Should all older adults with type 2 diabetes lower their HbA1c level to <7.5 right away so as to prevent dementia in the long run?

Dr. Linda Lam is Professor in the Department of Psychiatry at the Chinese University of Hong Kong (CUHK). Here she discusses her Age and Ageing paper Higher dementia incidence in older adults with type 2 diabetes and large reduction in HbA1c.

Dementia is a major health problem worldwide. It is particularly relevant to people with type 2 diabetes because their number is growing, they are twice as likely to develop dementia as those without, and there is currently no cure for dementia. While both high and low glycemic levels have been reported to increase the risk of dementia in older adults with diabetes, it remains unclear whether achieving an optimal glycemic target according to the current treatment guideline can help lower the risk in the long run.

We investigated whether lowering the glycemic level from a pre-existing high level to the target level recommended by the American Diabetes Association (ADA) within a year was associated with lower risk of dementia in older adults with diabetes. We studied over 2000 men and women aged 65 and above with type 2 diabetes but without dementia who lived at home and attended the Elderly Health Centre in Hong Kong. We followed them up regularly for 6 years to monitor their glycated hemoglobin (HbA1c) level, which reflected their average blood glucose levels for the past 3 months, and see who developed dementia. With reference to the ADA recommendation, optimal glycemic control was defined as HbA1c less than 7.5. Dementia was diagnosed by clinicians based on the standard diagnostic criteria as described in the 10th revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10) and Clinical Dementia Rating (CDR). We also assessed other health behaviours and problems such as whether the subjects had admissions to hospital since the last interview.

Consistent with previous studies, our findings showed that a high and increasing HbA1c level was associated with higher risk of incident dementia in 6 years. However, to our surprise, we found that among people whose HbA1c was greater than or equal to 7.5 at baseline, those who achieved the glycemic target within a year were associated with higher rather than lower incidence of dementia in 6 years. We repeated the analysis by comparing the risk of dementia between those whose HbA1c level was initially above the glycemic target and dropped by over 10% within a year and those whose HbA1c level declined by only 5-10% in a year. Our findings showed that the former, but not the latter, was associated with higher risk of dementia in 6 years, and this association was independent of other health factors.

Our results suggest that a large reduction in HbA1c could be a potential predictor and possibly a risk factor for dementia in older adults with diabetes. From a clinical perspective, healthcare professionals need to be attentive to both increasing and decreasing HbA1c levels and be cautious in optimizing or intensifying glycemic control in older patients with hyperglycemia. Future studies should examine what the optimal glycemic target for older adults with diabetes should be, and how it should be achieved so that the risk of diabetes complications can be minimized without adversely affecting cognition.