The capacity to do work or effect a physical change. Nutritionists refer to the fuel content of a food as its energy. There are many forms of energy, including light, sound, heat, chemical, electrical, and kinetic, and most of them play a role in the body. For instance, the retina converts light energy to electrical nerve impulses, making vision possible. Muscles use chemical energy obtained from food to produce kinetic energy, movement, and heat. Energy is measured in units called calories and joules. Because these units are extremely small, more practical units used in dietetics are the kilocalorie (kcal, 1,000 calories), and kilojoule (kJ, 1,000 joules). Carbohydrates and proteins provide 4 kcal per gramme (g), fats provide 9 kcal per g (see metabolism). In general, the energy liberated from the breakdown of food is stored as chemical energy in ATP molecules. The energy in these molecules is then available for processes that consume energy, such as muscle contraction.
Energy |
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