To lead a healthy and active life, the weight of the human body must be normal. It should not be over or underweight. BMI or body mass index is a term used scientifically to measure weight of an individual relative to the height. Thus, assessing if the person is overweight or underweight or normal. BMI also is an indicator of total body fat content. According to the National Institute of Health, if the BMI of a person is less than 18.5, the person is underweight. The BMI of a healthy weighing human ranges from 18.5 to 24.9. But if the BMI of a person is above 30, the person is regarded as obese.
Definition: The drop in the total mass of a human body is termed weight loss. It may also be defined in terms of reduction in BMI value of an individual. Weight loss may be an objective for overweight individuals, it is physiological loss of weight. However, in some medical conditions weight loss may be a pathological side effect. 
1Weight In Relation To Health
Weight is a direct indicator of the overall health of an individual. Maintaining a healthy weight may be difficult, but it does have many advantages as compared to being overweight or underweight. A primary contributor to weight and health is good and nutritious diet. Fit individual can work to their full capacity, physically, socially and economically.
On the other hand, an overweight human lives a low-quality life; it is a vicious cycle in which weight hinders physical activity, and physical inactivity can then cause more weight gain. Being overweight exposes a man to many different health issues. The human heart has to perform more work resulting in problems like high blood pressure, increase in cholesterol level, which at the same time makes the body more vulnerable to diabetes and other issues. Other most common diseases in an obese body, include various types of cancer, for example, breast cancer and colon cancer, gallbladder disease, strokes, breathing problems, and osteoarthritis which affects the joints of the body. Mental issues also become a major concern in obese patients, which includes anxiety, depression, and other health issues. Weight loss, hence, is a common objective in prevention and treatment of medical cases in obese patients. It can help deal with cardiovascular problems, cholesterol reduction and also also promotes a healthy blood flow through the body. All of this collectively can contribute to the general health and management of diseases.
Being underweight still is a risk factor for malnutrition. An underfed body becomes lazy and tired. The body may become deficient in vitamin D or calcium consequently resulting in osteoporosis because of less food intake. In children and teenagers, growth and development are affected because of insufficient nutrition. The immune system also slows down, which increases the risk of contracting various diseases and complications during surgeries. In females, a diet of low nutritional value, disturbs the menstrual cycle. Increased hair loss and teeth problems are also due to a less nutritious diet.