Ideally you shouldn't "miss meals" but maintain a healthy calorie intake that's dense with highly nutritive foods. Many chronically ill people have a higher kcal need because the body is in a state of continuous breakdown; viz., whatever illness one is dealing with such as congenital heart or lung disease or any autoimmune disease. Working out while chronically ill is I dare say a must in the sense you optimize your health through exercise, as well as boost your overall mental well being. I suspect the chronic illness will dictate the level and type of exercise one can do to achieve relative wellness. Don' t be the illness. Be you who just so happens to have "x".