Adults who can no longer fit into the trousers they wore when they were 21 are at risk of developing diabetes even if they are not technically overweight, an expert in the disease has warned.
Professor Roy Taylor, director of Newcastle University's Magnetic Resonance Centre, said that people's waist should be the same size now as it was when they were 21.
The study's findings showed that people of healthy weight and who have type 2 diabetes can reverse the condition by losing between 10 and 15 per cent of their body weight.
He said: “As a rule of thumb, your waist size should be the same now as when you were 21.
"If you can’t get into the same size trousers now, you are carrying too much fat and therefore at risk of developing type 2 diabetes, even if you aren’t overweight.”
In the study, eight out of 12 participants - who all had the condition despite having a "normal" body mass index (BMI) - managed to reduce the levels of fat in the liver and pancreas, and the activity of the insulin-producing cells in the pancreas was deemed to be "restored".
Professor Taylor, principal investigator of the study, said: "Doctors tend to assume that type 2 diabetes has a different cause in those who aren't overweight.
"This means that, unlike those who are overweight, those who are of normal weight aren't usually advised to lose weight before being given diabetes drugs and insulin.
"Instead, there's a tendency to start them on insulin and other medication at a much earlier stage.
"What we've shown is that if those of normal weight lose 10-15 per cent of their weight, they have a very good chance of getting rid of their diabetes."
The participants in the study, who had an average BMI of 24.5, followed a weight loss programme that included a low calorie liquid diet for two weeks - where each day they were only consuming 800 calories a day through soups and shakes.
This was followed by four to six weeks where they received support to maintain their new weight.
They completed three rounds of this programme until they lost 10 to 15 per cent of their body weight.
After weight loss was achieved, scans showed reductions in the fat in the liver and eight of the 12 participants saw their type 2 diabetes go into remission, which was defined as having blood sugar levels under control and patients no longer needing any medication.
Prof Taylor added: "These results, while preliminary, demonstrate very clearly that diabetes is not caused by obesity but by being too heavy for your own body. It's due to having too much fat in your liver and pancreas, whatever your BMI.
"In the liver, this excess fat prevents insulin from working normally. In the pancreas, it causes the beta cells to stop producing insulin.
"As a rule of thumb, your waist size should be the same now as when you were 21.
"If you can't get into the same size trousers now, you are carrying too much fat and therefore at risk of developing type 2 diabetes, even if you aren't overweight."
Dr Ahmad Al-Mrabeh, the study's first author and co-investigator, added: "Weight loss by calorie restriction is powerful in achieving remission of type 2 diabetes and it is remarkable to have 67 per cent remission in this non-obese group."
The online meeting also heard data on another study which found that the amount of fat stored in the liver is higher in people with type 2 diabetes, whatever their BMI.
It has previously been suggested that excess fat stored in the liver and pancreas drives the development of type 2 diabetes and loss of this fat is key to remission.
Researchers led by the team at Newcastle examined two groups of patients following a weight-loss programme, with scans used to measure levels of fat in the liver and pancreas before and after weight loss.
Liver and pancreatic fat were higher in those with diabetes, whatever their BMI, and these levels fell with weight loss.
The researchers suggested that seemingly "healthy" levels of liver fat can trigger type 2 diabetes.
They said that the way liver fat is assessed should change to also take into account a patient's BMI.
Meanwhile, a separate study presented to the meeting suggested that people who are "healthy obese" - obese people who do not have any problems with high blood pressure, blood fats or diabetes - are not at a greater risk of heart attack or stroke compared with healthy people of normal weight.
But this group are more likely to develop heart failure.
The study authors tracked data on nearly three million French people, including more than 270,000 who were obese, for at least five years.
People deemed to have "metabolically health obesity" had a 34 per cent increased risk of heart failure and a 33% higher risk of developing atrial fibrillation.
But they did not appear to have a higher risk of heart attack, stroke, or cardiovascular death than metabolically healthy people of normal weight.