Middle-aged spread may be normal for a person of 50 or older but weight gain is associated with shortened life span right up to the age of 75, it was claimed.
The study, involving almost a third of a million people, showed that ideally a person of 60 should weigh the same as someone of 30.
Dr June Stevens and a team from the University of North Carolina at Chapel Hill analysed deaths among healthy men and women taking part in the American Cancer Society's Cancer Prevention Study.
None of the volunteers had ever smoked, was sick or had a history of heart disease, stroke, cancer or recent unintentional weight loss. Account was taken of age, education, physical activity and alcohol consumption.
From the age of 30 to 74, the fewest deaths occurred among those who had a body mass index of between 19 and 22. BMI, a measure of obesity, is calculated by dividing a person's weight in kilograms by the square of their height in metres.
By comparison, a typical female fashion model has a body mass index of about 18.
The results, published in the New England Journal of Medicine, showed the optimal weight for a 5ft 4in woman to be between 111lb and 128lb pounds (about eight to nine stone) and for a 5ft 10in man between 133lb and 153lb (about nine-and-a-half to 11st).
A 5ft 10ins man between 30 and 44 faced a 20 per cent higher risk of death if he weighed 166lb (nearly 12st) than if he was 20lb lighter.
Dr Stevens said: "Weight gain throughout early and middle adulthood is actually the norm, but it's probably not a very healthy norm.
"The bottom line is 'weigh less and live longer' up until about age 75. After that, a little extra weight appears not to be a disadvantage. Unfortunately, it looks like you have to wait until you're 75 before you can enjoy a guilt-free piece of holiday fudge."Reuse content