A BMI between 15 and 18.5 falls in the underweight category and may signal nutritional deficiencies or underlying health issues, particularly if it is persistent or associated with fatigue, weakness, or low immunity. Being underweight can increase the risk of weakened bones, decreased immune function, infertility, and in severe cases, organ damage. While some individuals, such as elite athletes or those with naturally smaller frames, may fall into this range without complications, most people in this BMI bracket are advised to consult a healthcare provider. This range is below the medically accepted healthy zone, and working toward 18.5–24.9 is generally recommended for overall health and resilience.