Stained Microscopy Image of the Human Colon

Questions About Obesity, Surgery, and Colorectal Cancer

If you want an example of extremely unhelpful health reporting, look no further than two reports this week on obesity, surgery, and colorectal cancer. Two different studies. Precisely opposite findings on a difficult subject. Very little perspective. Taken at face value, one can only conclude that surgery might add to the risk of colorectal cancer in obesity. Or it might cut that risk by 35 percent.

But the worst malpractice, in our opinion, comes from an epidemiologist who comments on one of the studies. “Just keep calories in check,” says Graham Colditz of the Washington University School of Medicine in St Louis. Seriously, sir. Where is the data to back up that solution for preventing colon cancer?

What Is Clear

In all of this, it’s pretty clear that colon cancer risk is higher for people with obesity. It’s also clear that for people with significant obesity, bariatric surgery can be the most effective treatment available. Not a miracle cure. Not a simple course to take. But extremely helpful for many people

It’s also clear that the impact of surgery on colorectal cancer risk is extremely hard to study. And that’s probably why these folks get conflicting results.

One Cohort of Patients from Nordic Countries

The negative study in the news uses a cohort of 49,931 patients who had bariatric surgery. Those patients came from a total cohort of 502,772 patients with an obesity diagnosis. When the researchers compared the surgery patients to everyone else, they found a 13 percent greater hazard for colon cancer in the surgery group.

But here’s the thing. Even though the researchers tried to adjust for confounding variables, these were not matched groups. As the researchers note, selection bias is an issue. Residual confounding could explain the differences observed here. For example the authors don’t think BMI differences would explain the differences in risk. But they don’t have the BMI data necessary to prove or disprove their speculation.

A Large Meta-Analysis

The second study in the news this week was a large meta-analysis of studies with more than 1.2 million patients. It was this study that found a 35 percent reduction in colorectal cancer risk after surgey.

Of course, this study has limitations as well. It, too, is retrospective. Plus, it also has the risk of residual confounding that the other study suffers from. That’s because neither study is truly randomized.

So even though the authors of this second study say that the “literature is clear” on risk reduction for colorectal cancer, we take it all with a grain of salt. What is abundantly clear is the many other benefits of bariatric surgery. For patients who understand both the risks and benefits, it can be an excellent option.

Click here for the Nordic cohort study, here for the meta-analysis, and here for further perspective on obesity and colorectal cancer.

Stained Microscopy Image of the Human Colon, photograph © Hayfaa A. Alshammary / Wikimedia

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February 9, 2020

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