Underweight refers to a condition where an individual’s body weight is lower than what is considered healthy for their height and age, typically defined by a body mass index (BMI) of less than 18.5. Being underweight can result from a variety of factors, including poor nutrition, eating disorders, or medical conditions that affect nutrient absorption. It can lead to a weakened immune system, fatigue, decreased muscle mass, and in severe cases, complications like osteoporosis or fertility issues.

Maintaining a healthy weight is important because being underweight can have serious health consequences. It can impair the body's ability to fight off infections, slow down recovery from illness, and lead to nutrient deficiencies. In children and teenagers, being underweight can affect proper growth and development. In adults, underweight can increase the risk of developing bone problems, malnutrition, and a decrease in overall quality of life.

Several factors contribute to being underweight, including poor diet, high metabolism, chronic diseases (like cancer or hyperthyroidism), mental health issues (such as depression or eating disorders), and genetics. People who are under stress, have anxiety, or are experiencing emotional distress may also find it difficult to maintain a healthy weight.