Why is learning and memory dysfunction in Type 2 diabetes limited to older adults?

Review of the literature on the cognitive correlates and consequences of Type 2 diabetes reveals two very intriguing findings. Not only are verbal learning and memory skills most likely to be disrupted as compared to other cognitive skills (e.g. attention, executive function; psychomotor efficiency), but these mnestic deficits appear to be restricted to individuals with diabetes who are older than 60-65 years of age. Middle-aged adults with either Type 2 or Type 1 diabetes are apparently protected insofar as researchers have only infrequently reported learning and memory impairments in that age group. Why do older adults have such an increased risk of diabetes-associated memory dysfunction? In our view, this phenomenon is a consequence of a synergistic interaction between diabetes-related metabolic derangements and the structural and functional changes occurring in the central nervous system (CNS) that are part of the normal ageing process. To critically evaluate that possibility, we summarise what is known about learning and memory dysfunction in the adult with diabetes, examine the extent to which chronic hyperglycaemia may adversely affect the integrity of the CNS, and selectively review the literature on age-associated changes in brain morphology and cognitive function, paying special attention to the threshold theory of cognitive impairment.