Glucose metabolism in normal aging and Alzheimer's disease: Methodological and physiological considerations for PET studies.

Alzheimer's disease (AD) is an age-dependent neurodegenerative disorder associated with progressive loss of cognitive function. 2-[18F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) has long been used to measure resting-state cerebral metabolic rates of glucose, a proxy for neuronal activity. Several FDG PET studies have shown that metabolic reductions occur decades before onset of AD symptoms, suggesting that metabolic deficits may be an upstream event in at least some late-onset AD cases. This review explores this possibility, initially discussing the link between AD pathology, neurodegeneration, oxidative stress and AD, and then discussing findings of FDG PET hypometabolism in AD patients as well as in at-risk individuals, especially those with a first-degree family history of late-onset AD. While the rare early-onset form of AD is due to autosomal dominant genetic mutations, the etiology and pathophysiology of age-dependent, late-onset AD is more complex. Recent FDG PET studies have shown that adult children of AD-affected mothers are more likely than those with AD-fathers to show AD-like brain changes. Given the connection between glucose metabolism and mitochondria, and the fact that mitochondrial DNA is maternally inherited in humans, it is here argued that altered bioenergetics may be an upstream event in those with a maternal history of late-onset AD. Biomarkers of AD have great potential for identifying AD endophenotypes in at-risk individuals, which may help direct investigation of potential susceptibility genes.