I’ve received a posting from Dr. Cannell, of the Vitamin D Council, regarding several studies on Vitamin D and Cancer. While in general higher levels of Vitamin D are associated with lower risks of cancer, the effect appears to be blocked by high intakes of “pre-formed” vitamin A (retinol). This form of vitamin A is found in high doses in Cod Liver oil. Daily intake of 5000 Units of Retinol are probably OK (for adults, correspondingly lower for children), for those who feel strongly that they should take Cod Liver Oil.
Beta-carotene is a form of vitamin A which must be converted in the body to the active state, and (as far as we presently know) is not a problem.
The newsletter from Dr. Cannell (available on the website above) also contains the statement that “it takes the elderly up to ten times more time in the sun that the young to make an equivalent amount of vitamin D.” I’m not aware of the data that backs this statement up but do consider Dr. Cannell to be authoritative and certainly current on vitamin D research.