Your metabolic system is how your body turns food into energy and decides what to do with the rest. Every time you eat, your body has to burn this food for fuel now, store it as glycogen (carbs) for short-term use, store it as fat for long-term reserves, or use it as building material for muscle, hormones, and tissue. Those decisions are made on a meal-by-meal basis by a handful of hormones and organs working in coordination, the pancreas, liver, fat tissue, muscle, gut, and brain all talking to each other constantly.

Energy production

The body runs on a single energy currency called ATP (adenosine triphosphate). Every muscle contraction, every thought, every cell division, every protein built, every signal sent, all of it is paid for in ATP.

The body has three fuels available:

• Glucose is the fastest. It can be burned with or without oxygen, keeping muscles at force during a sprint even when oxygen supply can't keep up. Glucose comes from the carbohydrates you eat, from glycogen broken down in the liver and muscles, or from gluconeogenesis (the liver manufacturing glucose from amino acids, glycerol, and lactate).
• Fat is the slowest but the densest. One gram of fat yields more than twice the ATP of one gram of glucose, but fat can only be burned with oxygen, making fat the dominant fuel during low-intensity activity and during long fasts. Fat comes from dietary fat or from triglycerides stored in adipose tissue.
• Protein is rarely a primary fuel. It's expensive to convert and the amino acids are usually doing more important jobs as building blocks for muscle, enzymes, and hormones. The body falls back on protein for fuel only when carbohydrates and fat are inadequate. When this happens, the liver converts amino acids to glucose, and the source of those amino acids is often muscle tissue.

At any given moment, the body is burning a mix of fuels, and the ratio depends on:

• Activity intensity. Low intensity (walking, resting, sleeping) favours fat oxidation. High intensity (sprinting, heavy lifting) demands glucose because it's the only fuel fast enough to keep up.
• Recent food intake. A high-carb meal pushes the body toward burning glucose for hours afterwards while insulin is elevated. A fasted state shifts toward fat oxidation
• Glycogen status. Full muscle and liver glycogen biases the body toward burning glucose. Depleted glycogen biases it toward burning fat and sparing remaining glucose for the brain.

The hormone running this whole decision tree most of the time is insulin. When insulin is high, the body is in storage and glucose-burning mode. When insulin is low, the body is in fat-burning mode.

Calories

Your body burns calories in four ways.

Basal metabolic rate (BMR) is what your body burns just keeping you alive. Heart pumping, lungs breathing, brain running, organs working, body temperature regulated, cells dividing and repairing. BMR accounts for roughly 60-75% of daily calorie burn for most people and by far the biggest line item. BMR is determined by body composition with muscle is more metabolically active.

Non-exercise activity thermogenesis (NEAT) is every calorie you burn moving outside of formal exercise. Walking, fidgeting, standing, taking the stairs, doing dishes, pacing on the phone. NEAT accounts for roughly 15-30% of daily burn (highly dependent on what you do for work etc). NEAT is also the variable that drops the hardest when you diet.