathing, circulation, and cellular repair continue around the clock.

Understanding metabolism is essential for appreciating how the body maintains homeostasis and responds to dietary intake, physical activity, illness, and environmental changes.

The Two Main Types of Metabolic Processes

Metabolism comprises two complementary categories of biochemical reactions: catabolism and anabolism. These processes work in tandem to balance energy production with the synthesis of essential molecules required for cellular structure and function.

Catabolism involves the breakdown of complex molecules into simpler ones, releasing energy in the process. This is the body's primary method of generating ATP. Key catabolic pathways include:

• Glycolysis: the breakdown of glucose into pyruvate in the cytosol, yielding a small amount of ATP

• The citric acid cycle (Krebs cycle): further oxidation of pyruvate derivatives in mitochondria, producing electron carriers

• Oxidative phosphorylation: the main ATP-generating process occurring in mitochondria, driven by the electron transport chain

• Beta-oxidation: the breakdown of fatty acids in mitochondria to produce acetyl-CoA, which enters the citric acid cycle

Catabolic processes are particularly active during fasting, exercise, or periods of increased energy demand, ensuring a continuous supply of fuel for cellular activities.

Anabolism, in contrast, involves the synthesis of complex molecules from simpler precursors, a process that requires energy input (usually in the form of ATP). Anabolic reactions are responsible for:

• Building new proteins from amino acids for muscle growth and tissue repair

• Synthesising glycogen from glucose for energy storage in the liver and muscles

• Creating lipids for cell membranes and hormone production

• Producing nucleic acids (DNA and RNA) for cell division and genetic expression

Anabolic processes predominate after meals, during growth phases, and during recovery from illness or injury. Hormones such as insulin and growth hormone promote anabolism, whilst glucagon and cortisol favour catabolism. The balance between these two processes determines whether the body is in a state of net energy storage or expenditure, influencing body weight, muscle mass, and overall metabolic health.

Factors That Affect Your Metabolic Rate

Metabolic rate—the speed at which your body converts nutrients into energy—varies considerably between individuals and is influenced by a range of intrinsic and extrinsic factors. Understanding these can help explain why some people appear to burn calories more efficiently than others.

Body composition is one of the most significant determinants. Muscle tissue is metabolically more active than adipose (fat) tissue, meaning individuals with greater lean muscle mass typically have higher resting metabolic rates. This is why resistance training and maintaining muscle mass are important for metabolic health, particularly as we age.

Age also plays a role. While traditionally it was thought that metabolic rate declines steadily with age, recent research suggests that energy expenditure remains relatively stable through much of adulthood, with more pronounced changes occurring later in life. These changes relate partly to loss of muscle mass (sarcopenia) and hormonal shifts. Regular physical activity and strength training can help maintain metabolic health regardless of age.

Sex influences metabolism, with men generally having higher metabolic rates than women, largely due to greater muscle mass and lower body fat percentages. Hormonal differences, particularly in oestrogen and testosterone levels, also contribute to these variations.

Thyroid function is central to metabolic regulation. The thyroid gland produces hormones (T3 and T4) that regulate the speed of metabolic processes. Hypothyroidism (underactive thyroid) slows metabolism, leading to weight gain, fatigue, and cold intolerance, whilst hyperthyroidism (overactive thyroid) accelerates metabolism, causing weight loss, heat intolerance, and palpitations. Thyroid function is typically assessed first with a thyroid-stimulating hormone (TSH) test, with additional tests as needed.

Other factors include:

• Genetics: inherited variations in metabolic enzymes and hormone receptors

• Physical activity: exercise increases energy expenditure both during and after activity

• Diet: the thermic effect of food (energy required for digestion) varies by macronutrient, with protein requiring the most energy to process

• Environmental temperature: cold exposure increases metabolic rate as the body works to maintain core temperature

• Medications: certain drugs can influence metabolic rate and weight, including beta-blockers, systemic corticosteroids, and some antidepressants (never stop prescribed medications without consulting your GP)

• Sleep: poor sleep quality and insufficient sleep duration can disrupt metabolic hormones, particularly leptin and ghrelin, affecting appetite and energy balance

Common Myths About Metabolism Explained

Misconceptions about metabolism abound, often fuelled by marketing claims and oversimplified explanations. Addressing these myths is important for evidence-based health decision-making.

