Nutrition begins with food.Nutrition is the process by which the body nourishes itself by transforming food into energy and body tissues.

If a person takes in more food than is required to meet the body’s needs, the excess calories are eventually converted to fat—a form of stored energy found mostly within adipose tissue. That causes weight gain. Eating too little causes weight loss over time, because the body must use stored fat for energy. One pound (0.5 kilogram) of adipose tissue is equal to about 3,500 calories.

Maintaining a healthy weight is a balancing act. Food provides energy, and physical activity uses up energy. If weight loss is advisable, experts recommend both reducing one’s daily calorie intake and getting more exercise.