Some calorically dense foods that supply adequate amount of protein are great. That means things like full fat cheese, whole milk, non-lean cuts of meat, nut butters, full fat yogurt and cottage cheese. It means buttering their toast and veggies, using baked goods made with eggs and oil, and allowing plenty of snacks throughout the day. It means not letting them fill up on fruit juices, and high fiber carbohydrates so that they lack an appetite for eggs, milk, meat, and other dense foods. Fruits, veggies, and carbohydrates are critical in their diet, but they can be overdone.
Adequate nourishment is the cure, and treatment is a slow process.
The following diet plan may help guide you in making some appropriate food choices to help your little one gain a little weight, and get back on track. Iron fortified infant cereal made with whole milk, topped with brown sugar or maple syrup. Fruit smoothie made with whole milk vanilla yogurt, banana, and other favorite fruit whirled smooth in a blender. Although sugar is a 'fast burn' as your doctor said, there is a place for a little bit in her diet. If a small amount of sugar will make some food more palatable so that she will eat it, than that is fine.