Certain types of brain cells may be “picky eaters,” seeming to prefer one specific energy source over others, new research shows. The finding has implications for understanding the cognitive decline seen in aging and degenerative diseases such as Alzheimer’s and multiple sclerosis. Studying mice, the scientists showed that a specific energy source called NAD is important in cells responsible for maintaining the overall structure of the brain and for performing complex cognitive functions. NAD (nicotinamide adenine dinucleotide) is a molecule that harvests energy from nutrients in food and converts it into a form cells can use.