A recent article by the AP highlights a study performed by Imperial College of London which suggests that thin people may be just as "fat" as obese people.  The reason for this is invisible to the naked eye, but real nontheless.  "Being thin doesn't automatically mean you're not fat," said one of the doctors involved in the study.  Internal body fat, meaning body fat which surrounds the vital organs rather than being stored under the skin, is prevelent in otherwise thin people.  The study found that 45 percent of thin women and 60 percent of thin men had significant amounts of internal body fat.  So, what does this mean in regards to their health.  Well, it turns out that internal body fat may be even more dangerous than its visible counterpart.  While an apparantly obese person can actually be fit due to a rigorous lifestyle (think "NFL Lineman"), internal body fat implies an abscense of physical fitness and a proclivity towards degenerative disease.  The study suggests that significant amounts of internal body fat can cause heart and liver disease as well as diabetes.  

Where does the internal body fat come from? 

Internal body fat deposits result from eating an unhealthy, but low calorie, diet and shunning excercise.  In other words, dieting is not enough on its own.  Summarizing, one of the study's doctors stated, "If you just want to look thin, then maybe dieting is enough...But if you want to actually be healthy, then exercise has to be an important component of your lifestyle."