@ Dalees, it's because of your metabolism, I have the same thing, this is by no means bad, this is optimal for getting toned and getting the looks that the girls dig
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Your doctor however doesn't seem to know what he's doing, pure protein wont help you build muscles when you have the metabolism that you obviously do, if you want to do gain weight, go to the gym on a muscle building program (surely you can get a program made by the staff at the gym if you don't have any knowledge) eat healthy, drink alot of water, take Omega 3+6, and do two times the suggested amount of Weight Gainer shakes (60% Protein 40% Carbs) - on non training days do it after lunch and right before you go to bed, workout days do it right after workout and before you go to bed.
The way you describe your diet it seems as it will not gain you in any way, and it is by no means healthy, and for people with very high metabolisms like me and as I assume you, going with a healthy diet, working out and doing double dosis of Weight Gainer is the ONLY way to gain weight.
My diet persists of following:
07:00 Breakfirst = Oat Meal or Eggs + Fruit and Omega 3+6
08:00 Snack = Fruit + Creatine
10:30 Snack = Dark bread with cold meat
12:00 Lunch = Protein and carb filled lunch such as Spaghetti Bolognaise , In school I just do just dark bread with alot of cold meat and usually a Spaghetti salad. + Omega 3+6 and a Weight Gainer Shake.
14:00 Snack = Fruit
16:30 Snack = Protein Bar
19:00 Dinner = Protein filled dinner, meat with veggies usually everyday.
21:45 Snack = Weight Gainer and if it's a workout day Kassein Protein shake + grape seed vitamins.