VLDL, a precursor for LDL, is produced in liver. Both are more or less the same chemically, but differ in the amount of fat carried. LDL is VLDL but somewhat processed by body, HDL is a VLDL (LDL) completely processed by body.

Bile is used to process food in the gut. It does not go back into our system. Bile is still produced by liver even in long fasts.

Oatmeals is a kind of elimination diet, much like carnivore diet or rice diet. The later one also lowered cholesterol.

What oatmeal diet really does is it completely eliminates essential fatty acids in food. These fatty acids are critical in VLDL production and, thusly, oatmeal diet reduces LDL levels through less production of VLDL.