total fatty acids In biochemistry, a fatty acid is a carboxylic acid typically with a long aliphatic chain, which is either saturated or unsaturated. The majority of naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ranging from 4 to 28. Fatty acids are commonly derived from phospholipids ortriglycerides. Fatty acids are important sources of fuel because, when metabolized, they produce large quantities of ATP. A lot of cell types can use either fatty acids or glucose for this purpose. Long-chain fatty acids cannot cross the blood–brain barrier and so cannot be utilized as fuel by the cells in the central nervous system; nevertheless, free short-chain fatty acids and medium-chain fatty acids can cross the blood–brain barrier, except for glucose and ketone bodies.