When we gain weight, we must acquire more strength and when we lose weight, we must lose the strength. This is a fundamental medical principle.
If we gain weight and feel week, it is a disease and when we lose weight and gain strength, we are recovering from the disease. One is not supposed to gain more than 5kg of weight after the age of 20 years. Any weight gain after that will only be due to accumulation of fat which leads to insulin resistance.
Insulin resistance does not allow food to convert into energy. In the state of insulin resistance, whatever you eat, it is converted into fat. As it is not converted into energy and you feel week. When you reduce insulin resistance by drugs or walking the metabolism becomes normal and whatever you eat gets converted into energy and you start gaining strength.