Metabolism

Metabolism is the body’s process of creating and using energy to support functions such as breathing and digesting. Because we cannot create our own energy from the Sun like plants can, we need to consume food to get energy.
If our bodies were not  constantly metabolizing food, then  we could no longer live. Metabolism involves many complex chemical reactions in the body that enable a person to get a certain amount  of energy—measured in calories—from eating an apple or even more energy from a slice of pizza.

Unused energy is stored in the body as fat. Energy reserves are a great survival mechanism, particularly in the days when famine was a threat. In the United States, scarcity of food is no longer a serious threat to survival so people do not have an opportunity  to burn  these energy reserves or fat. Americans also lead much more sedentary lives than people once did, often going from car to office cubicle with little physical exertion. In contrast, a pioneer who was chopping wood for hours a day would have much less chance of being overweight given the pioneer’s high caloric needs. The abundance of food and reduced physical exertion in the average American’s life has contributed significantly to the problem of obesity in the United States.