Metabolism: What It Is and How Metabolism Works

Metabolism is a word used to explain almost everything: weight, fatigue, and "slow metabolism." Behind it are specific chemical reactions that turn food into energy and into material for cells. Let's sort this out calmly, without promises or scare stories.

What metabolism is and what metabolism means

Metabolism, or metabolic processes, refers to the chemical reactions that occur in a cell or in the whole body. During these reactions, molecules of food, medications, and biological substances are broken down under the action of enzymes. This produces energy and compounds needed for growth, and the body also gets rid of toxic substances. Each product of such a reaction is called a metabolite.

Simply put, metabolism is not a separate organ but the continuous work of many reactions. The speed of this work affects how much food the body needs.

What metabolism in the body consists of

Metabolism is divided into two stages. Catabolism is the breakdown of complex substances into simpler ones, usually with the release of energy. Anabolism is the reverse process: complex compounds are built from simple ones, and energy is spent on this.

Reactions occur in chains—metabolic pathways, where one biologically significant molecule is sequentially converted into another. Enzymes act as biological catalysts: they lower the activation energy of a reaction and help regulate pathways in response to changes in the cell.

The link between the stages is ATP, the universal energy source of cells: it is formed during catabolism and used during anabolism. Over a day, the body uses an amount of ATP approximately equal to its own body mass.

• Proteins are built from amino acids; among them are enzymes that accelerate reactions.
• Fats, or lipids, are part of cell membranes and serve as an energy source.
• Carbohydrates store and transport energy, for example starch and glycogen.
• Nucleic acids store and implement genetic information.

What metabolic rate depends on

Metabolic rate varies among people, and this is normal. Energy needs depend on age, weight, height, and how much a person moves. A reference point for adults is about 2000 kcal per day for a woman and about 2500 kcal for a man.

Weight is influenced not only by metabolism. Environment and available food, family habits and genes, lack of sleep, and emotions matter—sometimes people eat out of boredom, sadness, or stress. Some medications, such as steroids and certain antidepressants, can contribute to weight gain.

What "slow metabolism" means and how it is assessed

"I have a slow metabolism" is a common phrase when weight increases but the food seems to be the same. There is no single number that shows "slow metabolism": the overall picture is assessed. If more calories come in than are used, the body stores excess energy as fat; if fewer, it draws on its stores.

During an appointment, weight and height are reviewed (for example, through body mass index—it shows whether weight matches height), along with how much energy is expended per day, what medications the person takes, how sleep is, and whether there are chronic conditions.

Which diseases affect metabolism

Some conditions do indeed change metabolism and weight. Examples are Cushing's syndrome and polycystic ovary syndrome (PCOS): weight may increase with them. And excess weight increases the risk of type 2 diabetes, heart disease, kidney disease, and some types of cancer.

When to see a doctor

If your weight changes while your habits remain the same, or if you are only planning to start a weight-loss program, begin by talking with a doctor. They will tell you what weight is healthy specifically for you and help set goals. The doctor will also check whether the changes are related to medications or chronic conditions and, if necessary, refer you for evaluation.

Metabolism itself is not a reason to call an ambulance. Numbers 103 and 112 are needed when a condition is life-threatening: for example, with signs of stroke or heart attack.

Here is a simple idea for the week: once, look at the "energy" line on the packaging of a familiar product—it shows calories in kcal. Not to count everything, but so the numbers stop being an abstraction. This material is for reference and does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer