HgbA1c (hemoglobin A1c) is measure of average blood sugar levels over the previous three months. From a 2018 study:
In this community-based population, we observed a significant trend for cognitive decline over a 10 year period among individuals aged ≥50 years with normoglycaemia, prediabetes or diabetes at baseline. Additionally, HbA1c levels were linearly associated with subsequent cognitive decline in memory and executive function (but not orientation) irrespective of diabetes status at baseline.
Source: HbA1c, diabetes and cognitive decline: the English Longitudinal Study of Ageing | SpringerLink
h/t to Jan at The Low-Carb Diabetic
Steve Parker, M.D.
PS: You know what else helps prevent cognitive decline? The Mediterranean diet.
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Diabetic Mediterranean Diet 
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Being diabetic is bad for your health.