Plasma lipid concentrations are the net result of the balance between two opposite processes: the loading, i.e. the entry of new lipids into the plasma compartment through the ingestion (diet) and/or endogenous synthesis, and the unloading, i.e. energy utilization, incorporation into cell membranes, and storage. Even though many fatty acids are thought to be synthesized in the body, it appears as nearly all circulating fatty acids are, in fact, derived from the diet. In view of the wide dietary availabilty of such nutrients, the need to conserve energy likely minimizes endogenous synthesis. Consequently, the possibility exists to alter circulating the profile of fatty acids, including the "non essential" ones by dietary manipulations. In turn, a theory on the dietary vs endogenous contribution to circulating fatty acids, including those known as non-essential, is discussed based on critical interpretation of original data.