Brief Answer:
To gain weight you need to consume more calories every day.

Detailed Answer:
Hi,
Welcome to health care magic,

If your body mass index(BMI)is less than 18.5, you are underweight.

To gain weight you need to consume more calories every day from healthy foods and choose the right type of exercise.

Underweight might occur due to eating disorders or some chronic diseases.

You can have below mentioned diet.

1) Eat high calorie, high protein diet. Do not increase the intake of junk foods and greasy foods, this will contain unhealthy fats.
2) Eat healthy fats like omega-3 essential fatty acids sources like salmon, walnuts.
3) Eat good protein rich foods like lean meats, fish, poultry, egg white nuts, seeds, pulses and legumes.
4) Eat healthy carbohydrates like fruits, vegetables and whole grains.
5) Eat five to six small balanced meals per day rather than eating three large meals.
6) Protein powder and nutritional supplement drinks can be added.
7) Consume a lot of high-calorie snacks (not junk foods) such as cheese sticks, milk shakes, muffins, dried fruits, yoghurt.
8) Drink a lot of fluids that supply nutrients and calories like milk, fresh fruit juice and energy drinks.
9) Eat starchy vegetables like potatoes and corn more often.
10) Drink at least 2 glasses of milk, replace tea with milk.
11) Have eggs for breakfast. Custard is good for putting on weight.
12) Eat bananas and sapota milkshakes.

Keep a record of calorie intake each day. Try to increase calories intake by checking the record and continue to increase calorie intake until you gain weight.
Avoid caffeine and nicotine.

Exercises are also important for healthy weight gain. Resistance training exercises such as lifting weights will help to increase the muscle size.

If you have any doubt then feel free to ask.

Regards,

Dr.sandipkabra

---