Overview

You are a fire. You take in fuel (food) and oxygen, and you burn it to create heat and energy. Metabolism is that fire. It is the sum of every chemical reaction happening in your body right now. It is how you stay alive against the laws of thermodynamics.

Core Idea

The core idea is Energy Flow.

  1. Catabolism: Breaking things down (Digestion). Releases energy.
  2. Anabolism: Building things up (Muscle growth). Consumes energy. Metabolism is the balance between these two.

Formal Definition

The set of life-sustaining chemical reactions in organisms.

  • Basal Metabolic Rate (BMR): The energy you burn just by existing (breathing, beating heart) while doing nothing.

Intuition

  • Catabolism: Demolition crew. Smashing a brick wall to get bricks.
  • Anabolism: Construction crew. Using those bricks to build a new house.
  • ATP: The currency. The demolition crew gets paid in ATP, and the construction crew charges ATP to work.

Examples

  • Hummingbirds: They have the fastest metabolism of any vertebrate. They burn energy so fast that if they stop eating for a few hours, they starve to death. They have to go into hibernation (torpor) every night just to survive until morning.
  • Sloths: The opposite. They move so slowly because they have a tiny “budget” of energy. Digesting a single leaf takes them a month.
  • Exercise: When you run, you switch from aerobic metabolism (using oxygen) to anaerobic (without oxygen). This creates lactic acid, which makes your muscles burn.

Common Misconceptions

  • “I have a slow metabolism”: Most people’s BMR is surprisingly similar (based on size/muscle). Weight gain is usually about intake, not a broken engine.
  • Detox: Your body has a built-in detox system (Liver/Kidneys). You don’t need juice cleanses to “boost” metabolism.
  • Enzymes: The workers that make the reactions happen. Without them, digestion would take years.
  • Mitochondria: The power plants where the burning happens (Krebs Cycle).

Applications

  • Pharmacology: Drugs are metabolized by the liver. If you are a “fast metabolizer,” a drug might not work because you break it down too fast. If you are “slow,” you might overdose.

Criticism / Limitations

  • Complexity: The metabolic map looks like a subway map from hell. Thousands of pathways intersect. It is hard to tweak one without messing up another.

Further Reading

  • Lane, Nick. Power, Sex, Suicide: Mitochondria and the Meaning of Life. 2005.
  • Pontzer, Herman. Burn. 2021.