A dramatic study, published in the British Medical Journal, revealed that teens with obesity had a similar risk of premature death as those who smoked half a pack of cigarretes a day.  In this study, nearly 46,000 Swedish men were followed over 38 years.  The men who were obese at age 18 were two times more likely to die by age 60 compared to men who were of normal weight.  This was about the same risk faced by the normal weight men who smoked half a pack of cigarettes or more a day. 

Given that smoking is known to be the most important cause of preventable death,  it is quite dramatic that obesity is apparently as lethal.