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The mechanisms underlying weight gain or loss are rather complicated. We will however try to explain them here, at least partially.
The following diagram gives a good idea of the mechanisms involved:
Energy regulation consists in balancing the energy supplied by food against the energy used up by the body. The process is here symbolised by the 2 pans of a scale:
— On the left are the inputs: the energy derived from your diet, the protein, carbohydrates & lipids you have ingested.
— On the right are the outputs:
When inputs exceed outputs, the scales tip to the left; the excess energy is stored in the fatty tissues by the body in the form of fat and you gain weight.
Inversely, if you use up more energy than has been ingested, your body uses the stored fats and you loose weight.
The two main parameters of this delicate energy balance, diet and physical exertion, are influenced by several interacting factors related to our environment (society, family, school, work) and socio-economic status (purchasing power, lifestyle). Individual physiological predisposition also has an impact on energy regulation mechanisms.