Markup /qa/2310

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Apparently there's someone who's supposed to know about nutrition here, so I'll tell you an issue I have.

Right off the bat, I only weight 56Kg while being 1,80cm, so the problem is obvious. I'm pretty sure it's impossible for me to gain any muscle like this which really limits my upper body development (legs are "fine" in comparison). Also my cardio is beyond atrocious, I'll get very light headed after running for just 1Km. I must admit that my diet is just plain bad and very inconsistent, but even in the periods when I ate very well (5 big meals every day, high in meat, dairy and pasta) I never achieved more than 59-60Kg. When I go to the doctor and tell her that my weight and muscle mass is low, she'll tell me that it's fine and she's not concerned at all and when I tell this to my family they all say that I'll get heavier as I age and that I should stop worrying about my body image.

So what do I do? Do I need to eat even more? That didn't seem to work in the past and it takes away a lot of my time