Not can't but more difficult. The immune system will eventually break it down and rid itself of the byproducts, as it generally recognizes these molecules as foreign. Sometimes the foreign molecules are simply extreted by stool or urine. But there's a chance that some small percentage gets sequestered within adipocytes and stem cells before an immune response can 'catch it'. When those cells turn over, the body gets another chance to rid itself of those. An even smaller percentage of artificial isomers (ex: L-sucrose versus D-sucrose) stick around for a long time because they aren't affected by the intermediates and processes of metabolic cycles--this is the problem with creating artifical molecules not ever occurring in nature.

Lesson: don't put fake industrial chemicals in your body.