Note · Fundamental concepts
What is metabolism? How the body turns food into energy
Metabolism is every chemical reaction in the body: breaking food down for energy and building the molecules the body needs. At rest, an adult burns roughly as much as a 60 to 90 watt bulb.
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Contents · 6 sections
Metabolism is the sum of all the chemical reactions that keep a living thing alive. It has two halves: breaking food down to release energy and raw materials, called catabolism, and using that energy to build what the body needs, such as proteins, fat stores and new cells, called anabolism. In everyday speech, "metabolism" usually means how much energy the body burns, which for a resting adult is roughly the power of a 60 to 90 watt light bulb.
In short
- Metabolism is chemistry, not a single organ or process: thousands of reactions, each run by an enzyme, linked into pathways.
- Breaking down releases energy, which cells capture in a molecule called ATP; building up spends it.
- Most differences in how much energy adults burn come from body size and composition, not from a "fast" or "slow" metabolism.
What does metabolism actually do?
Every cell is busy all the time. It pumps salts across its membrane, rebuilds worn-out proteins, copies its DNA before dividing and keeps its temperature and chemistry steady. All of that is metabolism. The U.S. National Library of Medicine's MedlinePlus describes it as the process the body uses to get or make energy from food: digestion breaks proteins, carbohydrates and fats into sugars and acids, the body's fuel, which it uses at once or stores in the liver, muscles and body fat.
The reactions are organized into pathways, chains in which one enzyme's product becomes the next one's starting material, as Wikipedia's article on metabolism sets out. The two directions work together.
| Catabolism (breaking down) | Anabolism (building up) | |
|---|---|---|
| What happens | Large molecules split into smaller ones | Small molecules are joined into larger ones |
| Energy | Usually releases it | Usually uses it |
| Examples | Glucose broken down to carbon dioxide and water; fat burned between meals | Amino acids joined into muscle protein; glucose stored as glycogen; bone built |
| When it dominates | Fasting, exercise, illness | After meals, during growth, healing and training |
Metabolism is one of the traits on the checklist biologists use to tell living from non-living: a living thing takes in matter and energy and transforms it, and stops when the chemistry stops.
How does food become energy a cell can use?
The energy in food is held in chemical bonds. Cells can't use it directly; they move it into a small carrier molecule, ATP, which works like a rechargeable battery that powers almost everything a cell does. The main route is cellular respiration, which for glucose adds up to one equation:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy
One molecule of glucose and six of oxygen become six of carbon dioxide and six of water. It happens in three stages.
- Glycolysis, in the cell's fluid, splits glucose into two smaller molecules and makes a little ATP. It needs no oxygen.
- The citric acid cycle, inside the mitochondria, strips the remaining carbon down to carbon dioxide, which is the carbon dioxide you breathe out, and loads high-energy electrons onto carrier molecules.
- The electron transport chain, in the mitochondrion's inner membrane, passes those electrons to oxygen. The flow pumps hydrogen ions across the membrane, and they flow back through a protein turbine, ATP synthase, that makes most of the ATP.
Textbooks give a yield of roughly 30 ATP for each glucose fully burned. Fat and protein feed into the same stages after being broken into smaller pieces. Without oxygen, cells fall back on glycolysis alone, which is far less efficient and, in muscle, makes lactate.
The energy is the same energy measured on food labels. A food Calorie is a kilocalorie, about 4.184 kilojoules. Carbohydrates and protein provide about 4 kilocalories a gram, and fat about 9.
What is basal metabolic rate?
Basal metabolic rate (BMR) is the energy the body uses at complete rest, awake, after fasting, at a comfortable temperature: just to keep the heart beating, the lungs breathing, the brain working and the cells maintained. A 2021 study in Science led by Herman Pontzer notes that basal expenditure usually makes up about 50% to 70% of the total energy a person uses in a day. Digesting food and moving account for the rest.
