Like core body temperature (Fig. 40.5), blood glucose levels (Chapter 38), and blood pressure (Chapter 39), the energy balance of an organism is often maintained at a constant level. An animal in energy balance takes in over time the same amount of calories of energy from food that it uses over time to meet its metabolic needs. Energy balance can be thought of as a form of homeostasis. We consider sources of energy, or energy intake, and ways in which energy is expended, or energy use.

For animals, the source of energy is the diet. In turn, energy is used to do work—maintain tissues, grow, move about, and the like. Energy required for basic life processes accounts for the majority (about 70%) of energy use. Interestingly, digestion and absorption of food themselves require energy, and this use is included in the 70% figure. The balance (about 30%) can be used for physical activity, with higher levels of activity using more energy, as we saw.

When energy intake does not equal energy used, there is an energy imbalance. If an animal eats more food than it requires, energy stores such as fat deposits grow over time. The result is that the body shifts its metabolism mostly to anabolic processes that build energy stores. Many animals achieve a net positive energy balance during the late summer and fall when food is plentiful, before it becomes scarce in winter. Other animals maintain a constant energy intake throughout the year by migrating to areas with more abundant food and avoiding colder temperatures that require increased energy expenditure to remain active and warm. Still other animals hibernate, or become less active, to conserve their energy use over the winter.

Animals that cannot acquire enough food are in negative energy balance and become undernourished. During prolonged periods of an inadequate food supply or starvation, an animal consumes its own internal fuel reserves. Starvation forces animals to deplete their glycogen and fat reserves first, and then, if no food is found, to resort to protein stores, primarily in muscle tissue. This ultimately leads to muscle wasting. Undernourishment and starvation are particularly serious human health problems in many developing countries, especially those ravaged by war and political instability.

Humans, like most other animals, store excess food calories as fat. We do so because, over much of our evolutionary history, food was less abundant and more unpredictable in its availability than it is today. With the development of agriculture and domestication of livestock, food supplies rapidly increased, and many human populations were able to grow and consume increasing amounts of food. With the rise of mechanized agricultural food production and the availability of highly processed foods developed by the modern food industry, excessive intake of food calories has led to an increasing and now critical public health problem: obesity.

Obesity is now an epidemic in many industrialized nations. In the United States, about 36% of the adult population is considered obese. Obesity is a major public health concern because it increases the risk of diabetes, heart disease, and stroke, and contributes to a shorter life-span. For most animals, however, acquiring food and storing its products efficiently in the body allow them to have a fuel reserve to meet seasonal energy requirements. This rationing of stored energy remains an essential part of their metabolic and digestive physiology.