Healthnotes Newswire (October 19, 2000)—Scientists once believed that a lack of dietary fiber increased the risk of colon cancer.¹ The culprit appeared to be a diet high in white flour and low in vegetables and whole grains. More recently, studies revealed that consumption of meat—not a lack of fiber—was the main dietary component responsible for an increased risk of colon cancer.^{2 3 4}

The latest chapter in the colon cancer saga suggests that supplementation with ispaghula husk, a type of fiber, increases the risk of colon polyps among people with a history of precancerous colon polyps, according to a report just published in The Lancet.⁵ Increased colon polyp formation is a marker for increased risk of colon cancer.

Ispaghula husk, a close cousin to psyllium seed, is known to have anticancer effects in animals.⁶ Like psyllium seed, ispaghula increases stool weight and helps decrease constipation. All these factors led researchers to expect that ispaghula husk might reduce risks for precancerous polyps in the colon—a change that should reduce the risk of colon cancer.

Surprisingly, men given ispaghula supplements experienced more than a doubling of the risk of polyp formation compared with men in the control group—an increase that was statistically significant. Women assigned to take fiber supplements were reported to have only a small (11%) increase in the risk of polyp formation compared with women in the placebo group—a difference that might have been due to chance.

In the same trial, other people with a history of colon polyps were assigned to take calcium supplements. The researchers had hoped that calcium supplementation might lower the risk of precancerous colon polyps on the basis of findings from previous research.⁷ Men receiving calcium supplements experienced a 42% lower risk of forming a polyp compared with men in the control group, although this result may have been due to chance. Women who were assigned to take calcium supplements had no reduction in risk of polyp formation compared with women in the control group.

Several factors may have hampered calcium’s ability to prove itself an anticancer agent in the Lancet trial. By chance, a higher percentage of the calcium group had had multiple polyps before the trial began, compared with people in the fiber and placebo groups. This difference may have increased the risk of polyp recurrences in the calcium group, appearing to erase potential benefits that might otherwise have been attributed to calcium supplementation. Also, compliance was poorer in the calcium group compared with the other groups. More than 30% of the calcium pills were never swallowed.

Virtually all the protection against polyp formation seen in the group assigned to take calcium supplements occurred in those who had had polyps on the right side of the colon before the trial began. Researchers had no explanation for this unusual finding. More research is needed before scientists will conclude that calcium supplements help protect against polyps or colon cancer.

Do the findings of this trial suggest that fiber increases the risk of colon cancer? In terms of ispaghula husk and possibly psyllium seed supplements, the answer might be yes. The researchers noted that the increased risk of colon polyp recurrences did not occur in those people assigned to take the fiber supplement who had poor compliance. Rather, the increased risk occurred primarily in those who actually took the fiber as they had been instructed to do, suggesting that the problem is real and not due to chance.

The findings of this trial most likely do not suggest that supplementation with other fibers will increase the risk of polyp formation. In previous trials in which people were given wheat bran, the number or size of polyps formed was slightly reduced, although these positive effects were quite limited.^{8 9} The researchers of the current trial concluded that their findings “. . . should not prevent recommendations for high consumption of vegetables, fruits, and cereals, because this approach has potentially beneficial effects on other chronic disease, especially . . . heart disease.”

All 552 people who completed the trial had a history of precancerous colon polyps. High-fiber group members were given 3.5 grams per day of ispaghula husk, and those assigned to take calcium received 2 grams of calcium per day. Subjects were followed for approximately 18 months—not long enough to see differences in the risk of colon cancer itself. As a result, the researchers tracked the formation of new polyps—an accepted marker for colon cancer risk.