Poor Glucose control harms region of the brain involved with memory

Poor glucose control, or excessive consumption of sugar, can harm a brain region that helps form long-term memories, probably by reducing insulin sensitivity and causing poor mitochondrial function.

The Dentate Gyrus, which is a region of the hippocampal formation, helps the brain store memories. The Dentate Gyrus is also a brain region which creates new neurons through a delicate process called neurogenesis, in which progenitor stem cells specialize into fully functional neurons. Exercise has been shown in studies to boost neurotrophic factors which help form new brain cells in this region of the brain.

http://www.nytimes.com/2009/01/01/health/31memory.html