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Metabolism is the process performed by any living thing in order to continue to live. It includes animals' eating, digesting and breathing, plants' photosynthesis and respiration in both. It is normally a set of chemical reactions that obtain energy for life from the environment, these chains of reactions are termed metabolic pathways and eventually lead to the complete transformation of nutrients into energy or other materials that can be used by the body.
Science fiction origin
Metabolic pathways are often very similar even in dramatically different species, utilising similar food sources and enzymes to work. Central common pathways are present in all domains of life from complex animals to simple bacteria. These pathways can be tracked throughout evolution.
The speed of someone's metabolism is often to blame for people being overweight or underweight. Although this is partially true, the real cause is that they haven't matched their food intake to their rate of metabolism. Age, drug intake and time of day are all factors that affect metabolic rate.
Many woo-peddlers describe miracle weight loss plans that center around somehow modifying or rebooting "your metabolism", usually with very little reference to where the actual energy released goes. A simple search of Natural News provides a plethora of examples, many contradictory.