"This is why hemoglobin A1C is frequently used in studies that try to correlate blood sugar control to various disease processes like Alzheimer’s, mild cognitive impairment, and coronary artery disease. It’s well documented that glycated hemoglobin is a powerful risk factor for diabetes, but it’s also been correlated with risk for stroke, coronary heart disease, and death from other illnesses. These correlations have been shown to be strongest with any measurement of hemoglobin A1C above 6.0 percent. We now have evidence to show that elevated hemoglobin A1C is associated with changes in brain size. In one particularly profound study, published in the journal Neurology, researchers looking at MRIs to determine which lab test correlated best with brain atrophy found that the hemoglobin A1C demonstrated the most powerful relationship.[2]”

[1] Perlmutter, David. “Grain Brain.”

[2] “C. Enzinger, et al., “Risk Factors for Progression of Brain Atrophy in Aging: Six-year Follow-up of Normal Subjects,” Neurology 64, no. 10 (May 24, 2005): 1704–11.”