Metabolism and Energy Systems

What is Metabolism?

Metabolism refers to all the chemical processes your body uses to maintain life. These processes include breaking down nutrients, producing energy, building and repairing tissues, eliminating waste, and supporting all physiological functions. Your metabolic rate is the amount of energy (measured in calories) your body uses at rest and during activity.

Components of Daily Energy Expenditure

Your total daily energy expenditure consists of several components:

Basal Metabolic Rate (BMR)

Your basal metabolic rate is the amount of energy your body uses at complete rest to maintain basic physiological functions such as breathing, circulation, maintaining body temperature, and cell production. BMR accounts for approximately 60-75% of total daily energy expenditure in sedentary individuals.

Factors affecting BMR include:

• Body composition (muscle tissue is metabolically active; fat tissue is less so)
• Age (BMR decreases with age, approximately 2-5% per decade after age 25)
• Sex (males typically have higher BMR than females due to greater muscle mass)
• Genetics (natural variation in metabolic rate)
• Hormonal status (thyroid hormones, cortisol, and others affect metabolic rate)

Thermic Effect of Food (TEF)

The thermic effect of food, also called diet-induced thermogenesis, is the energy required to digest, absorb, and process nutrients from food. This accounts for approximately 10% of total daily energy expenditure.

Different macronutrients have different thermic effects:

• Protein: Highest thermic effect (20-30% of calories consumed)
• Carbohydrates: Moderate thermic effect (5-10% of calories consumed)
• Fats: Lowest thermic effect (0-3% of calories consumed)

Activity Energy Expenditure (AEE)

Energy expended during planned exercise and daily activities accounts for approximately 15-30% of total daily energy expenditure in most people. This is highly variable based on activity level and type of activity performed.

Non-Exercise Activity Thermogenesis (NEAT)

NEAT refers to energy expended during all activities that are not sleeping, eating, or structured exercise. This includes occupational activities, leisure activities, fidgeting, and maintaining posture. NEAT can account for 15-30% of total daily energy expenditure and varies considerably between individuals and occupations.

Energy Production: The ATP Cycle

Your cells use a molecule called adenosine triphosphate (ATP) to store and transfer energy. When ATP is broken down, it releases energy that powers cellular processes. Three main metabolic pathways regenerate ATP from nutrients:

Aerobic Metabolism (Oxidative Phosphorylation)

Under normal conditions with adequate oxygen, aerobic metabolism is the primary pathway for ATP production. Carbohydrates, fats, and proteins can all be oxidized to produce ATP. Aerobic metabolism is efficient and sustainable for prolonged activity.

Anaerobic Metabolism (Glycolysis)

During high-intensity exercise or when oxygen availability is limited, your body relies on anaerobic metabolism, primarily through glycolysis of glucose. This produces ATP quickly but less efficiently than aerobic metabolism and produces lactate as a byproduct.

Phosphocreatine System

For very high-intensity, short-duration activities (up to approximately 10 seconds), the phosphocreatine system provides immediate ATP regeneration. This system is rapidly depleted and requires several minutes of recovery to replenish.

Carbohydrate Metabolism

Carbohydrates are broken down into glucose, which enters the bloodstream and is used by cells for energy. Excess glucose is stored as glycogen in muscles and the liver. When glycogen stores are depleted, your body can synthesize glucose from non-carbohydrate sources through gluconeogenesis.

Fat Metabolism

Dietary fat is broken down into fatty acids and glycerol. Fatty acids are transported to cells where they are oxidized to produce ATP through beta-oxidation. Fat stores can be mobilized when energy is needed, and fat is a relatively stable, long-term energy source.

Protein Metabolism

Amino acids from dietary protein are used primarily for building and repairing tissues. However, when necessary, amino acids can be converted to glucose through gluconeogenesis or can be oxidized to produce energy. Using protein for energy is metabolically less efficient than using carbohydrates or fats.

Metabolic Adaptation

Your metabolism is not static. It adapts to your dietary intake and activity level through several mechanisms:

• Adaptive Thermogenesis: Metabolic rate adjusts in response to caloric intake. Significant caloric restriction can reduce metabolic rate; increased activity increases metabolic demands.
• Enzyme Activity: Your body upregulates or downregulates metabolic enzymes based on nutrient availability and demand.
• Hormonal Regulation: Thyroid hormones, cortisol, growth hormone, and insulin all influence metabolic rate.
• Muscle Mass Changes: Muscle tissue is metabolically active, so changes in muscle mass directly affect metabolic rate.

Individual Metabolic Variation

While the basic mechanisms of metabolism are universal, significant variation exists between individuals in metabolic rate and efficiency. Genetic factors, age, sex, body composition, and activity history all contribute to individual metabolic differences. This variation is normal and helps explain why similar dietary patterns may produce different results in different individuals.