Hi doctor i am male of 23 years age my bmi shows that i m underweight i dontget healthy whenever i started to gain weight suddenly again i become thin?

Underweight: Your weight is dependent on a number of things including calorie consumption vs calorie use. If your metabolism does not allow you to gain weight and you believe it is abnormal it should be checked out to make sure you do not have an endocrine or gastrointestinal problem that does not allow you to put on weight. Would begin by having a discussion about this with your primary care provider.

Not clear: Uncertain I am clear about your position. Inability to gain weight can be genetics or due to your health status. You may have a condition that accelerates weight loss - such as an over active thyroid or a condition that does not absorb food, what we call malabsorption. You physician can clarify this.