<unk>  is chemical<unk>  that animals (including humans) derive from their<unk>  to sustain their metabolism, including their muscular activity. Most animals derive most of their<unk>  from aerobic respiration, namely combining the carbohydrates, fats, and proteins with oxygen from air or dissolved in water. Other smaller components of the diet, such as organic acids, polyols, and ethanol (drinking alcohol) may contribute to the<unk>  input. Some diet components that provide little or no<unk> , such as water, minerals, vitamins, cholesterol, and fiber, may still be necessary to health and survival for other reasons. Some organisms have instead anaerobic respiration, which extracts<unk>  from<unk>  by reactions that do not require oxygen. The<unk>  contents of a given mass of<unk>  is usually expressed in the metric (SI) unit of<unk> , the joule (J), and its multiple the kilojoule (kJ); or in the traditional unit of heat<unk> , the calorie (cal). In nutritional contexts, the latter is always the "large" variant of the unit, also written "Calorie" (with symbol Cal, both with capital "C") or "kilocalorie" (kcal), and equivalent to 4184 J or 4.184 kJ. Thus, for example, fats and ethanol have the greatest amount of<unk>  per unit mass, , respectively. Proteins and most carbohydrates have about , though there are differences between different kinds. For example, the values for glucose, sucrose, and starch are respectively. The differing<unk>  density of<unk> s (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms. Carbohydrates that are not easily absorbed, such as fibre, or lactose in lactose-intolerant individuals, contribute less<unk> . Polyols (including sugar alcohols) and organic acids contribute and respectively. The<unk>  contents of a complex dish or meal can be approximated by adding the<unk>  contents of its components.
Food energy
Nutrient
Mechanical power
Human nutrition
Bone conduction
Sleep and metabolism
Gastrointestinal physiology
Coagulation