Metabolism: Myths and facts

Metabolism refers to biochemical processes that occur within any living organism – including humans – to maintain life.

These biochemical processes allow people to grow, reproduce, repair damage, and respond to their environment.

It is a common belief that slim people have a higher metabolism and overweight people have a slower metabolism. In fact, this is very rarely the case.

This MNT Knowledge Center article will discuss the facts behind metabolism, what it is, what it does, and how it is influenced.

Fast facts on metabolism:

When people use the word “metabolism” they are often referring to catabolism and anabolism.

Catabolism is the breaking down of compounds to release energy.

Anabolism is the building of compounds, which uses energy.

People’s body weight is a result of catabolism minus anabolism.

Metabolic rate

woman on scales

Despite what promoters of certain brands of “health” foods say, there is little people can do to significantly change their resting metabolic rate.

Long-term strategies, such as increasing muscle mass, may eventually have an effect.

However, determining a body’s energy needs, then adapting lifestyle accordingly, will have a quicker effect on altering body weight.

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What is metabolism?

Most people use the term “metabolism” incorrectly for either anabolism or catabolism:

Anabolism is the building up of things – a succession of chemical reactions that builds molecules from smaller components; anabolic processes usually require energy.

Catabolism is the breaking down of things – a series of chemical reactions that break down complex molecules into smaller units; catabolic processes usually release energy.


Anabolism allows the body to grow new cells and maintain all the tissues. Anabolic reactions in the body use simple chemicals and molecules to manufacture many finished products. Examples include the growth and mineralization of bone and increases in muscle mass.

Classic anabolic hormones include:

Growth hormone – a hormone made by the pituitary gland that stimulates growth.

Insulin – a hormone made by the pancreas. It regulates the level of sugar glucose in the blood. Cells cannot utilize glucose without insulin.

Testosterone – causes the development of male sex characteristics, such as a deeper voice and facial hair. It also strengthens muscles and bone.

Estrogen – involved in strengthening bone mass, as well as developing female characteristics, such as breasts.


Catabolism breaks things down and releases energy; it uses larger compounds to create smaller compounds, releasing energy in the process. Catabolism provides the energy our bodies need for physical activity, from cellular processes to body movements.

Catabolic reactions in the cells break down polymers (long chains of molecules) into their monomers (single units). For example:

Polysaccharides are broken down into monosaccharides – for instance, starch is broken down into glucose.

Nucleic acids are broken down into nucleotides – nucleic acids, such as those that make up DNA, are broken down to purines, pyrimidines, and pentose sugars. These are involved in the body’s energy supply.

Proteins are broken down into amino acids – in some circumstances, protein is broken down into amino acids to make glucose.

When we eat, our body breaks down nutrients – this releases energy, which is stored in molecules of adenosine triphosphate (ATP) in the body. ATP is considered to be “the energy currency of life.”

The energy stored in ATP is the fuel for anabolic reactions. Catabolism creates the energy that anabolism consumes for synthesizing hormones, enzymes, sugars, and other substances for cell growth, reproduction, and tissue repair.

Body weight

Body weight is a result of catabolism minus anabolism – the amount of energy we release into our bodies (catabolism) minus the amount of energy our bodies use up (anabolism).

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