Myth 1: "Eating small, frequent meals boosts metabolism." Whilst the thermic effect of food does increase metabolic rate temporarily after eating, the total daily energy expenditure from digestion depends on the total amount and composition of food consumed, not meal frequency. Research shows no significant metabolic advantage to eating six small meals versus three larger ones, provided total caloric and macronutrient intake remains constant. Meal timing should be based on personal preference, lifestyle, and hunger cues rather than metabolic concerns.

Myth 2: "Certain foods or supplements can dramatically speed up metabolism." Claims that specific foods (such as chilli peppers, green tea, or grapefruit) or supplements significantly increase metabolic rate are largely exaggerated. Whilst some compounds (e.g., caffeine, capsaicin) may produce modest, temporary increases in energy expenditure, the effects are too small to result in meaningful weight loss. Evidence for foods or supplements producing clinically significant, sustained increases in metabolic rate is limited. A balanced diet and regular physical activity remain the cornerstones of metabolic health, as recommended by NICE guidelines.

Myth 3: "A slow metabolism is the main cause of obesity." Whilst metabolic rate does vary between individuals, research indicates that people with obesity often have higher absolute metabolic rates due to the energy required to maintain larger body mass, though energy expenditure per kilogram may differ. Weight gain typically results from an imbalance between energy intake and expenditure over time, influenced by dietary habits, physical activity levels (including non-exercise activity thermogenesis or NEAT), genetic factors, and environmental influences. Blaming a "slow metabolism" oversimplifies a complex, multifactorial condition.

Myth 4: "You can't change your metabolic rate." Although genetics and age influence baseline metabolic rate, lifestyle modifications can have meaningful effects. Building muscle through resistance training, increasing physical activity, ensuring adequate sleep, and managing stress can all positively influence metabolic health. Addressing underlying medical conditions, such as thyroid disorders, is also crucial.

When to Seek Medical Advice About Metabolism

Whilst variations in metabolic rate are normal, certain signs and symptoms may indicate an underlying metabolic or endocrine disorder requiring medical evaluation. Recognising these red flags is important for timely diagnosis and management.

Unexplained weight changes warrant investigation. Significant, unintentional weight loss or gain—particularly when accompanied by normal or increased appetite—may suggest thyroid dysfunction, diabetes, or other metabolic conditions. If you experience a weight change of more than 5% of your body weight over 6–12 months without clear cause, contact your GP for assessment.

Persistent fatigue that does not improve with rest, alongside other symptoms such as cold intolerance, dry skin, constipation, or depression, may indicate hypothyroidism. Conversely, symptoms including unexplained weight loss, heat intolerance, tremor, palpitations, anxiety, or diarrhoea could suggest hyperthyroidism. Both conditions require blood tests, starting with thyroid-stimulating hormone (TSH) measurement, with additional tests for thyroid hormones (FT4, FT3) as indicated, following NICE guideline NG145.

Signs of diabetes should prompt urgent assessment. These include:

• Excessive thirst and frequent urination

• Unexplained weight loss despite normal or increased appetite

• Persistent fatigue and blurred vision

• Slow-healing wounds or recurrent infections

If you have these symptoms, seek same-day GP assessment. For suspected type 2 diabetes, HbA1c testing (≥48 mmol/mol is diagnostic) may be appropriate. However, for suspected type 1 diabetes, during pregnancy, in children/young people, or with rapid symptom onset, blood glucose testing is required instead of HbA1c. Seek immediate medical help (call 999) if you develop symptoms of diabetic ketoacidosis: abdominal pain, vomiting, deep breathing, drowsiness, dehydration, or fruity-smelling breath.

Other concerning features include:

• Persistent muscle weakness or wasting

• Changes in heart rate or rhythm (seek urgent care for severe palpitations or chest pain)

• Excessive sweating or heat/cold intolerance

• Changes in menstrual patterns (women)

• Difficulty concentrating or memory problems

• Swelling in the neck (potential goitre) – seek immediate medical help if you develop difficulty breathing or swallowing

If you have a family history of metabolic or endocrine disorders, or if you are taking medications that may affect metabolism (such as corticosteroids or lithium), discuss monitoring options with your GP. Regular health checks, including blood pressure, lipid profile, and glucose measurements, are important for detecting metabolic syndrome—a cluster of conditions increasing cardiovascular risk.

Patient safety advice: Do not attempt to self-diagnose or self-treat suspected metabolic disorders with over-the-counter supplements or restrictive diets. Never stop prescribed medications without medical advice. Always seek professional medical advice for proper investigation, diagnosis, and evidence-based management tailored to your individual circumstances.

Frequently Asked Questions