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The normal BMI is 18 to 24, while under 18 is described as being a low body mass index. People who have higher BMIs are divided into an overweight class from 25 to 29, an obese class from 30 to 39, and a morbidly obese class over 40. 

A below-average BMI is sometimes associated with medical problems. As orthopaedic surgeons, we understand that patients with low BMIs may have an increased susceptibility to problems, including stress injuries. It is important that you contact your primary care physician to discuss your BMI and how it relates to your specific body type, caloric intake, and its association with other medical conditions and problems.