Researchers found that among nearly 90 000 women aged 40 to 59, those who had ever used oral contraceptives were 17 percent less likely to develop colon cancer over the next 16 years. The findings, which appear in the International Journal of Cancer, are in line with evidence suggesting that oestrogen plays a role in colon cancer risk.

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Some studies, for example, have found that older women on hormone replacement therapy (HRT) have a lower risk of the disease. In addition, lab experiments have shown that oestrogen may inhibit tumour development in the colon by affecting cell growth, or by lowering levels of a cancer-linked hormone called IGF-1.

However, it's too soon to conclude that birth control pills offer colon cancer protection, according to lead researcher Dr Geoffrey C. Kabat, of Albert Einstein College of Medicine in New York. For one, he said, the risk reduction was small. In addition, the study found no "dose-response" relationship between oral contraceptives and colon cancer - that is, the risk reduction was not greater among women who'd used birth control pills for longer periods. In general, a positive dose-response strengthens the case for cause-and-effect relationships.

Other factors should be taken into account
It's also possible that there is something else about women who use birth control pills that makes them less susceptible to colon cancer, Kabat explained. They may, for example, be more physically active and weigh less - two factors that studies suggest may lower the risk of colon cancer.

Even though the researchers attempted to account for lifestyle habits and other factors in their analysis, Kabat said they cannot exclude those things as an explanation for their findings.

The results are based on 89 835 Canadian women taking part in a study on breast cancer screening that followed them for an average of 16 years.

During that time, women who had ever used birth control pills were less likely to develop colon cancer. However, this was not true of women who had used HRT, in contrast to what several previous studies have found.

According to Kabat, there are still "many questions to sort out" regarding hormone use and colon cancer, and the results of any single study have to be interpreted cautiously. More studies are needed, he said, to figure out what factors are important in colon cancer development. – (ReutersHealth) - February 2008

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