Compared to an adult of appropriate weight, an underweight adult will have a higher chance of suffering chronic fatigue.
An underweight person is a person whose body weight is considered too low to be healthy. Underweight people have a body mass index of under 18.5 or a weight 15% to 20% below that normal for their age and height group. Being underweight may be due to genetics, metabolism, drug use, lack of food, eating disorder, or illness.