If you feel healthy ( not tired), a thin body is an advantage. People are different and therefore, you need not be like your friend.
Having said that, how is your fat intake and do you have any fat digestion problem? Do you take a balanced food, deficiency of nutrients and vitamins can affect your health. You also need to do some regular exercise.
How old are you? Some people put on weight during the adolescent period, some others, after forty years of age.
While there may be many other reasons for not putting on fat, one of the possibilities is an overactive thyroid.
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