I am south indian boy studying BDS II year ,am facing very unavoidable critics from everyone that I was so lean being a doctor,my height is 165 and my weight is 53kg.. I am looking very lean although I am eating well.. please can anyone instruct me to avoid my critics..

Your best option is to simply avoid them, or don't pay attention to what they are saying. If your healthy it doesn't matter what they think then! Its your life, you live it the way you want to!

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If you are healthy and your doctor is not concerned, then don't worry about it.
If however you ARE under weight (and you DO seem to be under a bit, according to BMI) your best option is to see your doctor, possibly increase your protein and red meats, eat healthy, etc. but see your doctor.