February 18, 2006
A large study attempted to show whether or not calcium and vitamin D prevent colon cancer. It was a strange study, using low doses over a short period.
There are probably those who believe that the latest study on calcium and vitamin D shows that neither is good for anything. But we should hesitate before going to that extreme. One can also believe that the study was not suited to draw this conclusion. Or, as it is stated in a leading editorial in “The New England Journal of Medicine:” the conclusion should be interpreted in light of the study was complicated and in light of the probability that the doses of calcium and vitamin D were too low.
The debate regards the insidious and widespread cancers of the colon and rectum. Half of a group of 36,282 American women between the ages of 50 and 79 took part in a seven year study where they received daily supplements of 1,000 mg calcium and 400 units vitamin D to see if reduced their risk of these diseases. The supplements given are the same as two normal calcium and vitamin D vitamin tablets, which many take to strengthen their bones. After the seven years the researchers assessed the number of women who developed colon and rectum cancer. The result was disheartening: Whether the women received supplements or placebo had not effect on the risk.
There was a single positive find buried in the data. The women who had the least vitamin D in their blood during the study had with statistical certainty the greatest probability of developing colon cancer. This could indicate that vitamin D has a positive effect. There was also a tendency, but only a tendency, that these women had the greatest benefit from the supplements.
Quite a lot of things contribute to that this conclusion be taken with a grain of salt. This is partially due to that the study was very complex.
Possibly the most important objection is that it “only” lasted seven years. It is believed that colon cancer takes 10-20 years to develop before it is diagnosed. It the supplements prevent a new cancer from forming it is clear that for this reason no effects will be found as early as after seven years. This has been considered: Participants in the study will be monitored further for the next five years.
If the goal was to show a difference within the seven year period, those responsible should have at least ended the study by examining the intestines of all of the participants in order to find early cancer stages, or polyps. This did not occur. There was neither the money nor the resources necessary to do over 35,000 intestinal examinations. It was only possible to establish that the number of independently undertaken intestinal exams and the number of discovered cancers in the two groups were about the same. But maybe nothing more can be expected.
One confusing detail is that the study participants were allowed to continue taking the supplements that they had taken before the study along with the supplements that they received as a part of the study. On average they received 1,100 mg calcium and 350 units vitamin D, both close to the recommended dosages, before the study began. Many of them therefore must have received very large doses of calcium, over 2,000 mg, per day. Is it reasonable to guess that this is the reason for the slightly increased frequency of self-reported kidney stones? 2.4% of those who received supplements and 2.1% of those who received placebo, got kidney stones during the seven years.
Also, the average age was relatively low (62), which reduced the risk of cancer, and therefore weakened the study. It was further weakened by the fact that more than one out of four participants did not finish the study. Whether this dropout rate is because calcium pills can cause constipation is not considered in the article.
Just as important, the dose of vitamin D, as referred to in the editorial, may have been too low. Recently it has been estimated that about 1,000 units daily is necessary for most people in order to achieve any supposed cancer preventing effect. This amount of vitamin D (or more accurately 25-(OH)-vitamin D) is necessary to achieve a serum concentration of over 30 nano-grams per litre (75 nanomols per millilitre). Nevertheless, only a minority of the study participants received this amount.
What can be concluded from this? The editorial gives some suggestions for new studies. Much indicates that vitamin D, and maybe calcium, prevents cancer. But we still lack sufficient knowledge.
By: Vitality Council
1. Wactawski-Wende J et al. Calcium plus vitamin D supplementation and the risk of colorectal cancer. N Engl J Med 2006;354:684-96.
2. Forman M C et al. Calcium plus vitamin D3 supplementation and colorectal cancer in women. N Engl J Med 2006;354:752-4.
3. Garland C F et al. The role of vitamin D in cancer prevention. Am J Publ Health 2006;96:9-18.