If your body is losing weight it's a simple case of burning more calories than you are eating. That makes the goal simple: consume more calories. There are 3 types of food: carbs, protien and fat. Fat is by far the most calorie dense foods so go eat high fat foods. Consider nut butters, Kind bars, sausage sticks, milk products, and high fat breakfast breads such as biscuits and gravy. If you want muscle mass, eat high protein instead, but eat an ungodly amount of it and lift weights. 2 high calorie+high protien foods to consider if you need both are Nuts and Cheese Last note: processed sugary crap is addicting. That is not hyperbole, that's fact. Do NOT go hard on the sweets trying to get calories. This somewhat applies to carbs as well. Do not go hard or you will get addicted.