The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)

Energy Balance

Nicholas Clément (1779–1842), Claude Bernard (1813–1878)


In its attempt to maintain well-being, the body strives to achieve homeostasis, a concept Claude Bernard introduced in 1854 to describe a stable and constant internal environment. Such stability commonly includes body temperature and pH but also energy. Bioenergetics is the study of the flow of energy in living organisms. To achieve such balance, our energy intake must equal energy expenditure. The energy intake is determined by diet and includes the food energy (calories) and the amount of food consumed. Energy expenditure is based on physical or external work being performed and the internal heat produced. The internal heat includes: the basal metabolic rate (BMR), which is the amount of energy expended when at rest that is sufficient to enable vital organs and systems to continue to function normally; and the thermic effect of food—that is, the energy cost associated with biologically processing food for use and its storage for later expenditure.

As is all too obvious, gaining imbalances may occur when energy intake exceeds its expenditures, and it commonly results from overeating and a sedentary lifestyle. Such excess energy is primarily stored as fat, leading to weight gain. Conversely, a losing imbalance happens when the energy intake is less than the energy expenditure; this is the consequence of under-eating, digestive disorders, or other disease states.

In 1960, the International System of Units established a set of standards used in commerce and science, and these have been adopted worldwide by virtually all nations, with the notable exception of the United States. In a food-related context, the joule (J) or kilojoule (kJ) is the unit of energy. Food packages in the European Union use both the kJ and the metric system unit of energy, the calorie (c) or kilocalorie (kcal); in the United States, labels only designate the Cal (1 Cal = 1 EU kcal or 4.2 kJ). One Cal is defined as the amount of energy required to raise the temperature of one kilogram (2.2 pounds) of water by 1º Celsius (1.8º Fahrenheit). There are many claimants for the individual first to use the calorie in nutrition, but one prominent contender is Nicholas Clément in 1824.

SEE ALSO: Metabolism (1614), Homeostasis (1854), Bioenergetics (1957), Optimal Foraging Theory (1966).

The international unit of energy, the joule, is named after the English physicist James Prescott Joule (1818–1869), who in 1845 devised a “heat apparatus” (shown) that was able to estimate the “mechanical equivalent of heat”—that is, the work required to raise the temperature of a fixed volume of water by 1.8°F (1°C).