People who regularly take aspirin cut in half their risk of dying from colon cancer, according to the largest, most definitive epidemiologic study to investi­gate this link.  The finding suggests a relatively inexpensive strategy for bat­tling a scourge that currently claims the lives of about 50,000 Americans annually.

The new work also adds weight to the findings of a much smaller study re­ported earlier this year.  Although that retrospective study of about 6,000 men and women did not address cancer-sur­vival rates, it did indicate that regular aspirin use may reduce the incidence of colon cancer (SN: 3/16/91, p.166).

Spurred by those findings, epidemiolo­gist Michael J. Thun and his colleagues at the American Cancer Society in Atlanta correlated aspirin use and colon-cancer deaths among the 662,424 men and women they had been tracking since 1982 as part of the societys Cancer Prevention Study.  Upon entering the study partici­pants had answered a questionnaire cov­ering a range of behavioral, dietary and lifestyle factors, including frequency of aspirin use during the past year.

The risk of dying from colon cancer decreased with increasing aspirin use, Thuns team reports in the Dec. 5 NEW ENGLAND JOURNAL OF MEDICINE.  Colon-cancer death rates among the most fre­quent users-those taking aspirin 16 times or more per month were 60 percent as high as that seen in the study's aspirin abstainers.  And when the re­searchers controlled for risk factors such as diet, obesity and family history of colon cancer, the risk dropped to 50 percent of the colon-cancer death rate of the aspirin abstainers.

Its an interesting finding, says epi­demiologist John A. Baron of Dartmouth Medical School in Hanover, N.H.  He notes, however, that the new study does not prove that aspirin itself helps prevent colon cancer.  For examp1e, he points out that side effects of frequent aspirin use, such as Intestinal bleeding may have caused members of this subgroup to seek medical attention more frequently than other volunteers thereby increasing the likelihood that any developing colon can­cer would receive early diagnosis.  And early diagnosis increases the chance that a person with colon cancer will survive.

A large, randomized trial in which some people take aspirin and others take placebo pills would provide firmer evi­dence of aspirins protection against co­lon cancer.  But such a gold-standard study might be difficult to conduct, Baron says, because people who know about aspirins widely publicized heart benefits (SN: 7/27/91, p.55) might balk at the prospect of receiving placebo pills in­stead.*

For now, Baron and Thun remain cau­tious about advocating regular aspirin use for cancer prevention, since it can cause potentially dangerous side effects.**   At the same time, they note that aspirin may provide secondary anticancer bene­fits for people who already take it to manage arthritis pain or to reduce the risk of heart disease.

Even if epidemiologists can demon­strate that aspirin fights colon cancer, there remains the question of how it exerts this effect.  Like other nonsteroidal anti-inflammatory drugs (NSAIDs), aspi­rin inhibits the synthesis of compounds called prostaglandins, which spur body cells including colon cells to proliferate.  Thun suggests that aspirin might prevent rampant cell division a key attribute of cancers by interfering win prostaglandin production.

A number of laboratory studies have shown that aspirin and other types NSAIDs inhibit the growth of chemical induced colon tumors in rats and mice.  Other studies have shown that an NSAI called sulindac can shrink large-bow polyps in people.  Such polyps, though benign, can develop into cancers, Thun notes.  Taken together, the individual pieces of evidence provide scientists wit compelling reasons to further explore the link between aspirin and colon cancer, Thun contends.   KA. Fackelman, Science News, 1994, p. 374.***

*   The contravening variable of early medical attention due to intestinal bleed would can be weeded out if they looked not at survival rates but rates of getting colon cancer.  Early detection has little impact in a long-term study. 

**  This problem as well as the identification of the placebo are laid to rest with the use of enteric (coated aspirin), for it digests in the intestines, which is alkaline.  Since aspirin is a weak acid, its acidity neutralized, and thus as is to be expected the enteric aspirin does not cause stomach bleedingand there is little risk of intestinal bleeding. 

***  Subsequent studies have confirmed this prophylactic result and the science behind it.