You may not be aware of what NAD and FAD are. There are both known to be electron carriers, and they are expected to do many different roles. FAD stands for Flavin Adenine Dinucleotide whole NAD stands for Nicotinamide Adenine Dinucleotide.
NAD is known to be found in almost all living beings and is needed by the body, while FAD is known to contribute to a lot of metabolic reactions that are available. The main difference between the two is the number of hydrogen atoms that can accommodate. FAD can get up to two hydrogens, but NAD can only accommodate one. These two are both very useful in the Krebs Cycle.