Felling very tired, exhaustion, lethargy and describes about physical and/or mental state of being tired & weak. Even though physical & mental fatigue was different, the two will usually exist together if a person was physically exhausted for long enough, they may also be mentally tired.
Obese people were much more likely to experience the fatigue & often feel very tried for several reasons like having to carry lot of weight. Obese people are at higher risk of developing several diseases & conditions where the fatigue was a common symptom, like diabetes & sleep apnea. Being underweight mean there was less muscle strength; the very thin person will tire more easily. The successful treatment of the fatigue requires finding of the underlying causes & treating the causes successfully, weight loss was the best way to regain the control of the body & become more active.