Vincent77 you bring up a good point about the difference between natural form vitamins and chemical form.
There is a difference in bioavailability between the various forms, but the natural form isn't always the most bioavailable.
Take vitamin c, vitamin e, and the amino acid tocopherol for example.
Ester-c, ester-e, and d-alpha-tocopherol are all chemical derivitaves. But they absorb into the body better than the natural form due to them being bonded to another nutrient that allows it to pass through your stomache lining easier.