A chemical may be a chemical but is there a difference in the way the body behaves when its taken orally versus through the skin. For instance transdermal estrogen is metabolized better than estrogen that has to take a pass through the liver.
For instance transdermal estrogen is metabolized better than estrogen that has to take a pass through the liver.
Apparently it's not a matter of metabolism but its action on certain processes in the liver:
The dissociation of the GH/IGF-I axis by the oral route is likely to arise from impaired hepatic IGF-I production which causes increased GH secretion through reduced feedback inhibition. The route of oestrogen therapy confers divergent effects on substrate oxidation and body composition. The suppression of lipid oxidation during oral oestrogen therapy may increase fat mass while the fall in IGF-I may lead to a loss of lean body mass. The route dependent changes in body composition observed during oestrogen replacement therapy may have important implications for post-menopausal health and oestrogen use in general.
--Metabolic effects of oestrogens: impact of the route of administration.,
Ho KK, O'Sullivan AJ, Wolthers T, Leung KC.
Ann Endocrinol (Paris). 2003 Apr;64(2):170-7.
In the case of vitamin D, it is the precursor that is produced in the skin, not the vitamin. It is metabolized in the liver into the vitamin form. Oral vitamin D precursor is absorbed and similarly metabolized in the liver into the same active form.