Hello Nomad,
I don't think there is a specified number of calories for thalassemia patients. I think it should be determined on an individual basis. I agree with Andy that while growing, many thals require additional calories to help them maximize their height potential. Thals, like the general population, should wat healthy balanced diets. They should focus on getting adequate zinc (and other vitamins/minerals) in their diets. Calories per say should be based on whether an individual is under weight.
I often hear that thals experience lack of appetite just before transfusions. Perhaps at such times a nutritional/caloric supplement may be helpful.