Diabetes is bad for the brain, especially if you are overweight, a new study suggests.

Overweight diabetics may be at greater risk of developing irreversible brain abnormalities and cognitive problems.

MRI scans of 50 overweight or obese South Koreans with early-stage type 2 diabetes (TD2) showed more severe and progressive abnormalities in brain structure than those of the normal-weight participants.

The research, published in the journal Diabetologia, highlights a need for greater awareness of the combined effects of obesity and diabetes on the brain, the authors say.

"Our findings also highlight the need for early intervention aimed to reduce risk factors for overweight or obesity in type 2 diabetic individuals to preserve their brain structure and cognitive function," they wrote.

Chronic type 2 diabetes is already known to increase the risk of a range of health problems in multiple organs.

Complications in the brain caused by the disease may accelerate cognitive dysfunction and even increase the risk of dementia.

While the exact mechanism underlying how type 2 diabetes alters the brain is not fully understood, insulin resistance, poor blood sugar control and inflammation are suspected to play a role.

However, little is known about the effect excess weight in the presence of diabetes.

To investigate this combined effect on the brain, Dr In Kyoon Lyoo, at the Ewha Brain Institute at Ewha Womens University in Seoul and colleagues designed a study of 50 overweight or obese people age 30 to 60 who had been diagnosed with type 2 diabetes.

Fifty normal-weight people diagnosed with type 2 diabetes and 50 normal-weight people without diabetes also took part.

The researchers used magnetic resonance imaging (MRI) to examine each participant's brain structure.

Participants also were tested for memory, psychomotor speed and executive function, since these are known to be affected in people with type 2 diabetes.

The results aligned with the researchers' initial assumptions, said Dr Lyoo.

Clusters of grey matter - a major component of the central nervous system -were significantly thinner in the temporal, prefrontoparietal, motor and occipital cortices in the brains of diabetic participants compared with the brains of those in the non-diabetic group, the study found.

More thinning of the temporal and motor cortices - areas of the brain that control motor, executive and cognitive functions, concentration and body awareness - could also be seen in the overweight/obese diabetic group.

"These findings suggest that weight status may play additive roles in T2D-related brain and cognitive alterations," the authors wrote.

They did, however, caution that the study did not recruit overweight/obese individuals without diabetes and said further studies were needed.