There are various causes of underweight like inadequate food intake, prolonged meal time gaps, and increased physical activity weight increasing food intake. Calculate your BMI (Body mass index). If it is in normal range then there is no need to gain weight. Body mass index (BMI) below 18.5 is underweight. Conversely, more than 25 is considered overweight and over 30 is considered obese. BMI is calculated by formula BMI = Weight in Kilograms / (height in meters x height in meters). If you want to gain weight then gradual weight gain advisable. Advice 1. Start workout in gym at least for half an hour for 4 – 5 days in a week regularly. Start with a balanced mix of cardio, weight training and flexibility exercises 2. Take natural high protein like egg, soya bean, pulses, meat, and milk. Choose food with nutritional calories like potato, banana and dry fruits, chocolates and cakes 3. Drink 3 – 4 liters of water daily 4. Chew the food properly to make it easily digestible 5. Avoid excess intake tea and coffee after meal as they contain tannin which hinders absorption of minerals like iron I hope this information will help you. I will be happy if you ask another question Thanks