A person is considered “underweight” when has a Body Mass Index below 18.5. Underweight, such as obesity, indicates that the body is in a no-healthy situation.
The body, if properly fed and healthy, tends to remain in the normal weight parameters (BMI 18.5 to 25). You can become underweight only when you are not eating enough or if you have a disease that does not allow adequate food or interferes with the absorption of nutrients.