From a Neurology.org online Journal article:

Low and high levels of hemoglobin are associated with an increased risk of dementia, including AD, which may relate to differences in white matter integrity and cerebral perfusion.

Objective: To determine the long-term association of hemoglobin levels and anemia with risk of dementia, and explore underlying substrates on brain MRI in the general population.
Methods: Serum hemoglobin was measured in 12,305 participants without dementia of the populationbased Rotterdam Study (mean age 64.6 years, 57.7% women). We determined risk of dementia and Alzheimer disease (AD) (until 2016) in relation to hemoglobin and anemia. Among 5,267 participants without dementia with brain MRI, we assessed hemoglobin in relation to vascular brain disease, structural connectivity, and global cerebral perfusion.

To read more click on following link: https://n.neurology.org/content/neurology/early/2019/07/31/WNL.0000000000008003.full.pdf