An association between hyperglycemia (which is high blood glucose levels) and cognitive dysfunction have been found in relatively healthy older individuals (Umegaki H, et.al, 2017).
Insulin resistance which happens when the insulin sensitivity in your tissue decreases was found to be associated with memory impairment and the individuals with diabetes were worse. The research makes it clear that if you want to improve your memory or prevent it from getting worse, you need to implement strategies to improve insulin sensitivity.
Even in young adults, hyperglycemia is associated with subtle brain injury and impaired memory and attention (Weinstein G, et.al., 2015).
When you eat a meal consisting of high glycemic index carbohydrates, your blood glucose levels will increase too much. While you may tolerate that occasionally, eating that way every day will affect your insulin sensitivity, and it will not be good for your memory.
It has been proposed that toxins generated by insulin resistance transit across the blood-brain barrier into the brain, where they induce insulin resistance to the brain tissue, creating inflammation and cell death (De La Monte SM, 20120.)
Diets high in trans-fat and saturated fat adversely affect cognition. Fruit, vegetables, cereal, and fish are associated with lower risk of dementia and better cognition (Parrot MD, Greenwood CE, 2007).
As you get more insulin resistant, ingestion of rapidly absorbed, high-glycemic index carbohydrates increase oxidative stress and inflammatory compounds. Cereals were listed as associated with lower risk. But, you have to be careful with cereals because most cereals are not low glycemic index.
For that reason, cereals may adversely affect your insulin sensitivity and not be your best choice. That does not mean you should avoid all carbohydrates. There are some healthy, very low glycemic index carbohydrates like beans, lentils, and vegetables. Also keep in mind that fish is contaminated now, especially with mercury. Contaminants have shown to interfere with the benefits of fish. It is better to use a high-quality fish oil to increase your omega 3 fatty acid intake. If you eat fish, wild salmon is still your best choice.
This is a summary of the 3 reasons why improving insulin sensitivity will improve your memory.
Increasing insulin sensitivity and eating low glycemic index food, what you eat will not increase your blood glucose that high. It will also not increase oxidative stress and inflammatory compounds. You will end up with less cell death and brain damage and instead see the improved memory.
Umegaki, H., Makino, T., Uemura, K., Shimada, H., Hayashi, T., Cheng, X. W., & Kuzuya, M. (2017). The associations among insulin resistance, hyperglycemia, physical performance, diabetes mellitus, and cognitive function in relatively healthy older adults with subtle cognitive dysfunction. Frontiers in aging neuroscience, 9.
Weinstein, G., Maillard, P., Himali, J. J., Beiser, A. S., Au, R., Wolf, P. A., ... & DeCarli, C. (2015). Glucose indices are associated with cognitive and structural brain measures in young adults. Neurology, 84(23), 2329-2337.
Suzanne, M. (2012). Metabolic derangements mediate cognitive impairment and Alzheimer's disease: role of peripheral insulin resistance diseases. Panminerva medica, 54(3), 171.
Parrott, M. D., & Greenwood, C. E. (2007). Dietary influences on cognitive function with aging. Annals of the New York Academy of Sciences, 1114(1), 389-397.
Greenwood, C. E., & Winocur, G. (2005). High-fat diets, insulin resistance and declining cognitive function. Neurobiology of aging, 26(1), 42-45.