BMI is a quick way to indicate weight problems. Too thin is a BMI below 19; the preferred range is 19 to 24, overweight is 25 to 29, obese is more than 30, and morbidly obese is more than 35. However, for some people, mitigating factors need to be considered before a person is advised to change his or her diet. For example, muscle is heavier than fat, so fit adults could have a BMI of 26 and still be quite healthy.