A new study suggests an impressive reduction in cancer death after bariatric surgery, but it is not the final word.

There is an epidemic in this country that has been raging far longer than COVID-19, claiming around 300,000 lives a year in the United States. That epidemic? Obesity.

Obesity is a major contributor to cardiovascular disease and mortality, but less often discussed is the role of obesity in promoting cancers. Multiple cancer subtypes are, at least in part, related to obesity, including colorectal cancer, breast cancer, ovarian cancer, liver cancer, multiple myeloma and others. These associations have been well-described in the literature. Less well-described? Does losing weight prevent cancers?

Why don’t we have better data on the impact of weight loss on cancer incidence? Well, one major reason is that cancer takes a long time to develop and losing weight – and keeping it off – is really hard.

Currently, and to be fair this is a pretty loaded “currently”, the most effective method for sustained weight loss is bariatric surgery.

So – an obvious question – do people who undergo bariatric surgery reduce their risk of cancer?

We got a potential answer to that question last week from this article, appearing in JAMA, that leveraged the impressive electronic health record data infrastructure of the Cleveland Clinic hospitals to create a cohort of 5053 individuals who underwent bariatric surgery and 25,265 matched controls.

Now, let me be clear – this is not a randomized trial. People who get bariatric surgery are different than people who don’t, even after matching – as the authors did – on metrics like BMI, comorbidities, age, sex, and race. We need to keep that in mind as we examine the primary results.

Because the primary results are fairly dramatic.

Obviously, the people who underwent bariatric surgery had greater weight loss. No surprise there.

But the cancer outcomes are pretty impressive. With an average of around 6 years of follow-up, individuals who underwent bariatric surgery had a 30% reduction in obesity-associated cancer incidence, and nearly a 50% reduction in cancer-associated mortality.

There was even evidence of what you might call a dose-response effect. Bariatric surgery patients who lost more weight had a lower incidence of cancer than those who lost less weight.

If we take these results as true and causal, bariatric surgery would be a cancer-prevention intervention with a greater effect than getting heavy smokers to quit. There should be massive public service campaigns urging people with elevated BMI to go under the knife.

But causality can’t be assessed with a study like this, even with the matching and propensity scoring done here. The control patients had an elevated BMI, but did not undergo screening for bariatric surgery, which often requires dedicated weight loss attempts and even pre-surgical cancer screening, which could have the effect of “weeding out” those with early stage cancers before they get surgery.

Some signs that the groups are not truly comparable include the fact that the bariatric surgery group was much more likely to undergo cancer screening after surgery, and the fact that surgery was associated with a reduction even in cancers that are not thought to be obesity-associated.

All that being said, the challenges of a real randomized trial in this space – with the huge costs and long-term follow up may mean that studies like this are the best we’re going to get. And given the fact that obesity is associated with more than cancer, and the advent of some truly impressive medical therapies, the days of ignoring elevated BMI may be as numbered as the days of ignoring smoking status were forty years ago.

A version of this commentary first appeared on Medscape.com.