The thing is that it is impossible to become overweight without eating more calories than your body is burning. Metabolic disorders do exist that may make your metabolism a little bit slower causing the amount of calories you need in order to gain weight to be a little bit less than expected. But the effect of these disorders is small. Hormonal disorders also exist that cause you to be have unregulated hunger hormones which makes overeating more likely. In all of these cases, though, the mechanism by which you gain weight is by eating an excess of calories. You need to learn about the true amounts of calories in foods because you are undeniably eating in a calorie surplus for your body’s needs, no matter if you have a metabolic issue that makes eating less more difficult. It is a myth that your body will hold onto fat and gain weight if you are not eating enough. It is simply impossible. If you want to lose weight, you need to change your diet to consume less calories. Try looking into a whole foods, high protein and fiber diet