Ketone bodies are by products of metabolism of?

Free fatty acids are the precursors of ketone bodies in the liver. While an active enzymatic mechanism produces acetoacetate from acetoacetyl-CoA in the liver, acetoacetate once formed cannot be reactivated directly except in the cytosol, where it is used in a much less active pathway as a precursor in cholesterol synthesis. This accounts for the net production of ketone bodies by the liver. Ref: Botham K.M., Mayes P.A. (2011). Chapter 22. Oxidation of Fatty Acids: Ketogenesis. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.