In addition, various associations have been reported for carotenoids, including decreased risk of macular degeneration, cataracts, coronary heart disease, and a variety of cancers. These are all associative phenomena that have generated a great deal of interest. Much effort and energy and money has been put into trying to answer that question and the answer as of today is, we don't know. I think many of us would want to rephrase the question to ask about carotenoids in general rather than beta-carotene. John Bertram observed that carotenoids have a very potent effect of inducing the synthesis of one of the connexon proteins involved in gap junctions.
Gap junctions are very important for allowing cells to communicate with one another.
Bertram found that tumor cells are very deficient in gap junctions, and that when carotenoids are added to cultures of tumor cells, gap junction communication increases, and normal cells can now communicate with them and presumably regulate their growth. This very important observation was considered to be an intrinsic property of carotenoids, but that explanation is now in doubt. However, human intervention studies have not been supportive of a role for beta-carotene as an anticarcinogenic compound. For example, two studies from Greenberg's group, dealing with either skin cancer or colorectal adenomas, have not found any beneficial effect of supplemental beta-carotene. Alpha-tocopherol had no effect on lung cancer incidence or mortality.