Food Energy: Fuel for the Body
Food energy, derived from the chemical energy in food, powers various bodily functions in animals and humans. The majority of food energy is obtained through aerobic respiration, which combines carbohydrates, fats, and proteins with oxygen. Other dietary components, such as organic acids, polyols, and ethanol, can also contribute to energy intake.
Energy Content of Foods
The energy content of food is measured in kilojoules (kJ) or kilocalories (kcal). Fats and ethanol have the highest energy density, followed by proteins and carbohydrates. The energy density varies based on the proportions of carbon, hydrogen, and oxygen atoms in the food.
Estimating Food Energy
The energy content of a dish or meal can be estimated by adding the energy contents of its individual ingredients. However, the actual energy obtained by the body may be lower due to the presence of indigestible components like fiber. The Modified Atwater system estimates food energy by considering the amount of digestible dietary components, excluding fiber.
Dietary Sources of Energy
The typical human diet primarily consists of carbohydrates, fats, proteins, and indigestible components such as fiber. Ruminants have specialized bacteria that enable them to extract energy from cellulose. Organic acids and polyols also contribute to the energy content of the diet.
Energy Usage in the Body
The food energy obtained through respiration is utilized for various purposes, including:
- Basal metabolism of organs and tissues
- Maintaining body temperature
- Muscular force for posture and movement
The conversion efficiency of energy into muscular power is relatively low, due to losses during respiration, ATP generation, and mechanical work. For an overall efficiency of 20%, one watt of mechanical power equates to 18 kJ/h of energy.