Metabolism is a fundamental biological process that sustains life, encompassing all chemical reactions that convert food into energy and building materials for cells.

While metabolism is often discussed in the context of weight management and energy expenditure, many misconceptions surround its nature and function.

Metabolism Encompasses Both Anabolism and Catabolism

Metabolism consists of two complementary processes: anabolism, where small molecules are synthesized into complex ones for growth and repair, and catabolism, where complex molecules are broken down to release energy. This dynamic balance maintains biological function and cellular homeostasis. Understanding metabolism requires recognizing that it is not solely about burning calories but managing the intricate biochemical pathways that sustain life.

Glucose, Lipid Acids, and Amino Acids Are the Primary Energy Sources

Energy for cellular processes primarily derives from the metabolism of carbohydrates (glucose), lipid, and proteins (amino acids). Glucose metabolism contributes approximately 50-70% of the body's energy supply under normal conditions, with lipid acid metabolism providing an essential complementary source. Amino acid metabolism also yields energy, especially during prolonged fasting or intense exercise, illustrating metabolism's flexibility in fuel utilization.

Basal Metabolic Rate (BMR) Determines Largest Energy Expenditure Component

Basal metabolic rate represents the energy required to maintain vital physiological functions at rest, accounting for 60-70% of daily energy expenditure. Variations in BMR are influenced by factors such as age, gender, muscle mass, genetics, and hormonal status. Higher muscle mass increases BMR due to greater cellular activity. Importantly, BMR differs from total metabolic rate, which also includes energy expenditure related to physical activity and digestion.

Metabolic Rate Is Modifiable but Limited by Genetics

While genetics set a baseline metabolic rate, lifestyle factors such as physical activity, diet, and environmental influences can modify metabolism to some extent. Exercise increases muscle mass and mitochondrial efficiency, raising energy consumption even at rest.

Certain substances, including caffeine and nicotine, transiently elevate metabolic rate through stimulation of the nervous system. However, claims of drastically boosting metabolism through supplements or fad diets lack strong scientific support.

Metabolism Adapts to Energy Availability and Demand

The body’s metabolism is dynamic, adapting to fluctuations in calorie intake and energy requirements. During calorie restriction or fasting, metabolism slows to conserve energy, while overfeeding induces metabolic acceleration. This adaptive response maintains energy balance but can complicate sustained weight loss efforts. Metabolic adaptation involves hormonal regulation, mitochondrial efficiency shifts, and changes in energy substrate preference.

Mitochondrial Function Is Central to Metabolic Health

Mitochondria, the cell's energy powerhouses, play a critical role in metabolism by converting nutrients into usable energy in the form of ATP. Dysfunctional mitochondria contribute to impaired metabolism, linked to conditions such as obesity, diabetes, and neurodegenerative diseases. Emerging research highlights mitochondrial plasticity—the ability to adapt energy production pathways depending on oxygen availability and cellular conditions—as a key factor in health and disease.

Metabolism Influences and Is Influenced by Various Diseases

Disruptions in metabolic pathways are implicated in numerous pathologies beyond obesity, including cancer, autoimmune disorders, and cardiovascular disease. For example, cancer cells often exhibit altered glucose metabolism, known as the Warburg effect, favoring glycolysis even in oxygen-rich conditions to support rapid proliferation. Metabolic reprogramming is an area of active research, offering insights into disease mechanisms and therapeutic targets.

A 2024 review titled Putting metabolism centre stage in health and disease argues that metabolic impairments sit at the root of many major chronic diseases — including metabolic disorders, cardiovascular disease, liver disease, some cancers, and more.

Metabolism is a complex and multifaceted biological system involving the synthesis and breakdown of molecules to sustain life. Its key energy sources are glucose, lipid, and amino acids, with basal metabolic rate accounting for most daily energy use. Although genetics influence metabolism, lifestyle factors play important roles in modulating it.

Metabolic adaptability and mitochondrial health are crucial determinants of physiological resilience and disease susceptibility. Recognizing metabolism's breadth and impact helps dispel myths and highlights its vital role in health and medicine.