A growing body of evidence indicates that aging of the brain is strictly related to the decline of energy metabolism. In particular, in older adults, the neuronal metabolism of glucose declines steadily resulting in a growing deficit of ATP production. The decline is evoked by deficient NAD recovery in the salvage pathway and subsequent impairment of the Krebs cycle. NAD deficit impairs also the activity of NAD-dependent enzymes. All these open vicious circles of neurodegeneration and neuronal death. Some brain structures are particularly prone to aging and neurodegeneration. These are pathological foci of neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease. This review article summarizes the impacts and mutual relationships between metabolic processes both on neuronal and brain levels. It also provides directions on how to reduce the risk of neurodegeneration and protect the elderly against neurodegenerative diseases.