There are many wrong notions and theories about building muscle and the belief that thin guys cannot build muscle without the use of steroids is one of them. There are thousands of skinny guys out there who are ready to throw in the towel simply because they are unable to put on weight no matter how much they eat. But it's not their fault, because they are genetically programmed to burn more calories than others.
However, it is still very much possible for lean young men to put on weight and although consuming more calories than your body can burn is the most basic method of weight gain, it's equally important to combine the same with weight training in order to build mass effectively. This is because weight training enables the body to absorb more nutrients and increase the level of important hormones along with muscle mass.