Heart disease and diabetes are usually thought to be problems for the overweight and the obese, but thin people aren't immune from these conditions. It's very easy to be thin yet unhealthy, as naturally thin people might indulge in fast food, skip the gym and think they can get away with it. No matter the number on the scale, though, blood sugar levels and bad cholesterol counts can be rising. Thin people might also skip out on the check-ups that can detect these problems because they think they're in good shape.
Additionally, the very factor that keeps people thin may cause diabetes. People who are naturally very thin are usually that way thanks to genetics, and a 2011 study revealed that these "lean genes" might work by placing fat very deep within the body. Rather than carrying a spare tire, these people might be carrying fat around the heart or the liver. According to the study findings, fat located at these spots might put a person at higher risk for diabetes than visible fat [sources: Naish; Park].