A Canadian study that examined the relationship between body mass index and death among 11,326-adults over a 12-year period found, 70% of the underweight and 36% of those who are extremely obese, are more likely to die earlier than those of normal weight.  While, the slightly over-weights actually live longer than people who are of normal weight.
 
Those few extra pounds actually protect older people as their health begins to decline, but this should not encourage people of normal weight to gain a few extra pounds.
 
The reason why overweight people live longer is not exactly clear, but researchers theorize a few extra pounds provide a buffer when people fall sick.  Study co-author David Feeny explains that extra weight may be protective as one’s health declines, and you are less likely to lose a little weight and become frail due to health shocks like illness, since you had that little bit of extra to start with.
 
So, while being a bit overweight reduces your chances of dying, obesity does not, and that it’s actually better to be over-weight than under-weight.
 
More detailed studies on the link between weight and illness suggest, overweight people are more likely to die of heart disease, though they less likely to die of Alzheimer’s, Parkinson’s and other diseases.
 
But, it is to be noted that while overweight people are less likely to die, being overweight raises their risk of diseases like diabetes, heart disease and several forms of cancer, apart from the fact that excess weight impairs the quality of life.