Hello, The healthy way to increase weight is to eat good amount of wholesome food in good quantity, thereby increasing your calorie intake. It is known that 7,700 calories gained will put on as 1 kg in weight. If in spite of eating a calorie rich food, there is no weight gain, you should be investigated for a malabsorption syndrome which prevents the nutrients you eat from getting absorbed. Your weight is also determined by genetic factors. If you have very slim parents, you may also be slim inspite of all these.