Why can't i gain weight?

Nutrition & thyroid: For someone who's been thin all his/her life, thank your luck stars for a "fast metabolism". If you're concerned about your inability to gain weight, get a referral to & speak w/nutritionist to make sure you're eating enough calories & protein. Also check w/your family doc to make sure your thyroid is normal as is your gut & immune system. Unexplained weight loss is another animal completely.

Weight gain : Itl depends on a lot of factors.Age genetics dietary habits excercise stature body mass metabolism etc.If you feel you need to put on some weight and there are no other issues talk up your doctor hormonal levels checked some workup assuring no concerns no psychological issues affecting mood appetites then there are meds that stimulate appetite can be used all the best.