It occurs when the entire body mass diminishes. There is loss of body fluids when one is undergoing through weight loss. The reasons for the weight loss are voluntary efforts to decrease weight for improving the state of obesity, some disease which naturally sheds kilos from the body leading to extreme weight loss, and malnutrition. Malnutrition, which means poor appetite, occurs when one do not get three meals a day. It is because of not taking the balanced diet.
The loss in body weight which is unnatural and unexplained is called cachexia.
There are some medications which can help people overcome weight loss.