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IMAGE Cancer is the second major cause of death in the United States. While techniques of cancer diagnosis and treatment have steadily improved, they remain inadequate; and for this reason, scientists continue to investigate possible methods of cancer prevention .

Numerous supplements and herbal extracts have shown promise for helping to reduce cancer risk. However, the evidence isn’t yet strong for any of these potential “chemopreventive” agents. The major obstacle is a generic one: No matter what the treatment, whether natural or man-made treatment, it is difficult to determine whether a proposed cancer-preventing treatment actually works.

Only double-blind, placebo-controlled studies can prove a treatment effective; such studies are always expensive and somewhat difficult to perform, but in the case of cancer prevention they are extraordinarily expensive and extremely difficult. The primary reason is that the early stages of cancer develop slowly, often over a period of decades. This necessitates maintaining a study for many years. This multiplies costs, and if too many people drop out by attrition, the results of the study may become invalid. Another problem involves statistics. Cancer is relatively uncommon:.Over a given period of a few years, the probability of a healthy person developing cancer remains relatively small. When a study is examining relatively rare events, it must enroll a high number of people to produce mathematically meaningful results. This combination of long-term and large translates into “expensive and difficult.”

In fact, some of the largest and most expensive double-blind studies in history were those performed to test the efficacy of vitamin E and/or beta-carotene for preventing cancer. The results of these studies were unexpectedly negative: the supplements did not reduce and might even have increased cancer risk. These outcomes have had a chilling effect on the governments and foundations that finance such studies.

Nonetheless, natural products continue to be investigated as possible “chemopreventive” agents. Much of the evidence that makes it into the media involves studies conducted in a test tube. “Observational studies” constitute another type of widely reported evidence. In such studies, huge numbers of people are observed for a long period of time, and researchers try to find correlations between (for example) what people eat and what diseases they develop. Unfortunately, both test tube and observational studies are highly unreliable—it was studies of these types that led to the giant, ultimately negative studies of vitamin E and beta-carotene. Only double-blind studies can really prove a treatment effective, and these remain relatively few in the field of cancer prevention.

Calcium is one of the supplements still under study as a cancer-preventive treatment. Numerous observational and some double-blind studies have found evidence that calcium supplementation may reduce the risk of colon cancer. However, not all double-blind studies have been positive, and the issue remains in doubt.

New findings do support the possibility that calcium supplements have an anti-cancer effect, at least in women. This evidence comes from a study primarily designed to test the effectiveness of calcium and vitamin D for preventing osteoporosis in postmenopausal women. The researchers who conducted this trial additionally examined cancer incidence. The results were encouraging. In this four-year study of 1179 women, use of calcium, especially when combined with vitamin D, significantly reduced overall cancer risk.

Note, however, this study involved only women. There are some concerns that calcium supplements might increase risk of prostate cancer (in men).

For more information, including dosage and safety issues, see the full articles on calcium , vitamin D , and cancer prevention .

  • Lappe JM, Travers-Gustafson D, Davies KM, et al. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr . 2007;85:1586-91.