**5. Conclusions**

The metabolic cost of brain activity is high. Neuronal activity accounts for 80% of brain energy consumption. Glucose is an essential energy source for the adult human brain. It can be used to obtain energy and to produce metabolic intermediaries for biosynthesis of compound of biological interest. Glucose can also be stored in brain, in the form of glycogen. Glucose oxidation via glycolysis occurs in all brain cells. However, the ability of neuronal cells to activate this metabolic pathway is poor. Astrocytic glycolysis and glycogenolysis is activated in response to synaptic activation with the subsequent formation of lactate. Thus, during glutamatergic synaptic activity neurons preferably consume lactate released from glia. The energetic coupling between neuronal and astroglial cells is essential to meet energy brain needs in an efficient way. Astrocytes interact with each other and with endothelial cells from the blood brain barrier and smooth muscle cells surrounding arterioles. Under fasting conditions, during intense exercise and in suckling infants, ketone bodies are a significant source of energetic fuel for brain. Metabolic failure has been described in several neurodegenerative diseases. These diseases present atypical protein assemblies, failures in normal protein degradation pathways, induced cell death, impaired axonal transport and metabolic failures. Almost all cellular processes need energy. It is thus possible that metabolic failure is an early event in these pathologies. This hypothesis is supported by evidence that shows metabolic failure in pre-symptomatic patients suffering from diseases such as Alzheimer's disease and Huntington's disease.
