Metabolism is the complete set of chemical reactions happening inside every living cell to sustain life. The word comes from the Greek metabolē, meaning “to change,” and it covers everything from breaking down the food you eat into usable fuel to building the complex molecules your body needs to grow and repair itself. These thousands of interconnected reactions fall into two broad categories: those that break things down and those that build things up.
Catabolism and Anabolism
Every metabolic reaction belongs to one of two branches. Catabolism is the breakdown side: it takes large, complex molecules and degrades them into simpler ones like carbon dioxide, water, and ammonia. This process releases energy. When your body digests food, it’s running catabolic reactions, pulling apart carbohydrates, fats, and proteins so the energy locked in their chemical bonds can be captured and used.
Anabolism is the opposite. It uses energy to assemble small, simple molecules into the large, complex ones your body needs: DNA, proteins, stored sugars, and fats. Every time your body repairs a muscle fiber, copies genetic material before cell division, or stores extra glucose for later, anabolic pathways are at work. The two branches depend on each other. Catabolism supplies the energy and raw materials that anabolism requires.
ATP: The Cell’s Energy Currency
The energy released during catabolism doesn’t float freely through the cell. It gets packaged into a molecule called ATP (adenosine triphosphate), which acts as a universal energy currency. ATP stores energy in the bonds between its three phosphate groups. Those groups carry negative charges that naturally repel each other, making the bonds unstable and energy-rich. When a cell needs power for any task, it snaps off one phosphate group, converting ATP to ADP (adenosine diphosphate) and releasing a burst of energy in the process.
Nearly every energy-requiring process in your body, from contracting a muscle to sending a nerve signal to assembling a new protein, is powered by this same ATP-to-ADP conversion. Cells then recycle ADP back into ATP through metabolic pathways, creating a continuous loop of energy capture and release.
How Cells Extract Energy From Food
The main energy-extraction pathway, cellular respiration, happens in three stages. The first, glycolysis, splits one molecule of glucose into two smaller molecules called pyruvate. This step uses 2 ATP to get started but produces 4, for a net gain of 2 ATP per glucose molecule. It also generates electron carriers (NADH) that will be important later.
If oxygen is available, the process continues. Pyruvate enters the citric acid cycle (also called the Krebs cycle) inside the mitochondria, where it’s broken down further, releasing carbon dioxide and loading up more electron carriers. Those carriers then feed into the third stage, oxidative phosphorylation, where the real payoff happens. Electrons pass through a chain of proteins in the mitochondrial membrane, and the energy they release drives the production of a large batch of ATP. In total, aerobic respiration produces roughly 32 ATP molecules from a single glucose molecule, compared to just 2 from glycolysis alone when oxygen is absent.
This is why oxygen matters so much for energy. Without it, cells are stuck with anaerobic respiration, which captures only a small fraction of the energy available in glucose.
How Enzymes Control Metabolic Speed
Metabolic reactions don’t just run freely. They’re controlled by enzymes, proteins that speed up specific chemical reactions without being consumed in the process. Each step in a metabolic pathway typically has its own dedicated enzyme, and cells regulate the pace of entire pathways by adjusting enzyme activity.
One of the most important control mechanisms is negative feedback. When the end product of a pathway builds up to sufficient levels, it loops back and inhibits an enzyme earlier in the chain, slowing down its own production. This keeps concentrations within a narrow, useful range. A classic example involves the enzyme phosphofructokinase in glycolysis. When cells have plenty of ATP, the excess ATP binds to this enzyme and reduces its activity, effectively telling the cell to slow down energy production because it already has enough.
This kind of regulation, called allosteric regulation, works because enzymes can change shape when molecules bind to them at sites other than their active center. It gives cells remarkably precise, moment-to-moment control over their chemistry.
Hormones and Metabolic Rate
Beyond individual enzyme controls, hormones regulate metabolism at the whole-body level. Thyroid hormones are the most direct regulators of your basal metabolic rate (BMR), which is the energy your body burns at rest just to keep basic functions running. Thyroid hormones boost ATP production and stimulate mitochondria to generate more heat, increasing overall energy expenditure.
When thyroid hormone levels are too high (hyperthyroidism), the result is a hypermetabolic state: increased resting energy expenditure, weight loss, faster fat breakdown, and reduced cholesterol. When levels are too low (hypothyroidism), the opposite happens: resting energy expenditure drops, weight tends to increase, and fat breakdown slows. One mechanism behind this involves the mitochondrial membrane. Thyroid hormones increase proton leakage across the inner mitochondrial membrane, which means cells must burn more fuel just to maintain normal ATP output, producing extra heat in the process.
What Makes Up Your Daily Energy Use
Your total daily energy expenditure has three components. The largest, by far, is your basal metabolic rate, which accounts for 60% to 70% of all the energy you burn. This is the energy cost of simply being alive: pumping blood, breathing, maintaining body temperature, running cellular repair.
About 10% of your energy goes to the thermic effect of food, the energy needed to digest, absorb, and process what you eat. Not all macronutrients cost the same to process. Protein is the most metabolically expensive, raising your metabolic rate by 15% to 30% during digestion. Carbohydrates cost 5% to 10%, and fats cost just 0% to 3%. The remaining 20% to 30% of your daily energy powers physical movement, from walking to structured exercise.
How Metabolism Changes With Age
A widespread belief holds that metabolism slows steadily from your late twenties onward, but a landmark 2021 study published in Science, analyzing energy expenditure data from over 6,000 people, found a more surprising pattern. After adjusting for body size and composition, metabolic rate peaks at around one year of age, when it runs roughly 50% above adult levels. It then gradually declines through childhood and adolescence, reaching adult levels around age 20.
From there, metabolism stays remarkably stable between ages 20 and 60. The midlife weight gain many people experience during this window isn’t driven by a slowing metabolism. It’s explained by other factors like reduced physical activity and changes in diet. After age 60, metabolic rate does begin a genuine decline, reaching about 26% below middle-aged levels by the time people are in their nineties.
When Metabolism Goes Wrong
Because metabolism depends on thousands of enzymes working correctly, a defect in even one can cause serious problems. Inborn errors of metabolism are genetic disorders in which a single enzyme is missing or malfunctioning, blocking a step in a metabolic pathway. The consequences depend on which pathway is affected. Toxic intermediates can accumulate, essential products can go unmade, or both.
Phenylketonuria (PKU) is one well-known example: the body can’t properly break down the amino acid phenylalanine, which builds up to damaging levels in the brain if left untreated. Galactosemia prevents normal processing of the sugar galactose found in milk. Medium-chain acyl-CoA dehydrogenase deficiency disrupts the body’s ability to break down certain fats for energy. These conditions are individually rare, but collectively they affect a significant number of newborns, which is why most countries screen for them shortly after birth.
Other metabolic disorders aren’t genetic but develop over time. Type 2 diabetes, for instance, involves a breakdown in the body’s ability to regulate glucose metabolism through insulin signaling. Metabolic syndrome, a cluster of conditions including high blood sugar, excess abdominal fat, and abnormal cholesterol, reflects widespread disruption of normal metabolic regulation and significantly raises the risk of heart disease and stroke.