In older people, type 2 diabetes is associated with a decline in brain function over 5 years, study shows

However, contrary to previous studies, the decrease in brain volume often found in older people with T2D was not found to be directly associated with cognitive decline during this time period. Yet compared with people without T2D, those with T2D had evidence of greater brain atrophy at the beginning of the study.

Previous research has shown that T2D can double the risk of dementia in older people. In this new study, Dr Michele Callisaya (University of Tasmania, Hobart, TAS, and Monash University, Melbourne, VIC, Australia) and colleagues aimed to discover whether type 2 diabetes is associated with greater brain atrophy and cognitive decline, and whether the two are linked. It is the first study to compare decline in both cognition and brain atrophy between people with and without T2D together in the same study.

The trial recruited 705 people aged 55-90 years from the Cognition and Diabetes in Older Tasmanians (CDOT) study. There were 348 people with T2D (mean age 68 years) and 357 without (mean age 72 years) who underwent brain MRI (lateral ventricular and total brain volume -- measures of brain atrophy) and neuropsychological measures (global function and seven cognitive domains) at three time points over a mean follow-up period of 4.6 years.

The results were adjusted for age, sex, education and vascular risk factors including past or current smoking, heart attack, stroke, high blood pressure, high cholesterol, and body mass index. The authors reported there were significant associations found between T2D and greater decline in both verbal memory and verbal fluency.

Although people with diabetes had evidence of greater brain atrophy at the start of the study, there was no difference in the rate of brain atrophy between those with and without diabetes over the time course in this study. There was also no evidence in the study that the rate of brain atrophy directly impacted on the diabetes-cognition relationship.

In people without type 2 diabetes, verbal fluency slightly increased on average each year (0.004 SD/units per year), whereas it declined in those with type 2 diabetes (?0.023 SD/units per year). The authors say: "Such accelerated cognitive decline may contribute to executive difficulties in everyday activities and health behaviours -- such as medication compliance -- which in turn may poorly influence future vascular health and cognitive decline, and possibly an earlier onset of dementia in those with type 2 diabetes."

They add: "Contrary to our hypotheses and results from previous cross-sectional studies, the rate of brain atrophy over these 5 years of study did not directly mediate associations between type 2 diabetes and cognitive decline. It is possible that greater accrual of cerebrovascular disease than occurred in our study may be more likely to reveal whether there is such a relationship."

They conclude: "In older community-dwelling people, type 2 diabetes is associated with a decline in verbal memory and fluency over approximately 5 years, but the effect of diabetes on brain atrophy may begin earlier, for example in midlife, given the evidence of greater brain atrophy in people with T2D at the start of the study. If this is the case, both pharmacological and lifestyle interventions to prevent brain atrophy in people with T2D may need to commence before older age."