Lack of good foods and structure
When we train with resistance in the gym we tear muscle fibers, these tiny little micro tears in the muscle tissue is what we want to happen. Our nutritional intake among other factors is what determines whether those micro tears in the muscle grow back bigger and stronger, or whether it simply repairs itself back to the original state.
If your nutrition is lacking you may be lucky if your muscle repairs back to the original state. So what I am saying is that you can actually get smaller training in the gym. To grow muscle you need adequate protein, good fats and carbohydrates. Counting calories is not necessary at your age but learn to listen to your body and try not to allow yourself to go hungry for too long. You should be eating 4-5 meals per day including a lean protein source, good carbohydrates and fats along with a protein shake directly after training.
You don’t have to eat ridiculously high amounts of protein, one palm size of chicken or beef, two palm sizes of fish. Try to have some carbs before or even during training, either a piece of fruit, rice cakes or oats. Then have more carbs after you train with your protein shake.
If you’re a small build and struggle to put on size don’t be scared to add olive oil to your salads or vegetables or coconut oil into your oats and shakes. Good fats have numerous benefits and bump up your calories and can even help you build muscle as well as enhance performance.
Below is a brief list of foods I want you start putting into your daily eating:
Lean Protein sources
chicken, turkey, fish, beef, eggs, whey protein powder
coconut oil, olive oil, avocado, various seeds and nuts, eggs.
white and sweet potatoes, white and brown rice, pumpkin, wholemeal breads and wraps, fruit, vegetables, pastas, rice cakes.