Food is the plant and animal tissue as well as microbial cells consumed by organisms. These organic structures are broken down into digestible molecules that harbor chemical energy within their bonds. Food energy refers to the potential source of cellular energy available in the chemical bonds of food.
All living things consume energy in the process of growth and reproduction. The source of all energy available to life on Earth is the sun, a thermonuclear reactor spinning safely out in space. Through the process of photosynthesis, plants transform light energy into chemical energy, using some of that energy for their own cellular needs. Other life forms, including humans, consume plants to produce complex biological systems and in turn provide additional sources of chemical energy in their body tissues to other organisms. The hierarchy among these various organisms is called a food chain or food web.
The energy content of food may be calculated by burning the material until it is completed consumed. The products of the combustion reaction will be carbon dioxide, ash, water, and heat. The heat given off is captured and measured by carrying out the reaction in a heat sink, such as a water-jacketed combustion vessel called a calorimeter. This heat is the total energy available in the food and is the basis of the commonly used calorie. Note that the calorie used in food labeling is equivalent to 1,000 calories (1 kcal or 4.182 kilojoules) of energy, as measured.
In an organism’s digestive system, food is converted to chemicals that can be absorbed into the cells directly or indirectly from a circulating delivery system, like blood. Humans are capable of acquiring carbohydrates, short lipids, and proteins as sources of nutrition. Other factors that aid in tissue building are absorbed not for their energy content but as reaction catalysts, co-factors, or compounds the body cannot produce, such as essential amino acids.
Within the cell, carbon content of the food molecules is oxidized to carbon dioxide or other oxygen-containing molecules. The release of energy from the conversion is captured by a chain of electron-transferring molecules, including adenosine triphosphate (ATP). The molecules enable reactions that build structures made thermodynamically possible by lowering activation energies.
Food energy is the source of energy for current use and the basis of tissue building. Some food energy exceeds the body’s needs and is stored in various molecules for later use. The process of converting food energy into current cell activities or into cell growth is called metabolism. Different organisms even of the same species perform this task on a macro level, with varying efficiencies. When dieters say they are watching their calories, they mean they are trying to control their intake of food energy in order to avoid adding additional storage capacity for excess energy-storing molecules.