Gaining muscle means gaining weight, so you don't need to worry about wasting away while you build strength. The key to gaining weight is simply adding calories to your diet. But to ensure that these calories produce muscle, rather than fat, you need to perform resistance exercises and fuel your body with the right calories.
Increase your food consumption by about 500 calories per day. One pound of muscle is equal to about 3,500 calories so this will help you gain about a pound of muscle per week when combined with proper exercise. If you add too many calories you risk adding unwanted fat along with muscle.
Eat 0.8 grams of protein per pound of body weight per day -- about twice the recommended intake for sedentary people. You should be getting 15 to 20 percent of your total calories from protein. Choose lean sources of protein like skinless chicken breasts, tofu and fish.
Consume 20 to 35 percent of your calories from fat. Try to eat healthy fats from foods like canola oil, olive oil, nuts and avocados. Avoid eating unhealthy saturated fats from animal products like butter and chicken skin.
Eat your remaining 45 to 65 percent of your calories from carbohydrates. Try to get your carbohydrates from whole grains, fruits, vegetables and legumes.
Perform resistance exercises using free weights, weight machines, resistance bands or your own body weight two to three days per week. Perform six to eight exercises in each workout, targeting your major muscle groups. Start by performing 12 to 15 reps per exercise. If you're looking to build larger muscles, increase your weights and perform 8 to 10 reps.