Low-density lipoprotein (LDL) is a lipoprotein that transports cholesterol and triglycerides from the liver to peripheral tissues. LDL also regulates cholesterol synthesis at these sites.

Because high levels of LDL cholesterol can signal medical problems like heart disease, it is sometimes called "bad cholesterol" (as opposed to high density lipoprotein or HDL, the "good cholesterol"). It's important to note, however, that LDL cholesterol appears harmless until oxidized. That's why getting sufficient antioxidants in the diet is important for heart health.

Insulin's role in LDL cholesterol is also important to note. Insulin boosts an enzyme called HMG-CoA reductase which increases LDL cholesterol. But glucagon downregulates HMG-CoA. Because glucagon production is stimulated by protein and insulin is stimulated by carbohydrate, its important for heart health to choose high-protein, low glycemic foods. 

References

Inhibition of in vitro human LDL oxidation by phenolic antioxidants from grapes and wines. Teissedre, P.L. : Frankel, E.N. : Waterhouse, A.L. : Peleg, H. : German, J.B. J-sci-food-agric. Sussex : John Wiley : & : Sons Limited. Jan 1996. v. 70 (1) p. 55-61.; Effect of antioxidants on oxidative modification of LDL. Esterbauer H, Puhl H, Dieber-Rotheneder M, Waeg G, Rabl H. Ann Med. 1991;23(5):573-81.