Insulin is not required for glucose to enter brain cells. Alzheimer's disease, then, is not a type 3 diabetes. Later in the disease, the oxidation of glucose transporters does reduce glucose levels in brain cells (Mark et al. 1997).

Insulin resistance does contribute to Alzheimer's disease as it initially increases the amount of glucose in the brain because the glucose is not being "absorbed" in other parts of the body (Jacob et al., 2002). This results in high levels of myo-inositol, the precursor molecule to Alzheimer's disease (Hauser and Finelli 1963; Huang et al., 1995) and to the activation of phospholipase C gamma-gamma, an enzyme implicated in triggering Alzheimer's disease, primarily via the platelet-derived growth factor receptor (Dequin et al., 1998; Okuda et al., 1996). Polyphenols in various fruits, vegetables, and spices, and polyunsaturated fats such as fish oil partially inhibit phospholipase C gamma and thus provide some protection against the disease (Godichaud et al. 2006; Kang et al., 2003; Sanderson and Calder, 1998; Valente et al., 2009).

References:
Dequin Z, Dhillon H, Prasad MR, and Markesbery WR. Regional levels of brain phospholipase Cγ in Alzheimer's disease. Brain res. 811(1998): 161-5. Abstract

Godichaud S, Si-Tayeb K, Auge N, Desmouliere A, Balabaud C, et al. The grape-derived polyphenol reveratrol differentially affects epidermal and platelet-derived growth factor signaling in human liver myofibroblasts. Inter J of Biochem and Cell Biol 38 (2006): 629-37. Abstract

Hauser G and Finelli VN. The biosynthesis of free and phosphatide myo-inositol from glucose by mammalian tissue slices. J Biol Chem 238(1963): 3224-28. Abstract

Huang W, Alexander GE, Daly EM, Shetty HU, Krasuki JS, et al. High brain myo-inositol levels in the predementia phase of Alzheimer's disease in adults with Down's syndrome: a 1H MRS Study. J Clin Invest 95(1995): 542-6. Abstract

Kang MA, Yun SY, and Won J. Rosmarinic acid inhibits Ca[2+]-dependent pathways of T-cell antigen receptor-mediated signaling by inhibiting the PLC-y1 and Itk activity. Blood 101(2003): 3534-42. Abstract

Okuda Y, Adrogue HJ, Nakajima T, Mizutani M, Asano M, Tahci Y, et al. Increased production of PDGF by angiotensin and high glucose in human vascular endothelium. Life Sci 59 (1996): 1455-61. Abstract

Sanderson P and Calder PC. Dietary fish oil appears to prevent the activation of phospholipase Cγ in lymphocytes. Biochim et Biophys Acta 15(1998); 300-8. Abstract

Valente T, Hidalgo J, Bolea I, Ramirez B, Angeles N, et al. A diet enriched in polyphenols and polyunsaturated fatty acids, LMN diet, induces neurogenesis in the subventrical and hippocampus of adults mouse brain. J of Alzh Dis 18(2009). Abstract

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