Not every study is completely positive. In a large study, the National Cancer Institute found that blood levels of vitamin D higher than a certain moderate level were associated with a 72 percent risk reduction in colorectal cancer, but found no association between vitamin D blood levels and overall cancer mortality.
Studies on those taking more substantial doses of vitamin D have not been undertaken, however, and many researchers believe that the recommended daily allowance of 400 IU for vitamin D is woefully inadequate and that deficiencies are widespread.
There is epidemiological evidence as well. Pills and sunshine are the primary sources of the vitamin D (technically a hormone) and most of us no longer spend our working days outside.
Thus, the results of a 2006 study of cancer patients from over a dozen countries in the European Journal of Cancer were not too surprising. They pointed to a large difference in cancer risk between sunny and non-sunny countries for several different cancers.
Of course, prospective, retrospective, and observational studies are always a bit dubious because of the effect of confounding variables, non-uniform dosages, the confirmation biases of the experimenter (not unrelated to those of the columnist), subjects' poor memories and the like.
Their prevalence in the medical literature is one reason for the often seemingly contradictory conclusions announced. (How many epidemiologists does it take to change a light bulb? None. Their retrospective studies have already changed it.)
Thus, one of the most compelling studies of vitamin D is not of this sort. It is the 2007 four-year, double-blind, randomized, placebo-controlled trial conducted by Drs. Lappe, Travers-Gustafson, Davies, Recker and Heaney and involving approximately 1,200 women.
Cited at the beginning of this column, the study appeared in the American Journal of Clinical Nutrition and called for either 1,100 international units of vitamin D per day (not the the present RDA of 400 IU) and some calcium or else just the placebo.
The women were over 55 and from a nine-county area of rural Nebraska and the reduction of all-cancer incidence (I stress all-cancer incidence) for those taking the vitamin D supplements was, as noted, 60 percent for the four years and a whopping 77 percent for the last three years.
This bears reiteration. The women taking 1,100 IU of vitamin D developed less than one quarter of the cancers developed by women in the control group.
Since vitamin D is safe and cheap, many of those involved in the studies above suggest that there is every reason for people to increase their intake of vitamin D supplements. There is certainly every reason but one to investigate them further.
The purpose of my fictional introduction above is not only to defamiliarize vitamin D, but also to underline that reason. As is the case with aspirin, there unfortunately isn't much economic incentive to conclusively demonstrate the effectiveness of much higher doses of vitamin D.
The primary incentive is a moral one. We want people to enjoy sookhaphahpdee.
John Allen Paulos, a professor of mathematics at Temple University, is the author of the best-sellers "Innumeracy" and "A Mathematician Reads the Newspaper," as well as (just out in paperback) "Irreligion: A Mathematician Explains Why The Arguments for God Just Don't Add Up." His "Who's Counting?" column on ABCNews.com appears the first weekend of every month.