Patients who gained 10 to 55lbs cut their risk of premature death by a third.
The study presented at the American Diabetes Association in San Francisco last week, followed up 655 patients, aged an average of 28, with type-1 diabetes for 20 years.
BMI and waist circumference measurements were taken every two years. During follow-up, 147 participants died.
Diabetics who had a BMI over 30 were 33 per cent less likely to die than those with a normal BMI of 20 to 24.9.
Lead researcher Professor Trevor Orchard, from the University of Pittsburgh, proposed that gaining weight may be a sign that patients are getting enough insulin and appropriately controlling their disease.
But the results are not a firm recommendation for diabetics to put on weight, he added.
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