Obesity can reduce life expectancy by eight years in people with Type 2 diabetes, research suggests.
Obesity is comon among people with diabetes
A study by the University of Surrey and the Royal Free and University College Medical School looked at over 44,000 patients with 'adult onset' diabetes.
Benet Middleton, Chief Executive of the charity Diabetes UK said the findings were worrying as 80% of people with Type 2 diabetes are overweight.
He said the findings are a wake up call to the government to act on obesity.
Type 2 diabetes is the most common form of the disease, accounting for 80% of all diabetes cases in the UK, around 1.2 million people.
It develops when the body becomes resistant to the effects of insulin, and is most common in people over 40.
Obesity is a major risk factor in developing Type 2 diabetes.
The researchers looked at patient's body mass index (BMI) to classify those with obesity.
A person's BMI is calculated by dividing weight in kilograms by height in metres squared.
A BMI of over 30 is considered obese.
They found people with Type 2 who also had a BMI of 35 or more had an average life expectancy of 70, compared to 78 for those in the healthy BMI range of 20 - 24.
Researcher Professor Ross Lawrenson from the University of Surrey said "The life expectancy in this group was higher than we would normally expect, because we used an older cohort of people with an average age of around 63, and many were already in their late 70's."
Diabetes UK said people with diabetes already had a higher risk of complications and premature death which obesity made even worse.
Mr Middleton told BBC News Online: "Diabetes on its own can reduce life expectancy. Obesity can do so even further."
Lead author of the study, Henrietta Mulnier, at the University of Surrey, said this is the first study of this size to confirm obesity can cause increased mortality rates among people with Type 2 diabetes.
"The beauty of this study is that it was done in a large cohort, meaning the findings are more reliable," she told BBC News Online.
The research was presented at the Diabetes UK annual medical conference in Birmingham.
An increasing problem
Mr Middleton said: "We know that Type 2 diabetes is on the increase due to a change in lifestyle and diet.
"We are also seeing Type 2 diabetes among children which was previously unheard of."
He said people with diabetes need to manage their weight so they can reduce the risk of serious complications such as heart attack and stroke.
He added that the charity is keen to see much clearer action on obesity, such as better labelling of food, bans on junk food advertising, promoting healthy eating and awareness among parents.