The study looked at 900,000 men and women and found that those with a BMI in the normal range had a lower death rate.
The higher BMI ranges increased the likelihood of kidney and liver disease, as well as heart and lung disease.
A jump in BMI ranges – for example going from a BMI of 24 to one of 32, which means entering the obese range, could cut life span by three years.
Dr Gary Whitlock,who led the trial said: “Excess weight shortens human lifespan. In countries like Britain and America, weighing a third more than the optimum shortens lifespan by about 3 years.”