How your body converts food into energy and maintains basic functions.
Metabolism refers to all chemical processes your body performs to maintain life. It includes converting food to energy, synthesizing proteins, removing waste, and regulating body temperature. Metabolism operates continuously, even during sleep.
Your metabolic rate—the calories your body burns—is influenced by multiple factors. Understanding these factors helps explain why energy needs differ between individuals.
BMR is the energy your body uses for essential functions: breathing, circulation, cell production, and nutrient processing. It represents 60-75% of daily energy expenditure for sedentary individuals.
BMR is determined by body composition, age, sex, genetics, and hormonal status. Muscle tissue burns more calories at rest than fat tissue, which is why regular activity helps maintain metabolic health.
This is the energy required to digest, absorb, and process nutrients. It accounts for 8-15% of daily energy expenditure. Different macronutrients require different amounts of energy to process: protein requires more energy than carbohydrates or fats.
Exercise and daily movement account for 15-30% of daily energy expenditure. This includes intentional exercise and non-exercise physical activity (occupational movement, fidgeting, maintaining posture).
Your body adapts to environmental temperatures and caloric intake by adjusting heat production. This is a minor component of total expenditure in controlled environments.
Muscle tissue is metabolically active and burns more calories at rest than fat tissue. Individuals with higher muscle mass have higher basal metabolic rates.
Metabolic rate generally decreases with age due to loss of muscle mass and changes in hormonal status. This is partly why caloric needs may decrease in older adults.
On average, males have higher metabolic rates than females due to greater muscle mass and hormonal differences. This is a population average; individual variation is significant.
Genetic factors influence body composition, muscle mass, hormonal status, and how efficiently your body produces and uses energy. These contribute to natural variation in metabolism.
Thyroid hormones, insulin, cortisol, and others regulate metabolic processes. Hormonal imbalances can affect energy expenditure and nutrient utilization.
Severe caloric restriction can reduce metabolic rate as the body conserves energy. Adequate nutrition, including protein, helps maintain metabolic function.
Carbohydrates are broken into glucose, which enters the bloodstream. Excess glucose is stored as glycogen in muscles and liver, or converted to fat for long-term storage. Blood glucose regulation involves insulin and other hormones.
Proteins are broken into amino acids, which are used for tissue repair, enzyme production, and hormone synthesis. Excess amino acids can be converted to glucose or fat for energy storage. The body also requires nitrogen from amino acids.
Dietary fats are broken into fatty acids and glycerol, which are used for hormone production, cell structure, and energy. Excess energy from any macronutrient can be stored as body fat.
Your body adjusts metabolic processes in response to energy availability. During periods of adequate nutrition, metabolic processes operate efficiently. During prolonged caloric restriction, the body reduces metabolic rate to conserve energy—a protective adaptation.
This is why extreme restriction can make weight loss more difficult: as energy intake decreases, energy expenditure also decreases. Moderate approaches that preserve muscle mass through resistance training and adequate protein intake help maintain metabolic function.