Hello. This is an interesting question and I understand the reasons for asking it. Usually pre-formed vitamin A is given as retinyl esters and you are considering prescribing cis-retinoic acid. I am not aware that this is commonly done. When retinol or retinyl esters are taken there is an immediate and transient increase in trans- and 13-cis retinoic acids as well as the 4-oxo prducts. These then accumulate with time as expected to steady state. These steady state levels are highly variable between patients. I know this does not answer your question but it may be possible to project backwards to assess how much cis-retinoic acid is obtained from what approximate retinol dose. I am not aware of the controlling mechanism for the conversion of retinol to the active RAR forms but I imagine you want to bypass this by direct supplementation.

Thanks Gerald, I am actually not trying to bypass the normal metabolism of pre-formed vitamin A.  One cannot easily get pre-formed vitamin A at doses of 40,000 to 50,000 for daily use here in South Africa.  You may be aware of Dave Albert's work on reducing squamous cell carcinoma of the skin by using 40,000 i.u. daily so I need this for my patients and especially something that can be claimed from their medical aid.  I have used 50 -60,000 i.u. daily for the past 20 years.  I have super-strong bones (I also supplement vitamin D) and no age-related hearing loss. and I have never had any suggestion of vitamin A toxicity.  Thanks again.   Des