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To do anything you require energy and it is generated using the food you eat. The demand of energy in your body is relative to many factors like the climate and the terrain of the place you are living, your body structure, your occupation etc. For a calm and pleasant life there should be perfect harmony between the demand and supply. Normally you need not take separate efforts to identify your energy needs and related food intake, because your body keeps on telling it to you during every fraction of a second in your life. For a calm and pleasant life you have to protect your inborn ability to hear your body and maintain harmony between your body, mind and the external world.
Practice it and make it a habit. Then the nature will make it sustainable by triggering necessary genetic engineering!