 Fatty acids can also be broken down to acetyl @-@ CoA by beta @-@ oxidation . Each round of this cycle reduces the length of the acyl chain by two carbon atoms and produces one NADH and one FADH2 molecule , which are used to generate ATP by oxidative phosphorylation . Because NADH and FADH2 are energy @-@ rich molecules , dozens of ATP molecules can be generated by the beta @-@ oxidation of a single long acyl chain . The high energy yield of this process and the compact storage of fat explain why it is the most dense source of dietary calories . 
