If fatty acid synthesis and β oxidation were to proceed simultaneously, the two processes would constitute a futile cycle, wasting energy. We noted earlier that β oxidation is blocked by malonyl-CoA, which inhibits carnitine acyltransferase I. Thus, during fatty acid synthesis, production of the first intermediate, malonyl-CoA, shuts down β oxidation at the level of a transport system in the inner mitochondrial membrane. This control mechanism illustrates another advantage of segregating synthetic and degradative pathways in different cellular compartments.