It can be measured by tracking how much oxygen a person uses, but it is often estimated with a formula. One widely used formula is the Mifflin-St Jeor equation, published in 1990 and given in Wikipedia's article on basal metabolic rate:
BMR (kilocalories a day) = 10 × weight in kg + 6.25 × height in cm − 5 × age in years + s
where s is +5 for men and −161 for women.
A worked example
Take a woman aged 35 who weighs 65 kilograms and is 165 centimeters tall.
- 10 × 65 = 650
- 6.25 × 165 = 1,031.25
- 5 × 35 = 175, which is subtracted
- 650 + 1,031.25 − 175 − 161 = 1,345 kilocalories a day
For a man aged 35 who weighs 80 kilograms and is 180 centimeters tall, the same steps give 800 + 1,125 − 175 + 5 = 1,755 kilocalories a day.
To turn that into power, multiply by 4,184 joules per kilocalorie and divide by the 86,400 seconds in a day. The woman's estimate is about 65 watts; the man's about 85. A resting human body runs continuously at roughly the power of a household light bulb, almost all of it ending up as body heat.
Two cautions. The equation is an estimate for a typical person, built from measurements of a sample of adults; a particular person's measured rate can differ. And it describes rest only. Total daily use depends heavily on activity. This note explains the biology; it isn't diet or weight-loss advice, which belongs with a doctor or registered dietitian.
Is a "fast" or "slow" metabolism real?
Mostly, the differences people notice come from body size and makeup. A larger body has more living tissue to run, so it burns more. Fat-free mass, meaning muscle, organs and bone, is the strongest predictor of energy use. Organs such as the brain, liver, heart and kidneys burn far more per gram than muscle at rest, and muscle burns more than fat.
The largest study of the question is the one led by Pontzer, published in Science in 2021. It pooled measurements from 6,421 people aged 8 days to 95 years in 29 countries, using the doubly labeled water method, which measures total energy use over one to two weeks of ordinary life. After adjusting for body size and composition, it found four stages.
| Age | Energy use, adjusted for body size |
|---|---|
| Birth to about 1 year | Rises fast, to about 46% above adult levels |
| 1 to 20 years | Falls slowly, about 3% a year, to adult levels |
| 20 to 60 years | Stable, even during pregnancy |
| Over 60 | Falls about 0.7% a year; people in their nineties were about 26% below middle age |
That cuts against two common beliefs. Metabolism doesn't drop in a person's 30s or 40s: on average, size-adjusted energy use held steady from 20 to 60, so changes in weight over those years can't, on the whole, be put down to a slowing metabolic rate. And adult men and women did not differ once fat-free mass and fat mass were accounted for.
Still argued
How much people of the same size and makeup differ, and why, is still studied. Measured rates do vary between such people, and conditions such as an underactive or overactive thyroid change metabolic rate substantially. Researchers also disagree about how strongly the body lowers its energy use during dieting, and for how long. The averages above describe groups, not any one person.
How is metabolism kept in balance?
Metabolism is steered by hormones that respond to what the body needs, part of what homeostasis means. After a meal, insulin from the pancreas tells cells to take up glucose and store energy; between meals, other hormones release it from stores. Thyroid hormones set the overall pace of many reactions.
When one enzyme in a pathway is missing or doesn't work, the result is a metabolic disorder, in which some substance builds up or runs short. MedlinePlus lists disorders affecting the breakdown of amino acids, carbohydrates or fats. Phenylketonuria, described in what is a gene?, is one: a single enzyme that doesn't work, and a diet that works around it.
Check yourself
- Without looking back, explain the difference between catabolism and anabolism, with one example of each.
- Estimate the basal metabolic rate of a 60-year-old woman, 65 kilograms and 165 centimeters tall, and say what changed from the worked example.
- What did the 2021 study find about energy use between the ages of 20 and 60, and why does that matter for the idea of a slowing metabolism?
Sources
- Metabolic Disorders — MedlinePlus, U.S. National Library of Medicine
- Daily energy expenditure through the human life course — Pontzer and colleagues, Science, 2021
- Basal metabolic rate — Wikipedia
- Metabolism — Wikipedia