Regional brain glucose hypometabolism occurs during aging and may contribute to the onset of aging-related cognitive impairment in humans. Ketones are the main alternative energy substrate to glucose for the brain. Several studies show that brain ketone uptake and metabolism are unaltered in cognitively-normal older persons and in Alzheimer's disease (AD). Nutritional ketosis is reported to have a positive impact on cognitive performances in mild cognitive impairment and AD. Nevertheless, changes in regional brain glucose and ketone uptake in adults are poorly understood during diet-induced experimental ketosis. The investigators hypothesis was that during diet-induced ketosis, brain ketone uptake would be directly correlated with the elevation of blood ketone levels and inversely correlated to brain glucose uptake.