Metabolism and Energy Balance

Metabolism = all the body's chemical reactions. Recall that metabolism is the sum of all chemical reactions in the body, and it has two opposite directions:

• Catabolism: breaking down larger molecules into smaller ones, releasing energy. Example: breaking glucose down in cellular respiration to make ATP.
• Anabolism: building larger molecules from smaller ones, using energy. Example: building proteins from amino acids, or storing glucose as glycogen.

The body constantly balances these — breaking down nutrients for energy while building and maintaining tissues.

Nutrients to energy. After digestion delivers monosaccharides, amino acids, and fats to cells, the body uses them for fuel and building blocks:

• Carbohydrates (glucose) are the body's preferred, quickest fuel, broken down through cellular respiration to make ATP (recall the A&P I overview: glucose + oxygen → CO₂ + water + ATP).
• Fats are the body's most concentrated energy store, used heavily during rest and prolonged activity.
• Proteins are mainly for building and repair, used for energy only when needed (e.g., prolonged fasting).

When energy intake exceeds needs, excess is stored (as glycogen and especially fat); when intake is insufficient, stores are broken down for energy.

Metabolic rate. The metabolic rate is how fast the body uses energy. The basal metabolic rate (BMR) is the energy used at complete rest to keep basic functions going (breathing, heartbeat, temperature). BMR is influenced by factors like body size and composition, age, sex, and especially thyroid hormone (recall it raises metabolic rate). Physical activity adds to total energy expenditure above the BMR.

Energy balance. Energy balance compares energy taken in (food) with energy used (BMR + activity):

• Balanced: intake equals expenditure → stable body weight.
• Positive balance: intake exceeds expenditure → energy stored → weight gain.
• Negative balance: expenditure exceeds intake → stored energy used → weight loss.

This simple framework underlies body weight and its regulation, which also involves hormones like leptin and insulin (recall the endocrine unit). The detailed biochemical pathways (glycolysis, citric acid cycle, and so on) are developed in the biochemistry subject; here the key idea is that metabolism balances building and breaking down, powered by nutrients, and that energy in vs energy out determines whether the body stores or uses energy.

How it works

Metabolism at a glance:

Catabolism (break down) → releases energy (e.g., glucose → ATP)
Anabolism (build up) → uses energy (e.g., amino acids → protein)
Energy balance: intake vs (BMR + activity)
   intake > use → store (weight gain)
   intake < use → use stores (weight loss)
Thyroid hormone raises metabolic rate

Comparisons

Common confusions

• Catabolism vs anabolism. Catabolism breaks down (releases energy); anabolism builds up (uses energy).
• Carbs are the preferred fuel; fat is the main storage. Protein is mainly structural.
• BMR is resting energy use, not total (activity adds more).
• Weight change is about energy balance (in vs out), influenced by hormones and metabolic rate.

Memory aids

• "Catabolism = Cut apart (releases energy); Anabolism = Assemble (uses energy)."
• "Carbs first, fat for storage, protein for building."
• Energy balance: "In vs Out."

Quick review

• Metabolism = all body chemical reactions: catabolism (break down, release energy → ATP) and anabolism (build up, use energy).
• Carbohydrates are the preferred quick fuel; fat is the main energy store; protein is mainly for building/repair.
• Metabolic rate (including BMR) is influenced by body composition, age, and thyroid hormone.
• Energy balance (intake vs BMR + activity) determines weight: excess is stored (gain); deficit uses stores (loss). Detailed pathways are in biochemistry.