There are legitimate medical conditions that contribute to someone's inability to burn fat and makes staying healthy a challenge (I know one personally), but I firmly believe that a vast majority of people who are fat don't fall into this category. Here's a nice way to tell. If, when you were a kid, you were consistently considered overweight, and that continues into adulthood then you might just be naturally fat or have a medical condition that aids the cause. If, when you were a kid, you were "normal" or "skinny" and in adulthood have reversed form and are now considered fat... you're fat because of your life choices and could reverse that if you choose.