Metabolism out of control?

Have you had your thyroid checked? It regulates metabolism so you could theoretically be hyperthyroid. I come from a family that has always been painfully thin. I also can eat whatever I want. Sounds great, but I know what you're talking about. In my family, we have fat malabsorption issues as well. You can consider researching celiac disease and get tested for that because it can also cause fat malabsorption.