Since you exercise alot, you need more energy than you are consuming. Try to increase intake of carbohydrates which is essential for supplying energy to muscles in the form of glycogen. Include plenty of rice, broken wheat, wheat products like pasta, noodles, potatoes, bread etc. Do not lose hope. You just need to be patient as some people don t seem to put on weight no matter how much they eat because of high metabolic rate. Your metabolic rate too must be high ,that is why your body is able to burn calories efficiently, which is not a reason to worry about. Thats why you have difficulty putting on weight. Don t worry, gradually as you age, your metabolism will slow down and you will put on weight. Focus on being healthy and fit. Stay active and don t get too stressed about gaining weight.