Brief Answer:
Follow the Advises as below..

Detailed Answer:
Hello XXXX, Welcome to HCM..

I've gone through your query and understand your concern..

Now from your description , your BMI is 18.6 which is normal, so relax.. you're not underweight..

However, if you're still looking for weight gain, you need to start consuming foods like banana , almonds, pistachio, milk-shakes, custard, soy-milk , eggs(whole) etc.

But you must know that food and supplements alone can't let you gain weight properly, you need to start working out as well(30-45 min per day) as it'll help you to gain muscle mass..


If you have got a diet plan service on your portal, use it to obtain a personalized diet chart made by the dietitian so that you can follow it on day to day basis..


I hope this will clear your doubts .. feel free to ask if there's any more queries, i'll be glad to guide you..

Take Care
Regards

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