Common reasons for suddenly gaining a lot of weight are: 1. autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis, and some other diseases, such as nephrotic syndrome, which lead to sudden fatness due to the effect of drugs after using glucocorticoids. 2. hypothyroidism, in which the body is in a state of low metabolism, which can lead to the body gaining weight. 3. there are some other The endocrine system, such as Cushing’s syndrome, can be fat, combined with polycystic ovary syndrome can also make people suddenly fat. 4, in the diet life, if there are bad eating and living habits, such as eating too much, eating fats and oils, as well as high carbohydrate, high sugar food or drink too much, can lead to sudden fat. 5, there are also some people may be too much exercise before, because of a variety of reasons to suddenly reduce the amount of exercise. Because of a variety of reasons leading to a sudden reduction in exercise, can also lead to sudden obesity. 6, some sudden fat patients because of anxiety, stay up late and other mental factors.