Question: what decides how fast our metabolism is ?

Samantha Weaver answered on 6 Jul 2012:

This is a really good question, and is very complicated!

Metabolism is usually divided into two categories. Catabolism breaks down organic matter, for example to harvest energy in cellular respiration. Anabolism uses energy to construct components of cells such as proteins and nucleic acids.

The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to proceed quickly and efficiently. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell’s environment or signals from other cells.

The metabolism of an organism determines which substances it will find nutritious and which it will find poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals.[1] The speed of metabolism, the metabolic rate, influences how much food an organism will require, and also affects how it is able to obtain that food.

A striking feature of metabolism is the similarity of the basic metabolic pathways and components between even vastly different species. For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacteria Escherichia coli and huge multicellular organisms like elephants!

The term ‘metabolic rate’ refers to the energy (calories) you expend over a day just keeping your body functioning – your heart beating and your lungs breathing, for example. This is often called the basal or resting metabolic rate.

Scientists have measured the exact amount of calories overweight and healthy weight people burn while sitting or lying quietly. This was done by measuring the amount of oxygen breathed in and the amount of carbon dioxide breathed out.

Results from these studies have consistently shown that overweight people use more energy to keep their bodies working. This is because they have larger bodies with bigger muscles and internal organs.

However, after taking into account differences in body size, lean and obese people have been shown to have similar metabolic rates.

Basal metabolic rate can be influenced by body composition. Muscle requires more energy to function than fat. As we get older, we tend to gain fat and lose muscle. This explains why basal metabolic rate tends to decrease with age.

Similarly, two people of the same age and weight may have different metabolic rates if one is fitter (and has more muscle) than the other.

Of course, we also use energy to move around during the day. Basal metabolic rate and the energy required for physical activity make up our total energy expenditure, or total energy needs. Your total energy needs depend very much on how active you are.

To make sure you ‘wake up’ your metabolism, remember to eat breakfast every day. This is a great way to ensure your metabolism gets off to an early start. When you’re asleep, your metabolism goes into slowdown mode — by eating breakfast you let your body know that you are up and moving. If you wait to eat breakfast, your body stays in hibernation mode a few hours longer.