Cholesterol has a complicated cycle in the human body.
On the one hand, cholesterol is extracted from your food through the wall
of your intestine. On the other hand cholesterol is also broken down in the
body and ends up in your intestines. Furthermore, cholesterol is also absorbed
from what you eat in your intestines.
Plant sterols have a similar chemical structure as cholesterol. In the small intestine, plant sterols with cholesterol (from food or the cholesterol cycle) compete for absorption into the human body. The presence of plant sterols means neither cholesterol, nor plant sterols are absorbed into your blood stream. This causes a drop in bad cholesterol (LDL cholesterol).
In short, plant sterols cause a reduction of the LDL cholesterol and a reduction of the total cholesterol. HDL cholesterol (‘good cholesterol’) does not change.