Lipids are insoluble in water; therefore, they are transported as complexes of lipoproteins and specific apoproteins. These proteins also serve as ligands to specific receptors, they facilitate transmembrane transport, and they regulate enzymatic activity. Lipoproteins may be classified according to their density, as follows: chylomicrons, very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).