Your body burns (metabolizes) carbohydrates, protein and fat to produce energy and perform other necessary functions. This energy is measured in calories.
Your energy needs vary considerably according to your activity level, body size, sex and age. For example, an older, petite, sedentary woman may need only 1,600 calories a day, but a 25-year-old woman of the same size who is very physically active may need 1,800 to 2,200 calories a day.
On average, daily calorie goals are as follows:
1,600 — children ages 2 to 6, most women and some older adults
2,000 — average adult
2,200 — older children, teenage girls, active women and most men
2,800 — teenage boys and active men
(Source: Mayo Clinic website - www.mayoclinic.com)