According to SFGate, a 13-year-old girl needs around 1,600 calories a day to maintain a healthy body weight. However, girls who are underweight often need additional calories from nutrient-dense foods, such as medical nutrition shakes, peanut butter or nuts.
SFGate says that around 32 percent of children in the United States are overweight or obese. These numbers are based on the 2010 Dietary Guidelines for Americans. The National Institute of Mental Health also points out that around 3 percent of teens in the United States suffer from eating disorders like anorexia. If the 13-year-old girl is moderately active, which means walking up to 3 miles a day or playing team sports, the 2010 Dietary Guideline for Americans estimates that she needs 2,000 calories every day.