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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.

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.

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).

The excess energy is stored either as fat or glycogen in the muscles and liver. Glycogen is the main storage form of glucose in the body.

One gram of fat produces 9 calories, compared with 4 calories from a gram of protein or carbohydrate.

Calorie requirements

Body size and composition

A larger body mass requires more calories. People with more muscle in relation to fat will require more calories than individuals who weigh the same but have less muscle in relation to fat.

Therefore, people with a higher muscle-fat ratio have a higher basal metabolic rate than people with a lower muscle-fat ratio, if their weight is the same.


As people age, several factors emerge that result in a lower calorie requirement. Muscle mass drops, resulting in a higher fat-muscle ratio. Also, the following age-related factors reduce a person's calorie requirement:

  • Hormones - men produce less testosterone and women produce less estrogen with age - both hormones are involved in anabolic processes that consume energy.

  • Menopause - as women approach menopause, there is a drop in hormones that normally promote energy use. Many women find it harder to lose weight during this time.

  • Physical activity - older adults tend to be less physically active than when they were younger.

  • Sex - men have a higher metabolic rate than women because their muscle-to-fat ratio is higher. This means that an average man will burn more calories than an average woman of the same age and weight.

Improving diet and nutrition

The best way to improve diet and nutrition is to eat plenty of fruits and vegetables. Managing the number of calories consumed each day is an important factor in weight control, especially if attempting to lose weight.

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